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Another Human Powered Vehicle Part 15 - Fixing Tyre Patter

Chasing a re-occurring problem

by Julian Edgar

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At a glance...

  • Testing the new rear suspension
  • Rear damping
  • Solving the wheel patter
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This article was first published in AutoSpeed.

Last week we covered the development of a new rear suspension arm, required to overcome a problem where the rear wheel would patter on bumpy roads taken at speed. The new arm used much larger tubing diameter and appeared to be much stiffer than the original. But was it any stiffer? And would it fix the wheel patter?

Measuring Torsional Twist

The first step in proving whether or not the new arm was stiffer was to measure on-road torsional deflection.

As covered last week, this measurement is made by mounting a long vertical arm on the rear suspension. The arm is equipped with a whiteboard marker that draws on a piece of white laminated chipboard mounted on the carrier. Any twist in the rear suspension is shown by a horizontal line being marked on the board; normal up/down movement of the suspension draws a vertical line.

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With the previous suspension, the width of the scatter of points was about 15mm, implying a lateral tread movement of about 7.5mm. But with the new rear suspension arm in place, the width of the marked line was... wait for it... worse! In fact, at 30mm it was twice as bad!

So what the hell is going on? The problem is that the whiteboard on which the pen writes in anchored to the seat, rather than to the main backbone. Any movement in the seat frame shows up as twist in the suspension, even if the suspension isn’t twisting much at all. And with the roll stiffness of the suspension softened since the last test was done, the greater angle of lean put more twisting force on the seat, resulting in a greater seat deflection.

So while I thought this measurement approach would show changes in the stiffness of the rear suspension arm, it didn’t. However, physically grabbing the wheel and pulling back and forth clearly showed better stiffness – so would it be felt in the riding?

Riding

The next test was to assess the differences that could be felt when riding the machine. On dead smooth surfaces there was, as expected, no discernible change. However, when passing over slow speed cornering bumps, the rear felt far more settled. This is a really interesting change – what, precisely, does “more settled” mean?

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The change in handling feel was similar to that experienced on my Falcon when the rear suspension trailing arms and bushes were modified – see Frank's Suspension, Part 3

The Falcon modifications reduced rear roll steer – so I’d now suggest that the original trike rear suspension was roll steering.

Rear roll steer is hard to describe – it’s not a case of feeling the rear steering in a different direction (like it is when driving a four wheel steer car) but more a case of the whole machine feeling imprecise in steering and when tracking through corners. A cambered wheel tends to steer (it’s trying to roll around a virtual cone) and so the twist in the rear suspension, which caused camber variations, would have been causing some steering effect.

And what about on the high speed, downhill and bumpy corner? On my main test hill the pattering was reduced a lot – that’s the good news. However, on other high speed sections with closely spaced bumps the pattering could still occur – even in a straight line. And the latter implies a real problem....

Damping

With the rear suspension patter now identified as occurring in a straight line as well as around high speed corners, it was beginning to look more and more like it was a damping problem. That’s despite the patter frequency (say, 10Hz) being very much different from both the natural frequency of the suspension and also of the tyre.

  • Friction Damper

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In a bid to either solve or identify the nature of the problem, I built a friction damper. This comprised a section of V-belt and a small alloy V-belt pulley. The belt was partly wrapped around the pulley and held tensioned by a spring. One end of the belt was anchored to the rear suspension arm and the other end to the main frame. The pulley was mounted on the main frame and was prevented from turning.

On suspension bump, the belt easily slipped around the pulley. But the angles were so arranged that on extension (ie droop or rebound), the belt wedged itself in the V-groove of the pulley and so resisted being pulled through it. This gave very soft bump damping and stiff rebound damping. The ratio of bump to rebound damping, and the stiffness of the damping, could be adjusted by altering the amount of belt wrap and the tightness of the spring.

This elegant and light system worked fine in damping suspension movement. But even before full testing of pattering was undertaken, I decided that the reduction in ride quality that the V-belt friction damper caused was too great a trade-off. Especially in vibration and over small bumps, the ride quality was way inferior to having no damper.

  • Hydraulic Damper

I then tried a hydraulic damper. As covered briefly in Building a Human-Powered Vehicle, Part 4, for my previous suspension trike design I used ex-motorcycle steering dampers. However, this approach wasn’t entirely successful. Building a Human Powered Vehicle, Part 7 describes the problems I had. The main one was modifying the steering damper to give asymmetric bump/rebound damping. This involved fitting internal valving, something I found very hard to successfully do. The lack of space to modify the internal piston, and the way that variations in internal fluid flow speeds gave odd behaviour, made it a frustrating experience.

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However, if the same sort of steering damper could be found that had external valving, modification would be a lot easier. And, as it happens, the Yamaha R1 uses a steering damper that has this sort of external valving. The damper is very much like the ones I used previously except cast into the alloy housing is a passage that connects the two sides of the piston.

I bought one of the dampers (a very lucky and cheap find on eBay) and modified the valving to give much stiffer rebound than bump. Achieving this was relatively simple – just take out the standard screw-in valve block, shorten it and then place a spring and a rubber seal inside to form a one-way valve. Since in these designs some fluid is allowed to flow past the main piston, the hydraulic system still allowed rebound extension, but with a much stiffer force than on bump. By changing the oil viscosity, the total stiffness could be changed.

Unlike my previous designs, I thought that this damper was pretty well on the money. Very slow speed bump and rebound stiffness were of the same rate, but the higher the shaft speed, the stiffer rebound was in relation to bump.

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I fitted the damper, only to find that the standard oil was too thin: there was insufficient damping. I then replaced it with higher viscosity oil and went testing. But, interestingly, the presence of the damper again transmitted vibration and small bumps to the frame (this was especially visible in shaking of the rear vision mirror), and even more interestingly, at speed the rear wheel patter was still there!

Aaaghhhh.

Solving One Problem

It was time for a very careful re-think.

Altering the rear suspension member for one much stiffer had improved things but not fixed them. (But it had given significant benefit in other cornering conditions.)

Changing rear airbag and rear tyre pressures had also made little/no difference to the pattering problem.

The pattering could occur in both cornering and riding straight, but only at high speeds (ie downhill fast runs at, say, 50+ km/h) on roads where there were closely-spaced bumps.

And then I had a thought. A few days before I’d had the back of the trike up on blocks and, for some reason, had been spinning the back wheel very fast by turning the pedals in a tall gear. I’d then noticed that the back wheel seemed a bit out of balance.

What if the rear wheel’s balance was bad enough that at speed it was trying to bounce up and down – and road bumps simply worsened this behaviour?

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I raced down to the workshop and removed the big rear wheel reflector attached to the spokes. I then spun up the rear wheel and noticed that while it was still a tiny bit out of balance, it was vastly better in this respect. And a test ride proved the theory: the high speed rear wheel pattering was gone.

Yes, after all the effort, it was as simple as removing the rear wheel reflector....

The Maths

Surely a single reflector wouldn’t cause much of a problem, would it? I had a physicist do the maths for a 40 gram reflector and he found that at my top (downhill) speed, the effective mass of the reflector grew to no less than 3.2kg....

It’s Baaaaack

Feeling pretty confident that all was now solved, I trailered the trike to the Gold Coast where there are flat roads and cycle paths. There, in one easy afternoon, I rode 75 kilometres. Sitting at a constant 18 – 20 km/h, I was able to feel things that I never feel at home, where it seems I am always doing either doing 5 km/h up hills or 50 down the other side.

And what I felt was another version of the rear-wheel patter, this time occurring over short sharp bumps like road reflectors. Over this type of bump, when pedalling hard, the rear wheel could be felt to monetarily lose traction. On close-spaced sharp corrugations, the rear wheel skipped, and over big wave-like bumps the rear simply felt under-damped.

All of which added up to the absolute necessity for a rear damper...

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I went back to my modified R1 steering damper and re-assessed how I’d previously fitted it. Looking again at the installation, I realised that I’d pre-loaded the air spring – the full extension length of the damper had been less than the full extension length of the spring. Was this preload responsible for the worsened ride quality when the damper had been fitted? I installed it again, this time with an extension plate that prevented any preload being applied.

And this time the ride quality remained excellent but the rear was clearly much better damped.

Handling

Removing the reflector and fitting the rear damper has effectively solved the problem of rear wheel patter.

When going really hard (like cornering hard enough to being close to be up on two wheels at 30 - 40 km/h on a bumpy surface), the rear tyre can still skip sideways on the harshest bumps. But this new behaviour doesn't worry me at all. On the road it's a progressive warning of an impending loss of rear lateral traction, very much like a RWD car that power-oversteers out of bumpy corners. It feels completely unlike the previous patter, which always gave you the feeling of: “Oh shit – where’s the back going?” The new sideways behaviour is much more: “Ah, starting to go hard enough to be on the brink of oversteer....”

I should also make the point that all the behaviour I have been describing is at speeds that, on these surfaces, a non-suspension recumbent trike would find completely impossible – not only from a tyre adhesion perspective, but also from a point of view of pounding the rider so hard they’d find it hard to see and steer...

The front damping (caused by the change in track that comes with suspension movement) and the rear damping (caused by the hydraulic damper) are now also very much in synch. For example, passing over a sharp depression (eg one caused where a filler strip has sunk after a pipe has been placed across the road), the front and rear bump impact harshness are now very similar. You also never feel the rear or the front bobbing in an undamped way – the suspension is working all the time, but it’s not particularly noticeable in its action.

What you notice is that the ride quality is incredible – and there’s no damn rear wheel patter!

Next week: the conclusion – the final part in this series.

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