jc2 Posted March 3, 2011 Report Share Posted March 3, 2011 Most twinshock trials bikes have non-parrellel triple clamps, with the forks kicked out about 1.5degrees more than the steering stem rake. Can anybody tell me what the advantage is of this set-up (over parrallel triples)? I've read that they're supposed to be better at/near full-lock, but I don't know if thats correct or why/why-not. Can anyone enlighten us? Quote Link to comment Share on other sites More sharing options...
feetupfun Posted March 3, 2011 Report Share Posted March 3, 2011 There are a few reasons for the forks being not parallel with the steering head on many bikes (not just twinshocks) It is a way to provide the ideal steering trail without having to have the axle set forwards of the centreline of the fork tube axes (manufacturer may be limited to one fork leg design) It allows for the steering geometry to be closer to ideal through the suspension travel of the forks It reduces the mass moment of inertia of the front end compared with having the same forks but fitted into parallel triple clamps. This is due to the tubes being closer to the steering head axis (for a manufacturer stuck with using in-line axle forks) I can't see it making any difference to steering geometry at full lock Quote Link to comment Share on other sites More sharing options...
jc2 Posted March 7, 2011 Author Report Share Posted March 7, 2011 Thanks guys but it hasn't really got to the bottom of it. I understand all yr saying, Feetup, but w one exception all those things can be achieved with offset-axle forks/sliders which have the same total offset/trail, & as you know, many bikes have both offset axle sliders & non-parallel triples, eg, SherpaT, KT, RL, Cota etc. So there must be some other reason why manufacturers use it, which I suspect is the main reason. TrilasRfun, thats the sort of thing I've heard/read claimed before, but I don't see how & I've never seen it adequately explained despite having searched fairly widely. eg Foale & Willoughby say very little in their classic book on frame design (which has considerable content on steering). I can see what yr saying that the forklegs are not vert or backwards at full lock with runout yokes, but don't see that that makes any diff for the better. Steering geometry is a function of steering axis angle (ie rake) & total offset - however that total offset is achieved. The angle of the forklegs doesn't provide rake. It all gets a bit confusing (to me at least) on tight turning, but in the worst case scenario, on full lock, assuming the bike itself remains verticle, the rake becomes camber on the front wheel & there is no rake, & with any offset at all (however it is achieved), we actually have negative trail. Seems to me thats why any bike feels much less stable the closer you get to full lock, and with runout on the yokes it makes for more negative trail & therefore a worse scenario! (it seems to me) In fact, its usually improved/cured by providing more (+ve) trail. The only advantage I can see of runout on the yokes is that as the front suspension compresses, you lose less trail than with parallel triples. ie you don't lose as much of the centring effect so the bike is more stable under suspension compression than with parallel triples. But perhaps I've got it all wrong. Quote Link to comment Share on other sites More sharing options...
feetupfun Posted March 7, 2011 Report Share Posted March 7, 2011 Thanks guys but it hasn't really got to the bottom of it. The only advantage I can see of runout on the yokes is that as the front suspension compresses, you lose less trail than with parallel triples. ie you don't lose as much of the centring effect so the bike is more stable under suspension compression than with parallel triples. I thought I said that already Quote Link to comment Share on other sites More sharing options...
feetupfun Posted March 7, 2011 Report Share Posted March 7, 2011 regarding achieving the ideal offset. yes it can be done with parallel offset or axle ahead of fork tube centreline, but the lowest mass moment of inertia without having the axle way forward of the line of the tubes comes with a combination of angular offset at clamps and minimising the absolute offset at the clamps. Quote Link to comment Share on other sites More sharing options...
02-apr Posted March 7, 2011 Report Share Posted March 7, 2011 (edited) I recall reading, back in the mists of time, an article by Sammy Miller where he said that one of the benefits was that the rise and fall of the steering head as the bike went from lock to lock was reduced. Just how critical this is to good handling on a trials bike is up to the rider I suppose. Edited March 7, 2011 by 2/4 1 Quote Link to comment Share on other sites More sharing options...
jc2 Posted March 7, 2011 Author Report Share Posted March 7, 2011 I thought I said that already Yeh Dave, that was the "exception" I referred to above. Its just elaborating the general with some specifics. Quote Link to comment Share on other sites More sharing options...
jc2 Posted March 7, 2011 Author Report Share Posted March 7, 2011 (edited) regarding achieving the ideal offset. yes it can be done with parallel offset or axle ahead of fork tube centreline, but the lowest mass moment of inertia without having the axle way forward of the line of the tubes comes with a combination of angular offset at clamps and minimising the absolute offset at the clamps. Agreed. But the runout is usually only about 1.5deg. If that were the main reason its used, they'd surely use nearly straight-across triples w offset-axle sliders &/or considerable runout in the triples. Edited March 7, 2011 by JC2 Quote Link to comment Share on other sites More sharing options...
sherpa325 Posted March 8, 2011 Report Share Posted March 8, 2011 From my understanding of trials bike geometry designers try to achieve a 'trail' measurement of about 2 1/2 inches.If you go with parallel triple clamps and a non leading axle you will need a very steep angle to achieve this trail measurement. This can make the bike twitchy. I can recall reading an article by S H Miller on Bultaco geometry and I am pretty sure he developed the current system. The idea is to have a less steep angle [for stability] and then offset the bottom triple clamp and the front wheel to reduce the trail measurement, thus giving you light and nimble steering without having the disadvantages of a steep head angle, in theory the best of both worlds. Quote Link to comment Share on other sites More sharing options...
feetupfun Posted March 8, 2011 Report Share Posted March 8, 2011 Agreed. But the runout is usually only about 1.5deg. If that were the main reason its used, they'd surely use nearly straight-across triples w offset-axle sliders &/or considerable runout in the triples. If the triples are straight you can't get enough steering lock without the tubes hitting the frame Quote Link to comment Share on other sites More sharing options...
jc2 Posted March 8, 2011 Author Report Share Posted March 8, 2011 (edited) I recall reading, back in the mists of time, an article by Sammy Miller where he said that one of the benefits was that the rise and fall of the steering head as the bike went from lock to lock was reduced. Just how critical this is to good handling on a trials bike is up to the rider I suppose. That would be true to some extent, but its not dependant on runout in the yokes. The rising/lowering of the steering head is a result of both/either rake & offset. Thinking of best/worst-case scenario, if you have zero rake you'd have no rising/lowering of the steering head. But there would be no diff whether the offset is obtained with raked out yokes or in the triples/sliders. One would think that the sideways movement in the steering head (which is more a function of offset/trail) would upset balance more than the rise/fall of the steering head, which already changes with suspension/surface undulations. Edited March 9, 2011 by JC2 Quote Link to comment Share on other sites More sharing options...
jmck Posted March 27, 2011 Report Share Posted March 27, 2011 How are you determining the amount of, if any, "rake" built into the triple-clamps? I have not seen this measurement displayed in any service manual. Quote Link to comment Share on other sites More sharing options...
jmck Posted March 29, 2011 Report Share Posted March 29, 2011 (edited) I have measured a 1975 TY-250, a 1981 Bultaco Sherpa T, a 1973 Montesa Cota 123, a 1976 Montesa Cota 348, a 2009 Beta Evo-125, a 2010 Beta Evo 250 4-T, a 2006 Sherco 2.9, and a 2009 Gas Gas 125, and on all of them the triple-clamps are 100% parallel with the steering axis. Edited March 29, 2011 by JMcK Quote Link to comment Share on other sites More sharing options...
feetupfun Posted March 30, 2011 Report Share Posted March 30, 2011 (edited) maybe you are measuring something different to what we are talking about? I can take a photo and post it up if required to remove any confusion. It is the fork tubes that are set non-parallel to the steering axis Edited March 30, 2011 by feetupfun Quote Link to comment Share on other sites More sharing options...
jmck Posted March 30, 2011 Report Share Posted March 30, 2011 (edited) maybe you are measuring something different to what we are talking about? I can take a photo and post it up if required to remove any confusion. It is the fork tubes that are set non-parallel to the steering axis I believe I know what you are talking about, and the fork tube "rake" is controlled by the triple-clamps/steering stem. No production twinshock trials bike that I am aware of has an adjustable steering stem so the triple-clamp rake angle is held constant. Some KTM models have an adjustable 2 degree offset steering stem, but the rake is held constant. It's quite possible that one manufacturer or more has in the past built a trials bike with a "raked" triple-clamp but I have been a motorcycle mechanic for over 35 years and have owned two bike shops, one currently, and I have never seen one nor have seen in a service manual that dimension being called out. I am in the process now of designing my own twinshock chassis's, and have a fairly large database of chassis geometry numbers, so if you know of a particular trials bike that has a "raked" triple clamp I would be very interested in seeing the numbers. Edited March 30, 2011 by JMcK Quote Link to comment Share on other sites More sharing options...
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