Steer Clear - understanding steering for twinshocks

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Re: Steer Clear - understanding steering for twinshocks

Postby JC1 » Mon Aug 04, 2014 11:28 am

Effects of Trail cont'd:

It may seem like you couldn’t have too much trail, so that all the above is minimized, but you can. Its self-centring effect is “subject to the law of diminishing returns” (Foale).

Too much trail can also cause too heavy steering & speed wobbles (headshake) if it over-corrects a deflection sufficiently to initiate an oscillation (made worse at speed by buckled rims, out of shape/balance tyres, natural frequency/resonance of the steering mass etc). These are not very relevant at trials speeds but there are two other factors.

Trail actually increases the wheelbase when you steer, which you could do without in the tight stuff. That of course is not possible, but better to keep it to a minimum as the effect is considerable.

To understand this, consider again the reduction of trail to zero at a certain angle of steer, as discussed above. At that point, the wheelbase has extended by the amount of trail, since the contact point of the front tyre no longer trails the steering axis (by the amount of trail), it is right on the steering axis. So for eg a TY250 with 1295mm wheelbase & 90mm trail, at 65deg steer & the bike upright the wheelbase becomes 1385mm (1295 + 90). Quite a difference!

Compare the following twin-shocks:
KT250, 1384mm (1305 + 79) at 61deg steer
TY175, 1362mm (1265 + 97) at 66deg steer
TY250, 1385mm (1295 + 90) at 65deg steer
TL125, 1384mm (1280 + 104) at 67deg steer
TLR250, 1393mm (1315 + 78) at 61 deg steer
TLR200, 1401mm (1315 + 86) at 63deg steer
RL250, 1421mm (1345 + 76) at 59deg steer.

And if you add some lean, the wheelbase gets longer still. But less trail means less extension of the wheelbase when steering – ie a good reason to minimize trail for trials.


Apart from the self-centring effect, trail also develops an opposing force that tends to steer the bike. This is the main self-steering effect, enabling you to “steer with the footpegs”. To quote Tony Foale; “When a machine is banked into a bend, trail gives rise to two opposing effects: 1) directional stability, tending to make the machine run straight & 2) the self-steering effect tending to steer the bike into the bend. To achieve neutral handling, these effects have to be properly balanced”.

To isolate this from the effect of rake already explained, consider again zero rake (vertical axis) with some offset to give trail. Now lean the bike over and you can see that weight acting on the front end will produce a torque about the steering axis that steers the bike into the turn. You can also see that more trail & more weight on the front of the bike will each increase this effect. (You may also notice that with negative trail the bike will steer the opposite way to what you lean!)

Imagine how this self-steering effect might feel with too much trail. It could be excessive even at small lean angle. In reality it has much the same 'feel' as flopping-in caused by excess offset of the steering mass C of G. Both make it difficult to 'steer with your feet', or at least require adjustment to how much you lean. You often find you have to stop the wheel steering too far inwards compared to a better set-up, better steering machine, which upsets balance considerably at trials speeds.

NB - It seems to me that for trials, trail is one of the most critical parameters to get right.


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Re: Steer Clear - understanding steering for twinshocks

Postby JC1 » Wed Aug 06, 2014 9:06 am

Before we leave trail, it’s worth noting that what we call trail (measured along the ground) is not real trail. It’s more accurately called ground trail & is what we are familiar with because that’s what manufacturers quote for “trail”.

But real trail is measured perpendicular to the steering axis (Fig 3.2), not along the ground, for that is the true ‘lever arm’ upon which the self-centring torque acts. (Sometimes it is called normal trail, where “normal” means perpendicular rather than “usual” or “common”, but to save confusion we’ll use the term real trail.)

Real Trail JPG.JPG
Real Trail JPG.JPG (20.61 KiB) Viewed 8492 times

This may not sound like a significant distinction, but real trail is more useful when comparing trail figures at different rakes (see below). If we consider real trail, say of 70mm, it has the same self-centring effect at any rake. But if we just consider ground trail we need more of it with increased rake to have the same self-centring effect. (NB. I am not saying here that we get more ground trail with more rake. We do, but that is a different matter.)

There is a simple trigonometric relationship,between them:

Real trail = ground trail x Cosine of rake angle.

Some specs for diff models (rake, ground trail/real trail & axle offset) listed below:

TL125, 28.5deg, 104mm/92mm, 70mm
TY175, 27.5deg, 97mm/86mm, 70mm
RL250, 27deg, 76mm/68mm, 86mm
TY250, 26.5deg, 90mm/81mm, 70mm
TLR200, 26.5deg, 86mm/77mm, 74mm (TL250 same specs)
KT250, 26.5deg, 79mm/71mm, 80mm
TLR250, 24.5deg, 78mm/71mm, 69mm

TBC


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Re: Steer Clear - understanding steering for twinshocks

Postby JC1 » Sat Aug 09, 2014 8:48 am

The effects of offset – There are two effects of offset of the C of G of the steering mass. The first, as we have seen, affects flop-in. The weight of the steering mass acts through its C of G (Fig 2.12), so the greater its weight or its C of G’s offset the greater the flop-in effect will feel (at low speed, leaned over), & the greater the force required to correct it & steer 'against it'.
Steering Mass C of G.JPG
Steering Mass C of G.JPG (27.66 KiB) Viewed 8471 times


But secondly, there is an opposite effect too (surprise, surprise!): ie at sufficient speed, with too much offset of the steering mass the bike can tend to steer itself out of the corner. The centrifugal force of the steering mass acting outwards through its C of G can exceed the gravitational force of weight acting inwards, forcing the steering mass outwards & steering the bike to the outside of the corner. It may not be called “climbing-out”, but that’s effectively what it is.

Although this is minimal in trials because of the slow speeds, it still could be a factor for non-stop riders because of the tightness of the turns. Centrifugal force is not only proportional to speed & mass, it’s also inversely proportional to radius of the turn (ie higher in tighter turns).


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Re: Steer Clear - understanding steering for twinshocks

Postby JC1 » Tue Aug 12, 2014 8:40 am

Effect of rear suspension evolution on steering geometry:

Rake had settled at about 26.5 to 27deg & trail at 80-90mm in the 60s-70s, then took a bit of a leap to 24.5deg on the TLR250 (with 78mm trail) in 1983. But with the intro of the TY mono (1984) there was a further jump to 23deg rake & it seems to have remained thereabouts.

One can see many good reasons for the reduction in rake, on account of its many advantages (as above) but I think there is another reason also.

It seems to me that some of the reason for the steeper rake & shorter trail is in the evolution of rear suspension & its effect on steering. The laden sag (with rider on board) with long travel rear (LTR) suspension of 160-170mm is considerably more at the rear than for the earlier shorter travel (90-100mm), while the front travel (165-170mm) and sag remained much the same.

The most important steering geometry of your bike is with the rider on board in his typical stance. On a classic/twinshock with 6-7” front travel & 4” rear travel, even with the rearward weight bias, laden sag at the front will most likely be more than at the rear so rake & trail in action are likely to be less than unladen specs on such bikes.
With the larger rear sag of the LTR systems when laden, it’s the other way around - rake & trail in action will be more than unladen specs. With the early LTR systems that weren’t much longer travel the changes were subtle & less noticeable.

By the time of Honda’s TLR250 with its considerable rear travel (160mm), the rake was noticeably steeper to compensate & the trail was at the shorter end of the existing range. Then when the TY mono arrived, with more progressive rising rate LTR suspension which was softer still in the initial travel (cf Fig 8.10), there was more rear sag still, & the unladen rake & trail required further reduction to compensate.

Rear Suspension displacement vs load JPG.JPG
Rear Suspension displacement vs load JPG.JPG (27.95 KiB) Viewed 8458 times

No doubt this alone does not make up all the difference in rake, but it surely is significant.

That being so, can a twin-shock bike without the same rising rate long travel rear suspension benefit from modern geometry? Surely careful consideration would have to be given to ensure realistic rake & trail when laden. It requires more than just changing the rake to match that of a modern bike.

TBC


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Re: Steer Clear - understanding steering for twinshocks

Postby David Lahey » Tue Aug 12, 2014 9:41 am

It would be interesting to know if the manufacturers' quoted geometry is with the bike resting on its wheels, or with bike weight supported, because of the effect of unladen sag


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Re: Steer Clear - understanding steering for twinshocks

Postby JC1 » Fri Aug 15, 2014 9:29 am

Conclusions: The upshot of all this is that it’s a wonder any bikes handle as well as they do! Everything is a compromise, a trade-off of opposing effects, striving for an ideal optimum while the ‘goalposts’ keep shifting.

But we have several good reasons for minimizing rake, perhaps even radically. Foale concludes; “To sum up on steering geometry, perhaps it is time to rethink our ideas on the relationship between rake and trail and their effects on steering and handling. Experiments are needed to establish the practical effects of steeper steering axes.” (Which he did, with surprising results. See his books.)

Even more so for trials: Foale continues, “The detrimental effects of rake become more pronounced at greater steering angles” ie where trials often operates. We could well add, “and at low speeds”.

Ideally, if we could start with a clean slate, we’d chose the desired trail first, then design the bike with the steepest rake & least offset to achieve that trail. It’s no surprise that’s more or less the direction modern bikes have gone, if not to the extent they could go. Eg say for 70mm real trail with zero offset, rake would only need to be12deg.

Of course reality brings in other considerations, eg aesthetics (the buying public notoriously avoid anything looking too radical), ergonomics (you’ve got to be able to reach the controls comfortably to control it), weight distribution (how far back could a rider stand with only 12deg fork rake?), braking control (shudder, shudder, from too steep telescopics) etc.

Now let’s be a bit more reasonable. We’ll aim for 70mm real trail again, assuming we have long travel rear suspension. Typical 35mm twin-shock forks with offset axle have 30mm offset in the sliders so let’s use them. To minimize offset of the steering mass C of G we’ll have zero offset in the top yoke but let’s build-in some of the advantages of angled triple clamps, with a generous 2.5deg ‘kick-out’ which adds about 35mm offset. That gives us total axle offset of 65mm. For a 2.75x21 trials tyre that would require 24deg rake. That’s remarkably close to the TLR250’s geometry – which many consider to be at/near the peak of twin-shock design.


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Re: Steer Clear - understanding steering for twinshocks

Postby paulm » Fri Aug 15, 2014 10:16 am

David Lahey wrote:It would be interesting to know if the manufacturers' quoted geometry is with the bike resting on its wheels, or with bike weight supported, because of the effect of unladen sag


It's my understanding that quoted geometry is a calculation - i.e. in effect, with the bike supported - that's apparently one of he reasons why it's important to get sag adjusted to manufacturers' recommendations.

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Re: Steer Clear - understanding steering for twinshocks

Postby JC1 » Wed Aug 27, 2014 11:13 am

Thats my understanding too. I've plugged the relevant measurements into the appropriate formula & checked against manufacture's specs for many bikes over the years & that's what it indicates.


Moving on to Possible fixes:

Based on typical twin-shock dimensions here are a few rules of thumb for modifying:

- For every 25mm that you lower the front or raise the rear you lose 1deg rake.
- For every 1deg of rake loss, you also lose 7mm trail.
- For every 25mm you slide the forks up thro’ the yokes you also lose about 12mm wheelbase
- For a 2deg de-rake of a frame you also lose about 25mm wheelbase (when done the typical way).
- For every 2deg de-rake you raise the front of the bike 12mm & thereby lose 0.5deg of the de-rake (cos you move the front axle back & down, not just back)

So, as an aside, consider what compressing the forks by weight transfer (in action) can do for sharper steering, shorter wheelbase & tighter turns.


Now to consider a few specific twinshocks, with some possible fixes:

Firstly, the RL250. It is widely said that they “don’t steer”.

Specs: wheelbase 1345mm, ground clearance 290mm, rake 27deg, trail 76mm, (real trail 68mm)

That’s a loong wheelbase & short trail, with rake in the ballpark for twin-shocks of the era.

Looking at an RL side on it looks long, like it’s got a ‘stretched neck’. The bulk of the weight looks rearward. The crank is high (high C of G). The swing-arm angle looks steep (pivot too high?). The foot-pegs could be lower but their fore/aft location looks to be about right (at least relative to the back half of the bike!).

With an eye also to its specs, it looks like it would take an age to turn (wheelbase too long - a massive 50mm longer than a TY250), the front wheel wouldn’t stick (too little weight on it), it would tuck-under (too little trail), feel top heavy (crank too high), & mono-wheel too easily (C of G & swing-arm pivot a bit high).

Hardly surprising that “they don’t steer." (Along with a few other issues!)

It could do with more weight on the front wheel & shorter wheelbase by about 25-30mm. Cutting the frame to steepen the rake is one way to do that. A 2deg de-rake to 25deg reduces wheelbase 25mm to a more acceptable 1320mm & puts more weight on the front wheel, but the de-rake also raises the front end 12mm, thus raising the C of G further & losing 0.5deg of the de-rake (to 25.5deg). Real trail is reduced from 68mm to 59mm which is very short for a twin-shock with short travel rear suspension & it will make tucking-under worse than a stock RL.

One could also drop the forks 12mm after the de-rake, lowering the C of G to what you started with & reducing wheelbase to 1315mm, rake to 25deg & real trail to 56mm. But tucking-under will then be worse still, starting at just 56deg of steer.

Those changes will tend to make the bike less stable but it will have sharper steering & tighter turning. Is that better (overall)? Worse? Or just different?

Another option would be to drop (or shorten) the forks 20mm & reduce the RL's considerable axle offset by about 18mm (eg use TLR250 yokes?). That would reduce rake to 26.2deg & wheelbase to 1320mm, drop the C of G, & increase real trail to 81mm. That’s very close to TY250’s steering specs (26.5deg, 81mm) but with 25mm longer wheelbase than the TY – not bad specs for a twinshock. These changes would increase stability & still make the bike more responsive with sharper steering & tighter turning. Tucking-under will be much improved (ie reduced; it wouldn’t start till about 65deg steer instead of 59deg). That’s a much better option to my way of thinking & it didn’t require frame surgery.

Dropping the foot-pegs would assist handling with both options too (via lower C of G) but moving them back would be counter-productive at least on one count, ie weight distribution & its effect on steering. Actually, two aspects of weight distribution . Firstly we don't want to lose any of the extra weight distribution we've put on the front wheel with the front end mods improving grip there. Secondly the bike will still tend to loop easily, partly because of the swing-arm geometry & we don't want to worsen the 'loop factor' by moving the pegs back. The lowered C of G will improve the 'loop factor' a little, but the bike could also benefit from a lowered swingarm pivot. With the swingarm pivot too high (relative to the countershaft sprocket & rear axle) the chain torque reaction when you apply power will tend to extend the rear suspension, raise the C of G & cause wheelies. Thus it won't steer too well under power (if the front wheel barely skims the ground).


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Re: Steer Clear - understanding steering for twinshocks

Postby David Lahey » Wed Aug 27, 2014 7:22 pm

Hang on John, after all that detailed stuff about front end geometry and how it affects the steering, you've just suddenly came out with a whole pile of stuff about the back end of the bike, in particular something about the height of the swingarm pivot affecting the tendency of the bike to wheelie, but with no explanation :)

The bit about lowering the front of the RL and increasing the trail I'm sure would be a winner. It's one of the things I tried many years ago on my KT, at the same time that Mark Beechy was experimenting with his own KT. I know the KT is nice to ride with everything standard, but with the standard setup, the front end does require a lot of attention in loose turns. Mark and I fitted TY250 forks in the KT250 triple clamps, which lowered the front of the bike and also reduced the offset. The front end then felt wonderful (yes just like a TY250). I didn't like the look of the KT with Yamaha forks though, and the mudguard touched the exhaust, so I just fitted shocks longer than the standard shocks, and set the sag in the KT forks to 50%, and this also worked well.


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Re: Steer Clear - understanding steering for twinshocks

Postby JC1 » Thu Aug 28, 2014 11:54 am

Circumstantial thinking Dave. One thing leads to another & all of a sudden you're at the back end!

You can tell a lot about how a bike will handle by looking at the specs & a good side-on view of the bike. The RL's swingarm geometry is one of the things that stands out like the proverbial .....

I did put a brief explanation of its effect at the end of the last paragraph, but your'e right, it was a short-cut. I tho't it was time to move on from the science/maths into some practical applications.

I have edited that last paragraph above (in my last post) to explain it some more, but I'll see if I can find a suitable diagram to post that explains it further/better.


I do remember your experiments with the TY forks on the KT, Dave. The TY forks are quite a bit shorter than the KT's, & I've often wondered how it would have gone with longer stanchions (say off a DT) so that the guard didn't hit on full compression. I might yet try it on mine.

There's some observations on the KT's geometry/handling coming after the RL's.


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