Steer Clear - understanding steering for twinshocks

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Rod
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Re: Steer Clear - understanding steering for twinshocks

Postby Rod » Sat May 16, 2020 1:37 pm

This pic of the the same bike in a section says it all, looks like negative trail has kicked in and he is struggling in that sand. I first saw this pic a few weeks ago on TC and thought it looked awkward and now after reading through this informative post I can see why.
Modified RL.jpg
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Re: Steer Clear - understanding steering for twinshocks

Postby JC1 » Thu May 21, 2020 1:09 pm

Yes, well spotted Rod. I thought much the same of the pic.


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Re: Steer Clear - understanding steering for twinshocks

Postby Toompine » Thu Jul 02, 2020 9:37 am

I just stumbled onto this research a while ago and is an absolute treasure. I had a KT250 with the Tryals Shop 302 kit in 75, then bought three in about 1999 and had to let the two good ones go in the 2010 recession. Well in the last 6 months I am back into it and now have two plus a roller.

The things that were troublesome on the bike back in the day are obviously still there and wanting to make a good rider I have been looking for real data, not just garage stuff. The things that I most remember are a lack of stability running in creek beds (front wheel deflection off line) or in sand and the front end plowing in turns. Now I am beginning to understand how minimal trail could affect the former and high CG and pegs the latter feeling.

Your data has given me a totally different path to consider and likely a much more successful one. I have had to read most everything two or three times but it is quite clear now

Again thanks.



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Re: Steer Clear - understanding steering for twinshocks

Postby JC1 » Mon Jul 06, 2020 4:41 pm

I'm glad you found it useful. All thanks to Tony Foale and his book, who also kindly gave permission to use his diagrams.
I had to read some sections of his book n times too before I fully got it. But it's so enlightening when you do.

With the KT, in search of more trail, it might first be worth looking at your bike's ride height with you on board in your typical stance. That's the easiest thing to adjust. Even just moving your weight rearwards in creek beds might help.

The KT has very soft fork springs made worse by sag over the years, and if your bike is oversprung at the rear for your weight, or if your rear shocks are too long you would have noticeably reduced trail when riding.

I actually like the steering of my KT with a nice new modern soft compound tyre (Michelin), 330mm shocks, pegs moved back and down about 15mm each way and a little more preload on the forksprings, tho I can occasionally feel it tending to tuck under from too little trail. My preference for twinshock trials bikes is trail between 80 and 90 mm (manufacturers specs, ie unladen), preferably at the lower end for lighter feel. The KT is right at the lower end of that and the TY250 at the upper end. Even 5-7mm difference in trail is noticeable for most riders. You can feel the difference in the steering between a TY250 (90mm) and the TY175 (97mm). I can't notice the 250 flopping in at all from too much trail but it is quite noticeable on the 175.

So if you can raise the front of your bike even 20mm when laden or lower the rear you should be able to feel a noticeable difference in increased trail. The rear can only realistically be lowered if your current shocks are too long or your springs too stiff for your weight. But your front can more easily be raised, either by more preload or (better still) stiffer springs. Or even with a longer damper rod (eg a spacer under the damper-rod, even if only temporarily to try it).

I seem to recall that David Lahey (on the forum here) runs TY250 springs in his KT forks, which if I recall correctly are firmer than the KT springs. Dave is a bit heavier than me and it seems to work well for him. (I suffice with a little more preload for my 80kg in riding gear) Dave has also lowered his footpegs more than I have and he may add his comments on how he likes it. Over to you Dave...


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Re: Steer Clear - understanding steering for twinshocks

Postby David Lahey » Mon Jul 06, 2020 6:12 pm

Before I start on what I've done, I need to say why I did it.
In my opinion the stock standard KT has a centre of gravity that is bordering on being too high. It also has a forward weight bias and a shallow steering rake angle. It also has footpegs a long way off the ground. These things combine to give a wobbly ride and heavy steering. If the rake angle is made steeper by lifting the rear end via longer shocks, the centre of gravity becomes seriously high. If the rake angle is made steeper by sliding the fork tubes up in the clamps, a slight benefit is available but only a very small amount of adjustment is available before the mudguard hits the lower triple clamp on compression. If the steering rake is made steeper by running a lot of sag (like what you get with standard springs and standard preload), then the suspension action is poor due to too much sag. It is not possible to increase the trail easily either because even standard the wheel and front guard are very close to hitting the exhaust and frame when the forks are compressed, so doing something like fitting in-line axle forks or different triple clamps to increase the trail is problematic. I have tried this as an experiment and the steering feels great but it is very distracting when the guard keeps pummeling the exhaust header.
Yes the forks on my KT are much firmer in action than standard. The springs are Magical brand and of the type sold for the TY250. They are significantly stiffer than standard TY250 springs. I've changed the damping too using Yamaha Majesty TY250 damping rods, which gives a very nice damping action.
There are some other things too that make this particular KT250 pleasant to ride.
There has been a change made to the main frame loop near the rear of the fuel tank that has reduced the fork rake without moving the front wheel closer to the exhaust or frame.
The footpegs have been moved as low as possible without sticking down lower than the frame rails and they are very low profile pegs. This greatly helps with balance and control.
The frame change was done in a way that suits the use of 340mm shocks which keeps the centre of gravity in a nice spot.
There are other ways to make them nice to ride. One I saw had the steering tube moved forwards in the frame (by quite a lot) which allows for the rake angle to be made steeper and also for increased trail, without the wheel hitting the exhaust.


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Re: Steer Clear - understanding steering for twinshocks

Postby JC1 » Wed Jul 08, 2020 12:54 pm

Good point about the KT's forward weight bias. Again, the problem seems to be when laden. If my data is correct unladen specs for the KT (44/56%) are very similar to other twinshocks (TY250 45/55; RL 46/54; 240 Fantic 46/54; SWM Jumbo 44.5/55.5), but footpeg location is considerably more forward than e.g. the TY250, and the KT certainly feels front heavy when on the pegs compared to other twinshocks. They could do with the pegs rearset further.

With regard the high C of G, I have often wondered if it would be worth modifying the engine cradle to lower crank so that the underside of the cradle is level and have been tempted to try a plastic tank, perhaps relocate the CDI unit from under the tank to behind the sidecovers, fabricate lighter seatbase and airbox and even redo the exhaust like a Sherpa as on the KT below, most of which requires major work and I don't particularly want to lose the look of the KT.
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Re: Steer Clear - understanding steering for twinshocks

Postby JC1 » Fri Jul 24, 2020 10:24 am

JC1 wrote:.
It’s worth looking into this ‘loop factor’ in a little more depth. It’s a bit of a sidetrack but it does affect steering to some degree (under power).

The natural tendency of a vehicle under acceleration is to ‘squat down’ on the rear suspension due partly to the change in weight distribution since the vehicle’s C of G is considerably above the horizontal driving force. Thus it creates that ‘squat’.

We might assume that the drive force acts where the rubber hits the road, but it actually acts on a bike horizontally through the rear axle. And since the pull of the chain also has an effect, depending on the geometry of the swingarm & chain-sprocket set-up, the drive force can (& often does) produce a torque that extends the rear suspension. Since this is counter to the squat effect, it is understandably called anti-squat (or more correctly, anti-squat torque).

Apart from engine HP & grip on the road (eg sticky tyre, grippy surface, or otherwise) there are numerous variables that affect it: ie front & rear sprocket diameter, swingarm pivot position, swingarm angle & front sprocket position. The diag below ‘reveals all’.

When it all boils down in the geometry, it all depends on the dimensions 'a' and 'b'. They are the most critical. (In a sense the length and angle of the swingarm are irrelevant. What counts is the distance 'b'.)


Chain Torque JPG.JPG
Chain Torque JPG.JPG (14.08 KiB) Viewed 3167 times

You can see that the chain pull force ‘F1’ acting at a distance ‘a’ from the pivot produces an anti-clockwise torque (F1 a) compressing the suspension (ie squat torque) & will be greatest on full compression when ‘a’ is maximum. But the thrust force ‘F’ (ie drive force) acting at a distance ‘b’ from the pivot produces a clockwise torque (F b) extending the rear suspension (ie anti-squat torque) & will be greatest at full extension when ‘b’ is maximum.

Now it’s the overall resultant that is important. When F b > F1 a, the overall nett effect is anti-squat torque (Fb – F1a), but when F1 a > F b the overall nett effect is squat.

It is effectively an over-centre mechanism. At some point of suspension compression on most bikes the anti-squat torque (Fb) is going to be zero. That happens when b=0, ie when the swingarm is horizontal. But what happens to it on further compression? It becomes an additional squat torque added to that of the chain-pull squat torque.

Apart from ‘a’ & ‘b’ changing as the suspension is compressed, they are also dependant on the sprocket diameters & the height of the swingarm pivot relative to the front sprocket & rear axle. That’s why the pivot location is critical to squat/anti-squat (& why some factory GP bikes have adjustable pivot locations for diff gearing combinations, track/tyre/stickiness, suspension settings etc).

If you get the pivot too high you get too much anti-squat, thereby worsening the ‘loop factor’ by raising the C of G as the suspension extends under power, & also stiffening the rear suspension under power. But if you get it too low you get excessive squat & soften the rear suspension under power. (In fact on high powered bikes you tend to pull the tyre off the track & cause excessive wheelspin.)

Now as long as ‘b’ > 0 (ie the rear axle is below the swingarm pivot) there is going to be some anti-squat torque produced under power even though it reduces towards zero as the swingarm approaches horizontal.

Going back to the RL, it seems that its swingarm is not horizontal until the rear suspension is almost bottomed!

Add to that the RL’s ‘frisky’ engine & you can see why the high swingarm pivot potentially has such importance/effect. (You can also see that running longer shocks is likely to make the loop factor doubly worse, by both raising the C of G directly & increasing the anti-squat effect.).

While having some anti-squat torque still produced at/near full compression is not necessarily a problem in itself (since it’s the nett effect that matters), it probably does indicate that the RL has excessive anti-squat at laden ride height, raising the C of G & tendency to loop under power.

Apart from lowering the swingarm pivot, one could overcome it somewhat with shorter/softer shocks (which unfortunately also lowers ground clearance & slows the steering) or with a roller under the top chain-run to raise it a little further above the swingarm pivot, thus increasing ‘a’ (in Fig 5.15 above) & hence also the pro-squat torque, to overcome more of that anti-squat.


I recently noticed, on looking at some pics of Beamish RLs, that the swingarm pivot is noticeably lower on the Mk2s with the production RL engine compared to the standard RL. Obviously they came to see a problem there too. (The Mk 1 Beamish is on par with the std RL, and the MK3 with the completely different black engine appears to have gone back to a higher pivot)

Beamish Suz, low swingarm pivot.jpg
Beamish Suz, low swingarm pivot.jpg (84.54 KiB) Viewed 3167 times

RL250  drive side.gif
RL250 drive side.gif (180.18 KiB) Viewed 3167 times


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tat ty
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Re: Steer Clear - understanding steering for twinshocks

Postby tat ty » Fri Jul 24, 2020 2:08 pm

The explanation of the loop factor did effectively loop my brain. I do have a reasonable mathematical ability, but I'm sure I'll need to read it at least 5 more times before there is a chance that I may "unloop"

I dip my helmet to JC1.

So here is the question that I've needed resolved and I suspect JC1 may have the explanation.

In the mid 1980s an American company named ATK produced a revolutionary dirt bike. Either side of the swing arm pivot (above and below) was an idler pulley, which ensured that the chain ran parallel to the swing arm.

My hunch is that the system (Anti-Tension Kettenantreib) had to do with the loop factor. So the question is out.

The bike also had a disc brake operating on the countershaft sprocket, which may or may not be interrelated.

TO THE INTRIGUING TRIALS PART OF THE STORY ..........

Appearing on of all places Australian Ebay approximately 5 years ago was a trials bike hand made as a prototype by an Australian rider who had spent time competing (not trials) in the US.

The bike had been won in a raffle by the South Australian vendor. Perhaps the raffle resulted from dispersal of the deceased estate of the bloke who built the bike.

That bike used the same ATK system and countershaft disk brake.

No regretfully I did not buy the bike but always wondered.

Someone may know it.

Alastair.



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Re: Steer Clear - understanding steering for twinshocks

Postby Kurt » Fri Jul 24, 2020 2:45 pm

I remember reading about it at the time, a cool setup.
It would be interesting how it would affect the feel of the brake.

Old link for some photos:
http://www.classicmotorcycleconsignment ... anks-rotax



tat ty
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Re: Steer Clear - understanding steering for twinshocks

Postby tat ty » Fri Jul 24, 2020 4:08 pm

Yes ... Certain that was the bike I was thinking of. I was obviously wrong about the chain/swing arm set up, but it does have the disc brake on the countershaft.

Alastair




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