trapezeartist Posted January 14, 2017 Report Share Posted January 14, 2017 What's the theory behind the rear suspension on a trials bike? There has been a trend towards leaning the suspension units forward on twin-shock bikes. This implies a falling rate as the angle between the unit and the swinging arm reduces as bump deflection increases. On the other hand, dual-rate springs imply a rising rate. I find it hard to work out what's going on with monoshocks as the linkage mechanism is partially hidden. It would need careful measurement and drawing to sort that out. But perhaps someone already knows. Quote Link to comment Share on other sites More sharing options...
b40rt Posted January 14, 2017 Report Share Posted January 14, 2017 Most dual rate springs on twin shocks, are soft and softer. Quote Link to comment Share on other sites More sharing options...
pschrauber Posted January 14, 2017 Report Share Posted January 14, 2017 At first you need the right springs that matches the bike weight and your weight with gear to your rear shocks and the fork. You can determine the right springs if these deliver the right sag for trials riding static and dynamic. static = standing bike without weight of the rider, dynamic = standing bike with the weight of the rider while standing on the bike (with full gear). If so everything is OK and any personal adjustments then can be made by changing compression, rebound or damping. The main effort is to get the right springs according to your and the bikes weight Quote Link to comment Share on other sites More sharing options...
beta_blocker Posted January 14, 2017 Report Share Posted January 14, 2017 Back in the late 1970's and '80's and on when motocross was going through its monoshock conversions from the twin shocks, what engineers were trying to create was a rising rate suspension so that it would allow supple compliance at small bumps and allow the system to also handle large impacts from big jumps. They seem to have all settled around 12" of travel and use linkages and oil dampened valving with multiple adjustments to control the motion. Current Trials suspensions with linkages are imitating these types of suspensions but with only with a little over half the travel and with a few newer exceptions and some better after market shocks, fewer adjustments. Proper set-up is very beneficial and more important then some might think and can improve your ability to maintain control over obsticals, turns and large impact objects. if you haven't read this article I recommend it as it is one of the few I've found that is not only focused on trials suspensions, but is clear and informative on setting yours up. https://www.joomag.com/magazine/trials-enduro-news-december-2014-volume-49-issue-12/0716551001416757092?page=61 1 Quote Link to comment Share on other sites More sharing options...
pmk Posted January 14, 2017 Report Share Posted January 14, 2017 Not sure if your reference regarding shock mounting and the angles involves the upper or lower mount? If the lower mount is moved forward on the swingarm that was / is called forwarded mounted. When the upper mount is moved forward, typically that is called laydown. With the travel being not very long, the swingarm arcs are small and it takes a pretty serious excess forward mount to have bad falling rate. Linkage designs are more intricate, based on the location of the pull rod mount points and how much they position and rotate the relay link. The comments about spring rate and proper sag is true. Those factors can take any motorcycle, whether road or off road and make it stellar or a turd. Unless well versed and experienced in suspenion design, leave it stock or with proven link mods and focus more on proper springs and quality damping. 1 Quote Link to comment Share on other sites More sharing options...
cleanorbust Posted January 14, 2017 Report Share Posted January 14, 2017 Some of the thinking behind altering twinshock trials suspension is I think more around raising the rear end as opposed to changing the leverage/moment of the suspension itself, the aim being to achieve the effect of steepening the head angle. Fitting longer units also achieves this. Quote Link to comment Share on other sites More sharing options...
pschrauber Posted January 14, 2017 Report Share Posted January 14, 2017 (edited) Back in the days when shocks where nothing else than mechanic springs with some damping far away of being progressive you could archive some progressive reaction when they where mounted with lower angle That's why Ossa did it with their Gripper model and these (not very long lasting) Telesco shocks. Luckily these times where over and progressive springs are no available. Edited January 14, 2017 by pschrauber Quote Link to comment Share on other sites More sharing options...
beta_blocker Posted January 14, 2017 Report Share Posted January 14, 2017 Two books that might you may enjoy if your interested in suspension theory and chassis design are: Race Tech's Motorcycle Suspension Bible by Thede and Parks Motorcycle Chassis Design by Foale and Willoughby Quote Link to comment Share on other sites More sharing options...
pschrauber Posted January 14, 2017 Report Share Posted January 14, 2017 Very useful information about how to dail in and adjust the suspension to your trials bike which covers modern and twinshock bikes and too rear suspension (shocks) and front suspension (forks). Static and dynamic say and so on ... You can look up the complete article which was released in two parts in Trialsport Magazin No. 360 and No.361. Quote Link to comment Share on other sites More sharing options...
trapezeartist Posted January 14, 2017 Author Report Share Posted January 14, 2017 Thanks, guys. There are lots of things I would like to reply to. I entirely get the stuff about "sag" even if it is the complete opposite of when I was trying to set-up a racing car with "zero-droop". I also get the point about adjustability of bump and rebound damping. My issue is the change of wheel rate with deflection. I suppose I was hoping someone would explain why falling rate geometry on a twin-shock is a good thing. I suspect the most meaningful answer comes from pmk who seems to be saying "falling rate is there but it doesn't matter as much as other things". Perhaps falling rate is an inevitable consequence of the packaging and the effort to jack-up the back of the bike for longer suspension travel and steeper head angle. Dual-rate springs go some way to beating that, and maybe that's why mono-shocks pushed twin shocks off the scene so abruptly. I've read the trials-enduro-news article before, and very good it is too. I'll read it again sometime soon. I'll have to polish up my German before I can get much out of Trialsport Magazin. Quote Link to comment Share on other sites More sharing options...
guy53 Posted January 15, 2017 Report Share Posted January 15, 2017 I don't think that it's a good idea to raise the rear end of a bike to steepen the fork angle is a good idea. When you work on one end of a motorcycle, you have to work on the other end to compensate, unless you work on a really bad handling moto. Guy Quote Link to comment Share on other sites More sharing options...
pmk Posted January 15, 2017 Report Share Posted January 15, 2017 Want to add, if you start plotting the leverage ratio curves at various points, you may find many twinshock machines have a very slight almost microscopic rising rate in regards the suspension geometry. If the machine utilized a dual spring or progressive wound spring, it artificially has a rising rate in regards to the spring force. As for gaining a progressive damping curve to artificially accomplish a rising rate of damping, you must look inside certain dampers. Researching the KTM PDS modern off road machine you will find internally they utilize a needle that alters the damping curve based on the dampers position of the shaft. This design not only adds progression but is position sensitive as well. This trials thing is new to me, however dirt bikes are not. My own twinshock I am working to finish as a rider is a 73 OSSA MAR. The original rear coilover shocks were Betors and on the machine when I got it. They had progressive wound springs. On account of corrosion on the shafts, the units have been replaced. The replacements are 10mm longer. Not concerned about that. The shocks are vintage Curnutts. These were a popular upgrade on dirt bikes back in the 70's. Internally these Curnutts use metering needles in some ways similar to the KTM WP PDS. So, my 73 vintage OSSA, with the slightest of slightest mechanical rising rate based on suspension geometry, will artificially produce rising rate on account of progressive wound springs and position sensitive damping within the damper itself. Will it be better or worse, time will tell. FWIW, trying to compare a race car chassis and suspension design to a motorcycle is often not a wise idea. The full chassis dynamics is considerably different in how the wheel movement arcs, the effects of drivetrain squat / anti-squat, and more. The RaceTech text book is a good read and reference book. While I do not agree with everything Paul Thede states, the book is well written and does a good job explainig moto suspension. Quote Link to comment Share on other sites More sharing options...
trapezeartist Posted January 15, 2017 Author Report Share Posted January 15, 2017 5 hours ago, pmk said: FWIW, trying to compare a race car chassis and suspension design to a motorcycle is often not a wise idea. I wasn't comparing, I was contrasting; exactly because you are doing very different things to achieve very different objectives. Quote Link to comment Share on other sites More sharing options...
still trying Posted January 15, 2017 Report Share Posted January 15, 2017 I have wondered about this "leaning the shocks forward" and wondered why, and the only thing that makes sense to me is that by moving the top mount forward reduces the amount of effect the springs rate of increase has, As for sag, This has a big effect on the angle of the swing arm. If the frame pivot is above the axle, then there is a component of the driving force pushing up at the frame pivot, (jacking up the rear end and stiffening the suspension, steepening the steering). This could be useful when trying to get the rear to launch up something and not useful when riding small bumps when soft suspension is more desirable. So consider this against riding style and terrain you typically encounter. And as mentioned it needs to also to fit with spring rate and the shock setup. SO, Three things to get to combine; swing arm angle, shock design and rate, spring design and rate(s).. And once you get that sorted see how its bugged up the front end and go round again. Quote Link to comment Share on other sites More sharing options...
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