Suspension Travel

So, what is the actual question?

Question is what should be the suspension travel of the rear suspension?? polaris ATV's have a suspension travels ranging from 8-10in for both front or rear suspension. Do they have CV joints with better movement when suspension travel is high as 8in. Last baja we had the problem of our axle getting out of the gear box and my team reasoned out that travel was high of the rear suspension. 

Edited by Rishabh - Jul/31/2012 at 12:59am

It sounds like your problem is with axle plunge rather than joint angle limitation. You need to carefully select your final drive location to minimize plunge and then you can keep the rear suspension travel. For reference, we are able to get away with 12 inches both front and rear and I would recommend 10 or more for all teams if they can do it.

There is no correct geometry.  You need to define goals for your system, and then prioritize the compromises you'll have to make. 

What is the track like for baja India? I highly doubt you need to design your own springs

Ok,so according to you my priority should be to design the suspension with roll stiffness high enough to prevent roll, and 6in 4in bump and rebound setting comply for rear suspension too for indian baja??? we used a mahindra alfa gearbox and I think the problem was that gearbox was not designed for that much travel because maybe it's not meant for off-roading, perhaps that was the reason the axle was coming out!! I was adviced by the members that proper positioning of the drive train would solve the problem,but even then can a alfa gearbox give a travel(10in) without the axle being plunged out?? 

Edited by Rishabh - Aug/01/2012 at 12:24pm

Rishabh wrote: Ok,so according to you my priority should be to design the suspension with roll stiffness high enough to prevent roll, and 6in 4in bump and rebound setting comply for rear suspension too for indian baja??? we used a mahindra alfa gearbox and I think the problem was that gearbox was not designed for that much travel because maybe it's not meant for off-roading, perhaps that was the reason the axle was coming out!! I was adviced by the members that proper positioning of the drive train would solve the problem,but even then can a alfa gearbox give a travel(10in) without the axle being plunged out??

If you bring your inner suspension points closer to the centerline of the car you can use longer cv shafts and get less cv angle change per inch of travel. The rear of your chassis (between suspension points) is rather wide and you are either going to hit the limit of the cvs maximum articulation or their maximum plunge. By tuning where the cv joints are in space you can minimize plunge, but that still isn't going to help with the articulation. Otherwise you would have to lower your static ride height at least in the rear to still get high travel while being within the limits of the cvs. I understand you may be contrained by an oem gearbox for your inner cv spacing, but longer links and shorter chassis is usually a good rule of thumb to follow when trying to design for high amounts of travel. Less cv angle will also help a little with drivetrain efficiency. Do you guys have a number of degrees that's the cv joint can handle? We had this problem two years ago in which the rear suspension was fine at ride height but at full droop it destroyed the cvs. We ended up having to run limit straps to bandaid the problem

Calculated Maximum Cornering speed:

Edited by Rishabh - Aug/03/2012 at 2:26pm

It depends on the atv, but many FWD passenger vehicles don't have all that much articulation on the inner cvs but have alot more on the outers since they need to be able to travel vertically and turn. We used honda foreman CV joints which only have articulations in the low 20s on the inners depending on what plunge you are running. There are companies such as gorilla axle who provide high articulation CV joints but you can avoid this problem by making your shafts longer. Keep in mind that increasing your rear track width in relation to your front track width will have a tendency to increase understeer.

Edited by Purduebaja - Aug/05/2012 at 5:09pm

Yes, design your car properly, respecting the angles and travels.

http://www.lotuscars.com/gb/engineering/engineering-software

Andy.R wrote: The cv is only as limiting as the length of your trailing arm, I have played with the model of having 20 inches of travel in the rear, granted it did not meet spec, but I was still within the CV limits of 20deg.

How'd you manage that? Even with a 32" axle that's barely possible. It's hard to push 16" of travel with 30* CV angle limit and a long (for SAE Baja) 22" axle.

I spoke to soon, went back and modeled it. I was wrong I had 16.3 inches of travel with .2 inches of plunge.  and that was within 21deg of cv travel.  My bad. 

he he he, still that's pretty good suspension travel! I assume that it's at the wheel?

If you have that much travel, try using a limiter strap on the trailing arm.

lots of worm-clamps and gorilla tape

using Adams software, you can play with your suspension geometry and simulate its cyclical motion. with a member to act as the drive shaft, you can measure the plunge over the travel. from this you can proceed one of two ways:

I would assume it just unscrews with a valve tool just like the one in a tire. the tool is cheap and can be found at all major auto parts chains. you can often get a small box of new valves in a box with the tool.

In lotus you just type in the coordinates (x,y,z) of the COG. This can be obtained through various methods which include but are not exclusive to weighing+math, or from a complete car CAD model. 

you'll have to set a goal of where you want it to be and then use old cars as a validation of you goal. 

if you estimation is done well, and you use previous knowledge of COG of other cars, it should be close.