I probably should not be surprised to have had this problem having disregarded the two of the main safety caveats from page 3E of the Rider Safety Check section of the owners manual:
- Never use off-road. Use only on smooth surfaces.
- Do not ride on highways, in traffic, or on unsafe streets.
Perhaps the real lesson here is to always read the small print.
Given that our only access to a road is along about 100m of track I probably should not have bought it.
On the other hand, if you take these safety instructions literally, then ... um excuse me but exactly where can you ride it?
Put it on a bike rack and drive everyone to the park for a nice little ride round I suppose.
Kind of defeats the object of having it really.
I think I'll just go and tidy up the mess left by that Chocolate Fireguard we got the other day ...
Wednesday 16 December 2009
Sunday 13 December 2009
The problem and how to stop it happening.
Why we bought a Trail-Gator.
Our son having reached the age where he could ride a small bicycle with stabilisers but not for any great distance and not yet safely on the road, we looked at buying some form of bicycle towing attachment.
These fall into two main categories: so-called Tag-a-Longs or Trailer Bikes and Towbars.
The problem with the Tag-a-Longs or Trailer Bikes is that whilst they are a good way to get to somewhere safe for your child to ride and are a life-saver once the point of "too-tired-legs" has been reached, they are actually of no use for your child to ride on their own in the periods in between (unless they learn wheelying and unicycling skills at the same time).
Towbars solve this problem by allowing you to tow your child on their own bike and then letting them ride it around until it's time to tow it back again.
After a bit more looking around we found the Trail-Gator which seemed to be about the only thing on the market at that time (around November 2008) that met our requirements.
The reviews for the Trail-Gator that we could find then seemed pretty favourable albeit with some saying that it was very fiddly to fit and set up and one or two that mentioned that occasionally the towed bike shifted out of alignment behind the towing bike. However, although the full fitting instructions can be found here, without having seen one in the metal it was difficult to judge how much of a problem this might be.
So about the middle of November 2008, we bought one, together with the detachable stabiliser wheels.
Some notes about the original assembly.
The reviews were certainly right about it being fiddly to set up! It really needs two people - or even three - for some of the time: one to do the fitting, the other(s) to hold whichever of the bikes you're working on.
The instructions are quite detailed but do need to be read through at least once to get the hang of how the kit is to be put together.
Similarly, the towbar attachment clamp on the towed bike is held on by two U-bolts and may require one or more of the supplied shims to set the lift of the front wheel above the ground. The fact that the whole kit is designed to fit the widest possible range of bikes means that it fits none of them ideally well and there is inevitably a lot of free play in all the fittings. Therefore all the parts are free to shift around as you are tightening them up, making exact positioning a time consuming task. This clamp is particularly tricky to fit as it may well take more than one attempt to find the best, most stable position for it. Probably adding the advice to clout the attachment clamp a few times with a rubber mallet to make sure it really is settled securely into it's intended resting place might not be a bad idea.
Some of the "one-size-fits-all" solutions involve the use of deformable aluminium sleeves to adapt a couple of the other clamps to a range of tube diameters and shapes. These are OK but are a bit of a one-shot. If you don't get it in the right position the first time but have tightened the clamp even part way then you may find you have deformed the sleeve beyond any chance of a second attempt.
So, armed with a click-type torque wrench and background of many years of motorcycle maintenance I set to work and a few hours later had one fully assembled Trail-Gator and one very excited small boy.
The first outing
Three of us set off from home along about a 100m stretch of dirt track (what constitutes our drive!) followed by about 300m of road, 1.5km of farm track and then around 3km of road. A round trip of roughly 10km. All of this on the flat as we live in the Vale of York.
All went well with the Trail-Gator being surprisingly easy to handle and even some useful pedalling going on behind me. Discussion during a picnic at the halfway point revealed that the towed bike was leaning a little to one side but since there is a certain amount of combined - and unavoidable - play in the aforementioned universal joint assembly at the towing bike end, in the towbar attachment to this joint and in the towbar length adjustment pin this didn't raise any alarm bells. The effect was duly noted and I would attempt to reduce the tilt or make it so that it was not biased to one side or the other once we got home again.
At this point I should admit that I did make one mistake on this first ride. I did not come out with the necessary tools to make any adjustments. On the other hand, I don't normally ride a bicycle with a rucksac full of spanners, a socket set and a torque wrench strapped to my back.
Silly me.
This is what happened next
About one third of the way back home I heard some shrieking and scuffing sounds and looked behind me to tell what I had thought was a cheery boy squealing with delight to stop messing about and dragging his shoes on the road. To my horror I realised that both he and my wife were screaming at me to stop and he was dragging his right shoe on the road because was no longer being towed in line behind me but at an angle of about 45 degrees to my right with his bike canted over at something like 30 degrees also to the right.
This left him terrified and clinging helplessly onto his dangerously tilting bike at almost a metre out from my bike towards the middle of the road as if he were going to overtake me
Fortunately it was a quiet stretch of road and I was able to come to a slow, controlled halt without tipping either of us off. After calming one very frightened little boy and a pretty scared mum, I had a look at the bike-Trail-Gator-bike assembly.
It was easy to see what had happened: the towbar attachment clamp had rotated round the steering tube of the towed bike. The correct position is illustrated in Figure 1 and accompanying photos with the rotated position in Figure 2 and it's accompanying photos.
After detaching the towbar at the towing bike end and a bit of grunt work using it as a lever I was able to turn the clamp back to the central position again. After a lot of encouragement, I was able to get our son back on his bike and we set off again very slowly only to find that the same thing was happening again almost immediately.
This time when we stopped, we detached his bike, stowed the towbar on mine and flipped the stabilisers down on his so he could ride by himself. All the way during the slow pedal home I was getting more and more angry with myself that I must have done something wrong or been careless in fitting the Trail-Gator.
Once we got home I went over it checking the fittings with the instructions in one hand and the torque wrench in the other. And not surprisingly, the U-Bolts on the steering tube attachment clamp could be tightened up a little more to the right torque.
Hang on though: those U-Bolts were tightened to the recommended torque before we set off. I'd made absolutely sure of that. So why were they loose now?
What went wrong?
Looking closely at the steering tube showed that where the clamp had rotated, it had rubbed the paint off the painted mating faces of the clamp (i.e. not from the unpainted U-Bolt itself) and the steering tube at the points where they touched (the contacts points) under the pressure of the clamping action. That would have reduced the effective diameter of the steering tube at the same time as increasing the effective diameter that the clamp had been tightened to by some fraction of a millimetre (how much I don't know, I didn't try to measure the combined depth of paint). That would be enough to reduce the tension in the U-Bolts and so slacken the clamp slightly.
The combined weight of the towbar, the towed bike and the towed rider acts through the centre of gravity of this assembly. A bit of simple basic mechanics will show that this is somewhere about or slightly in front of the towed rider.
The towbar is only rigidly attached at the towed bike steering tube so if the clamp slackens then the towbar is no longer rigidly fixed at either end of the assembly. The steering tube is angled so that the top of the tube is closer to the rider than the bottom (by the angle of the steering rake plus it is lifted up and so tilted further back by the towbar). This is an inherently unstable mechanical arrangement. Therefore once the clamp slackens, when the assembly tilts even slightly to one side or the other, a component of the downward force exerted by combined weight of the assembly will apply a sideways force to the joint formed by the steering tube and the clamp. This will then cause the steering tube to try to rotate inside the clamp.
At this point the layers of paint at the points of contact start to disintegrate and so loosen the joint enough for it to start to rotate.
Once this rotation starts, the tilt increases and the sideways force increases rapidly so the joint rotates quite quickly.
The angle of the steering tube means that as the towbar and towed bike swings out from where it should be in line with the towing bike, the towed bike is also tilted over at an angle from the vertical. This also increases the sideways force exerted at the point of rotation of the steering tube in the clamp and so pushes the towed bike further out to one side of the towing bike and further tilts it over.
Ironically it is this sequence of events that contributes - even with no rider on either bike - to making the initial assembly and alignment process very difficult to do without at least one other person to hold all the component parts steady!
Why did it go wrong?
As the clamp is tightened during assembly, the torque applied to the bolts may not be enough to crush the paint at the points of contact enough to ensure good metal to metal contact and so remove the possibility of slackening occurring as these paint layers compress.
So ensuring that all the paint at these points is removed and re-tightening the U-Bolts should ensure that a reliable secure joint is made, right?
Wrong.
There are only 10 points of contact in the whole of the clamp. Two on each side of the upper and lower V-plates (that's 8) and one between the back of the steering tube and each U-Bolt (that's 2). On each there is only a very small area of contact because they all touch onto the surface of a cylinder: the steering tube. Hence there can be a lot pressure exerted at each point but there is very little surface area across which to exert any friction to oppose any tendency to rotate.
Worse than that, careful observation of the clamp as it is tightened shows that it bends slightly. This is illustrated in Figure 3.
There are two problems with this.
The first is that it means there is some springiness in the joint which will allow the towbar to slightly swing up and down and side to side as the shock loads generated during normal riding are transmitted through the clamp.
This in turn will eventually wear the joint faces of the points of contact so allowing slackening again.
The second is that this same slight movement combined with the possibility of repeated retightening - perhaps as part of a regular inspection and servicing routine - could easily lead to the clamp being permanently bent out of its original shape. Such permanent distortion weakens the clamp's resistance to further bending and so makes it less resilient to the shock loads referred to above. In fact if the clamp is not retightened regularly, any bending will relax the pressure on the joint faces and so slacken the joint.
It is possible that extreme overtightening of the clamp could prevent it rotating but only because it distorts the steering tube itself.
This is illustrated in Figure 4.
Clearly this would be undesirable and could be very dangerous because it may weaken the structure of this region of the bicycle.
A solution
What is needed is a way to stop the towbar attachment clamp rotating about the steering tube.
After considering a number of options of varying complexity I arrived at the relatively simple, lightweight and unobtrusive plan of fitting two rods each fixed to holes already present in the clamp at one end to the seat tube at the other end.
This is illustrated in Figure 5a.
By fixing each rod to the seat tube with its own fastening this effectively forms a triangle as illustrated in Figure 5b. The triangulation can be seen in the photo below.
Each rod is fixed to the seat tube by a separate fastening to form an almost ideal triangle. It is this triangle that prevents the clamp rotating: any tendency to turn in one direction is prevented by the tension in one of the rods pulling on the edge of the clamp. By symmetry, this is true for rotation in either direction.
Fixing both rods to a single fastening would allow a small amount of rotation of the clamp because the effective shape of the rods and fastenings would no longer form a triangle but a trapezium shape, which is not as mechanically rigid. Although this movement is negligible it was decided to adopt a vertically offset single fixing for each rod to simplify construction and remove any possibility of unnecessary movement which may contribute to wear and fatigue of the fastenings.
Although this form of fixing creates a small vertical offset between the seat tube ends of each rod, this is considered acceptable since there is negligible force being applied in the vertical plane through these two points.
ii) in the following instructions, the terms front and back refer to the directions as seen by the rider of the towed bike when facing forwards.
iii) in the following instructions, the terms right and left refer to the sides as seen by the rider of the towed bike when facing forwards.
Materials you will need together with some suggested sources:
8 off M6 nuts
Wickes Product Code 511126
2 packs of this will get you 10 off M6 nuts and 10 off M6 standard washers:
4 off M6 standard washers
See above.
4 off M6 large ("Penny") washers
1 off (or, depending on the distance from the clamp to the seat tube, possibly 2 off) 1m lengths of M6 threaded rod (studding)
B&Q: EAN: 3232630218402.
Approximately 30cm of Multi Purpose Fixing Band or "Builders Band". The exact length depends on the diameter (and shape if it oval) of the seat tube. Fortunately, although Builders Band is only available in rolls of several metres, is not expensive.
Screwfix: Quote No: 38619.
Wickes: Product Code 246015.
Tools
Small hacksaw (to cut threaded rod and builders band)
(Strong wire cutters may be used as an alternative way to cut builders band)
File (to round off edges on builders band)
2 off 10mm open ended spanners (a 10mm spanner fits an M6 nut)
A pair of long nosed pliers (to hold the washers)
An indelible felt tip marker pen (to mark the length of the threaded rod)
Before starting:
a) familiarise yourself with the completed assembly shown in the various drawings and photos shown throughout this article.
b) assemble the towbar onto the clamp and make sure that the towbar is correctly aligned with the centre line of the towed bike before fitting the rods. If necessary, refer to the original fitting instructions to adjust the clamp.
To make the rods
Start by marking one end of the threaded rod.
Starting from the front of the bike, slide the marked end of the threaded rod through either of the rounded slots in the clamp and then holding the rod so that it is at right angles to the flat rear face of the clamp, slide it through until the marked end just touches the point on the surface of the seat tube facing closest to the clamp (when you do this, the rod will lie along a diameter of the seat tube).
Holding the rod in this position, mark it about 1.5cm - 2cm beyond where it protrudes from the front face of the clamp.
By doing this you should have marked a length of rod that is about 1.5cm - 2cm longer than the distance between the seat tube and the clamp.
Remove the rod from the bike and cut the rod at this second mark.
Use this cut piece of rod as a template to cut a second piece to the same length.
Round off any sharp edges at the cut ends with the file.
To make the straps
Next, make the two straps to hold the ends of the threaded rods onto the seat tube.
You will need to work out the length of builders band you need for these two straps as this will vary with the diameter of the seat tube on your bike.
It may be simplest to do this by cutting a length that you think will be about right and then using this to estimate the correct length by doing a trial fit with this sacrificial piece.
They need to be long enough that the ends overlap by one hole at each end but allow enough space between the the strap and the seat tube to get a washer and a nut in place to secure one end of the threaded rod. The overlapping holes must be the larger ones along the centre of the builders band and not the smaller ones along the edges. See photos below.
Once you have worked out the length you will need, cut two strips of builders band to the same length, round off any sharp edges and corners with the file and form them roughly as shown in photos above. The band is thin enough to allow this forming to be done by hand using the long nosed pliers to hold it when making the sharper bends near the ends. When fitted, this shaping will allow the two ends of the bands to fold over neatly and minimise any protrusion of the outside end.
Form the first strap around the seat tube as shown above.
Fitting the rods
Although it may be slightly easier to follow the order of assembly described below, it does not matter in which order you fit the rods. However, you do need to get the left and right sides assembled in the correct positions otherwise you may foul the towbar quick release mechanism. See photos below.
To fit the left hand side rod.
Thread two nuts, one about 5cm in from each end of the rod. Next, slide a large washer along to the nut at one end and a standard washer at the other.
At this stage your rod should look like the one on the bench in the photo in the "Tools" section above.
Feed the end of the rod with the large washer on it through the rounded slot in the back of the left hand side of the clamp and then fit the other end (with the small washer on it) through the overlapping holes in the lower of the two formed seat tube straps. The tension in the strap should then hold the end of the rod in place.
Sorry but this next bit is fiddly and you will probably need the pair of long nosed pliers.
Holding the strap in one hand, slip a washer over the end inside the strap and then offer up the nut inside the strap whilst twisting the rod to engage the thread. Screw on the nut until the rod just protrudes from the back of the nut as in the photo below.
Hold the free end of the rod at the bottom of the slot in the clamp and then slide the strap up or down the seat tube until the rod is at right angles to the clamp.
Screw the nut on the outside of the strap all the way down to the band and then tighten using a pair of spanners to hold the inner nut in place. Continue tightening until at least one half a turn after the two ends of the band have been fully compressed against each other.
Ensure that the clamp end of the rod is still at the bottom of the rounded slot and at right angles to the clamp.
Fit a large washer followed by a nut to the rod from the front face of the clamp.
Tighten the nut at the front face of the clamp until the rod is just held in place at the bottom of the slot by the tension in the rod. Do not tighten any further at this stage because this may start to twist the clamp around the steering tube.
To fit the right hand side rod.
Fitting the right hand rod is the same as for the left hand rod except that:
i) the clamp end of the rod must be positioned to the top of the rounded slot on the right hand side of the clamp and;
ii) locate the seat tube end of the rod in the upper of the two seat tube straps;
iii) slide the upper of the two seat tube straps down the seat tube so that it butts up against the lower strap.
Finishing off.
Once both rods are held in place by the nuts at the front of the clamp, tighten these nuts alternately, about half a turn at a time. This keeps about the same tension in each rod. It is important to do this because any imbalance in the tension may twist the clamp round the steering tube and upset the towbar to towed bike alignment.
Continue tightening the nuts equally until lightly tapping the rods with a spanner produces an audible ringing. Then to ensure that the tension is the same in each rod adjust the nuts - slackening one and/or tightening the other as necessary - until the pitch of the ringing in each rod is the same.
To lock the clamp end of the rods in place, tighten the nuts at the back of the clamp on each rod until the washers are pinched up (i.e. they can no longer be moved by hand). Holding each nut at the front of the clamp with one spanner, tighten the nut at the back with the other by a further one turn.
Finally, check the tightness of all fixings, pack a bag of tools, go for a test ride and then check them again.
Then you're done.
Confidence restored ...
Our son having reached the age where he could ride a small bicycle with stabilisers but not for any great distance and not yet safely on the road, we looked at buying some form of bicycle towing attachment.
These fall into two main categories: so-called Tag-a-Longs or Trailer Bikes and Towbars.
The problem with the Tag-a-Longs or Trailer Bikes is that whilst they are a good way to get to somewhere safe for your child to ride and are a life-saver once the point of "too-tired-legs" has been reached, they are actually of no use for your child to ride on their own in the periods in between (unless they learn wheelying and unicycling skills at the same time).
Towbars solve this problem by allowing you to tow your child on their own bike and then letting them ride it around until it's time to tow it back again.
After a bit more looking around we found the Trail-Gator which seemed to be about the only thing on the market at that time (around November 2008) that met our requirements.
The reviews for the Trail-Gator that we could find then seemed pretty favourable albeit with some saying that it was very fiddly to fit and set up and one or two that mentioned that occasionally the towed bike shifted out of alignment behind the towing bike. However, although the full fitting instructions can be found here, without having seen one in the metal it was difficult to judge how much of a problem this might be.
So about the middle of November 2008, we bought one, together with the detachable stabiliser wheels.
Some notes about the original assembly.
The reviews were certainly right about it being fiddly to set up! It really needs two people - or even three - for some of the time: one to do the fitting, the other(s) to hold whichever of the bikes you're working on.
The instructions are quite detailed but do need to be read through at least once to get the hang of how the kit is to be put together.
- It is important to note that the assembly requires the use of a torque wrench to accurately set the tightness of nuts on several of the fittings.
- Particularly in view of the problems we encountered it is worth stating right now that if you do not have access to - or do not know what is meant by - a torque wrench then you should not be trying to assemble this kit yourself.
Similarly, the towbar attachment clamp on the towed bike is held on by two U-bolts and may require one or more of the supplied shims to set the lift of the front wheel above the ground. The fact that the whole kit is designed to fit the widest possible range of bikes means that it fits none of them ideally well and there is inevitably a lot of free play in all the fittings. Therefore all the parts are free to shift around as you are tightening them up, making exact positioning a time consuming task. This clamp is particularly tricky to fit as it may well take more than one attempt to find the best, most stable position for it. Probably adding the advice to clout the attachment clamp a few times with a rubber mallet to make sure it really is settled securely into it's intended resting place might not be a bad idea.
Some of the "one-size-fits-all" solutions involve the use of deformable aluminium sleeves to adapt a couple of the other clamps to a range of tube diameters and shapes. These are OK but are a bit of a one-shot. If you don't get it in the right position the first time but have tightened the clamp even part way then you may find you have deformed the sleeve beyond any chance of a second attempt.
So, armed with a click-type torque wrench and background of many years of motorcycle maintenance I set to work and a few hours later had one fully assembled Trail-Gator and one very excited small boy.
The first outing
Three of us set off from home along about a 100m stretch of dirt track (what constitutes our drive!) followed by about 300m of road, 1.5km of farm track and then around 3km of road. A round trip of roughly 10km. All of this on the flat as we live in the Vale of York.
All went well with the Trail-Gator being surprisingly easy to handle and even some useful pedalling going on behind me. Discussion during a picnic at the halfway point revealed that the towed bike was leaning a little to one side but since there is a certain amount of combined - and unavoidable - play in the aforementioned universal joint assembly at the towing bike end, in the towbar attachment to this joint and in the towbar length adjustment pin this didn't raise any alarm bells. The effect was duly noted and I would attempt to reduce the tilt or make it so that it was not biased to one side or the other once we got home again.
At this point I should admit that I did make one mistake on this first ride. I did not come out with the necessary tools to make any adjustments. On the other hand, I don't normally ride a bicycle with a rucksac full of spanners, a socket set and a torque wrench strapped to my back.
Silly me.
This is what happened next
About one third of the way back home I heard some shrieking and scuffing sounds and looked behind me to tell what I had thought was a cheery boy squealing with delight to stop messing about and dragging his shoes on the road. To my horror I realised that both he and my wife were screaming at me to stop and he was dragging his right shoe on the road because was no longer being towed in line behind me but at an angle of about 45 degrees to my right with his bike canted over at something like 30 degrees also to the right.
This left him terrified and clinging helplessly onto his dangerously tilting bike at almost a metre out from my bike towards the middle of the road as if he were going to overtake me
Fortunately it was a quiet stretch of road and I was able to come to a slow, controlled halt without tipping either of us off. After calming one very frightened little boy and a pretty scared mum, I had a look at the bike-Trail-Gator-bike assembly.
It was easy to see what had happened: the towbar attachment clamp had rotated round the steering tube of the towed bike. The correct position is illustrated in Figure 1 and accompanying photos with the rotated position in Figure 2 and it's accompanying photos.
After detaching the towbar at the towing bike end and a bit of grunt work using it as a lever I was able to turn the clamp back to the central position again. After a lot of encouragement, I was able to get our son back on his bike and we set off again very slowly only to find that the same thing was happening again almost immediately.
This time when we stopped, we detached his bike, stowed the towbar on mine and flipped the stabilisers down on his so he could ride by himself. All the way during the slow pedal home I was getting more and more angry with myself that I must have done something wrong or been careless in fitting the Trail-Gator.
Once we got home I went over it checking the fittings with the instructions in one hand and the torque wrench in the other. And not surprisingly, the U-Bolts on the steering tube attachment clamp could be tightened up a little more to the right torque.
Hang on though: those U-Bolts were tightened to the recommended torque before we set off. I'd made absolutely sure of that. So why were they loose now?
What went wrong?
Looking closely at the steering tube showed that where the clamp had rotated, it had rubbed the paint off the painted mating faces of the clamp (i.e. not from the unpainted U-Bolt itself) and the steering tube at the points where they touched (the contacts points) under the pressure of the clamping action. That would have reduced the effective diameter of the steering tube at the same time as increasing the effective diameter that the clamp had been tightened to by some fraction of a millimetre (how much I don't know, I didn't try to measure the combined depth of paint). That would be enough to reduce the tension in the U-Bolts and so slacken the clamp slightly.
The combined weight of the towbar, the towed bike and the towed rider acts through the centre of gravity of this assembly. A bit of simple basic mechanics will show that this is somewhere about or slightly in front of the towed rider.
The towbar is only rigidly attached at the towed bike steering tube so if the clamp slackens then the towbar is no longer rigidly fixed at either end of the assembly. The steering tube is angled so that the top of the tube is closer to the rider than the bottom (by the angle of the steering rake plus it is lifted up and so tilted further back by the towbar). This is an inherently unstable mechanical arrangement. Therefore once the clamp slackens, when the assembly tilts even slightly to one side or the other, a component of the downward force exerted by combined weight of the assembly will apply a sideways force to the joint formed by the steering tube and the clamp. This will then cause the steering tube to try to rotate inside the clamp.
At this point the layers of paint at the points of contact start to disintegrate and so loosen the joint enough for it to start to rotate.
Once this rotation starts, the tilt increases and the sideways force increases rapidly so the joint rotates quite quickly.
The angle of the steering tube means that as the towbar and towed bike swings out from where it should be in line with the towing bike, the towed bike is also tilted over at an angle from the vertical. This also increases the sideways force exerted at the point of rotation of the steering tube in the clamp and so pushes the towed bike further out to one side of the towing bike and further tilts it over.
Ironically it is this sequence of events that contributes - even with no rider on either bike - to making the initial assembly and alignment process very difficult to do without at least one other person to hold all the component parts steady!
Why did it go wrong?
As the clamp is tightened during assembly, the torque applied to the bolts may not be enough to crush the paint at the points of contact enough to ensure good metal to metal contact and so remove the possibility of slackening occurring as these paint layers compress.
So ensuring that all the paint at these points is removed and re-tightening the U-Bolts should ensure that a reliable secure joint is made, right?
Wrong.
There are only 10 points of contact in the whole of the clamp. Two on each side of the upper and lower V-plates (that's 8) and one between the back of the steering tube and each U-Bolt (that's 2). On each there is only a very small area of contact because they all touch onto the surface of a cylinder: the steering tube. Hence there can be a lot pressure exerted at each point but there is very little surface area across which to exert any friction to oppose any tendency to rotate.
Worse than that, careful observation of the clamp as it is tightened shows that it bends slightly. This is illustrated in Figure 3.
There are two problems with this.
The first is that it means there is some springiness in the joint which will allow the towbar to slightly swing up and down and side to side as the shock loads generated during normal riding are transmitted through the clamp.
This in turn will eventually wear the joint faces of the points of contact so allowing slackening again.
The second is that this same slight movement combined with the possibility of repeated retightening - perhaps as part of a regular inspection and servicing routine - could easily lead to the clamp being permanently bent out of its original shape. Such permanent distortion weakens the clamp's resistance to further bending and so makes it less resilient to the shock loads referred to above. In fact if the clamp is not retightened regularly, any bending will relax the pressure on the joint faces and so slacken the joint.
It is possible that extreme overtightening of the clamp could prevent it rotating but only because it distorts the steering tube itself.
This is illustrated in Figure 4.
Clearly this would be undesirable and could be very dangerous because it may weaken the structure of this region of the bicycle.
A solution
What is needed is a way to stop the towbar attachment clamp rotating about the steering tube.
After considering a number of options of varying complexity I arrived at the relatively simple, lightweight and unobtrusive plan of fitting two rods each fixed to holes already present in the clamp at one end to the seat tube at the other end.
This is illustrated in Figure 5a.
By fixing each rod to the seat tube with its own fastening this effectively forms a triangle as illustrated in Figure 5b. The triangulation can be seen in the photo below.
Each rod is fixed to the seat tube by a separate fastening to form an almost ideal triangle. It is this triangle that prevents the clamp rotating: any tendency to turn in one direction is prevented by the tension in one of the rods pulling on the edge of the clamp. By symmetry, this is true for rotation in either direction.
Fixing both rods to a single fastening would allow a small amount of rotation of the clamp because the effective shape of the rods and fastenings would no longer form a triangle but a trapezium shape, which is not as mechanically rigid. Although this movement is negligible it was decided to adopt a vertically offset single fixing for each rod to simplify construction and remove any possibility of unnecessary movement which may contribute to wear and fatigue of the fastenings.
Although this form of fixing creates a small vertical offset between the seat tube ends of each rod, this is considered acceptable since there is negligible force being applied in the vertical plane through these two points.
- Notes:
ii) in the following instructions, the terms front and back refer to the directions as seen by the rider of the towed bike when facing forwards.
iii) in the following instructions, the terms right and left refer to the sides as seen by the rider of the towed bike when facing forwards.
Materials you will need together with some suggested sources:
8 off M6 nuts
Wickes Product Code 511126
2 packs of this will get you 10 off M6 nuts and 10 off M6 standard washers:
4 off M6 standard washers
See above.
4 off M6 large ("Penny") washers
1 off (or, depending on the distance from the clamp to the seat tube, possibly 2 off) 1m lengths of M6 threaded rod (studding)
B&Q: EAN: 3232630218402.
Approximately 30cm of Multi Purpose Fixing Band or "Builders Band". The exact length depends on the diameter (and shape if it oval) of the seat tube. Fortunately, although Builders Band is only available in rolls of several metres, is not expensive.
Screwfix: Quote No: 38619.
Wickes: Product Code 246015.
Tools
Small hacksaw (to cut threaded rod and builders band)
(Strong wire cutters may be used as an alternative way to cut builders band)
File (to round off edges on builders band)
2 off 10mm open ended spanners (a 10mm spanner fits an M6 nut)
A pair of long nosed pliers (to hold the washers)
An indelible felt tip marker pen (to mark the length of the threaded rod)
Before starting:
a) familiarise yourself with the completed assembly shown in the various drawings and photos shown throughout this article.
b) assemble the towbar onto the clamp and make sure that the towbar is correctly aligned with the centre line of the towed bike before fitting the rods. If necessary, refer to the original fitting instructions to adjust the clamp.
To make the rods
Start by marking one end of the threaded rod.
Starting from the front of the bike, slide the marked end of the threaded rod through either of the rounded slots in the clamp and then holding the rod so that it is at right angles to the flat rear face of the clamp, slide it through until the marked end just touches the point on the surface of the seat tube facing closest to the clamp (when you do this, the rod will lie along a diameter of the seat tube).
Holding the rod in this position, mark it about 1.5cm - 2cm beyond where it protrudes from the front face of the clamp.
By doing this you should have marked a length of rod that is about 1.5cm - 2cm longer than the distance between the seat tube and the clamp.
Remove the rod from the bike and cut the rod at this second mark.
Use this cut piece of rod as a template to cut a second piece to the same length.
Round off any sharp edges at the cut ends with the file.
To make the straps
Next, make the two straps to hold the ends of the threaded rods onto the seat tube.
You will need to work out the length of builders band you need for these two straps as this will vary with the diameter of the seat tube on your bike.
It may be simplest to do this by cutting a length that you think will be about right and then using this to estimate the correct length by doing a trial fit with this sacrificial piece.
They need to be long enough that the ends overlap by one hole at each end but allow enough space between the the strap and the seat tube to get a washer and a nut in place to secure one end of the threaded rod. The overlapping holes must be the larger ones along the centre of the builders band and not the smaller ones along the edges. See photos below.
Once you have worked out the length you will need, cut two strips of builders band to the same length, round off any sharp edges and corners with the file and form them roughly as shown in photos above. The band is thin enough to allow this forming to be done by hand using the long nosed pliers to hold it when making the sharper bends near the ends. When fitted, this shaping will allow the two ends of the bands to fold over neatly and minimise any protrusion of the outside end.
Form the first strap around the seat tube as shown above.
Fitting the rods
Although it may be slightly easier to follow the order of assembly described below, it does not matter in which order you fit the rods. However, you do need to get the left and right sides assembled in the correct positions otherwise you may foul the towbar quick release mechanism. See photos below.
To fit the left hand side rod.
Thread two nuts, one about 5cm in from each end of the rod. Next, slide a large washer along to the nut at one end and a standard washer at the other.
At this stage your rod should look like the one on the bench in the photo in the "Tools" section above.
Feed the end of the rod with the large washer on it through the rounded slot in the back of the left hand side of the clamp and then fit the other end (with the small washer on it) through the overlapping holes in the lower of the two formed seat tube straps. The tension in the strap should then hold the end of the rod in place.
Sorry but this next bit is fiddly and you will probably need the pair of long nosed pliers.
Holding the strap in one hand, slip a washer over the end inside the strap and then offer up the nut inside the strap whilst twisting the rod to engage the thread. Screw on the nut until the rod just protrudes from the back of the nut as in the photo below.
Hold the free end of the rod at the bottom of the slot in the clamp and then slide the strap up or down the seat tube until the rod is at right angles to the clamp.
Screw the nut on the outside of the strap all the way down to the band and then tighten using a pair of spanners to hold the inner nut in place. Continue tightening until at least one half a turn after the two ends of the band have been fully compressed against each other.
Ensure that the clamp end of the rod is still at the bottom of the rounded slot and at right angles to the clamp.
Fit a large washer followed by a nut to the rod from the front face of the clamp.
Tighten the nut at the front face of the clamp until the rod is just held in place at the bottom of the slot by the tension in the rod. Do not tighten any further at this stage because this may start to twist the clamp around the steering tube.
To fit the right hand side rod.
Fitting the right hand rod is the same as for the left hand rod except that:
i) the clamp end of the rod must be positioned to the top of the rounded slot on the right hand side of the clamp and;
ii) locate the seat tube end of the rod in the upper of the two seat tube straps;
iii) slide the upper of the two seat tube straps down the seat tube so that it butts up against the lower strap.
Finishing off.
Once both rods are held in place by the nuts at the front of the clamp, tighten these nuts alternately, about half a turn at a time. This keeps about the same tension in each rod. It is important to do this because any imbalance in the tension may twist the clamp round the steering tube and upset the towbar to towed bike alignment.
Continue tightening the nuts equally until lightly tapping the rods with a spanner produces an audible ringing. Then to ensure that the tension is the same in each rod adjust the nuts - slackening one and/or tightening the other as necessary - until the pitch of the ringing in each rod is the same.
To lock the clamp end of the rods in place, tighten the nuts at the back of the clamp on each rod until the washers are pinched up (i.e. they can no longer be moved by hand). Holding each nut at the front of the clamp with one spanner, tighten the nut at the back with the other by a further one turn.
Finally, check the tightness of all fixings, pack a bag of tools, go for a test ride and then check them again.
Then you're done.
Confidence restored ...
We have since ridden several tens of km over a variety of roads, cycle routes and farm tracks and there has been no movement in the steering tube to towbar clamp alignment.
Friday 4 December 2009
A good idea, badly implemented.
This blog is being written to describe the solution to a particular problem that myself and some other people have encountered with the Trail-Gator.
You can see this problem mentioned in some of the reviews on Amazon.
Time permitting, I will be posting pictures and instructions on what the problem is and what I did to prevent it from happening.
You can see this problem mentioned in some of the reviews on Amazon.
Time permitting, I will be posting pictures and instructions on what the problem is and what I did to prevent it from happening.
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