Board logo

Suspension Design….Help!
orton1966 - 1/1/11 at 09:22 AM

As some might have seen from my other posts I’ve decided I want to design and make an exo-style midi, kind of like the Warner R4 but with my own twists. Importantly it’s a clean sheet design, including suspension, including custom upright but boy is the suspension hurting my brain!

I’ve created a model in solid works that (if I’ve done it correctly) allows me to change the position of all 4 pivot centres, wheel size/offset and wishbone lengths. I can then change ride height to simulate bounce/droop and roll angle. When I do these it recalculates camber angle, track and roll centre height, see screen shot below:


I’ve read several books, Herb Adams, Staniforth, Tony Pashley etc. and despite many common points there is equally some slight contradiction, however the general consensus seems to be lower wishbone as long as possible (except Staniforth), roll-center close to static (Adams seemingly prioritising this in bounce/droop, Staniforth in Roll). Obviously there are some design constraints/ decisions already made, these being:

The choice of 15” wheels probably 7” wide with 195 by 50 tyres front, possibly moving up to 205’s for the rear.
Being a midi and with a conscious decision to sit as far forward as possible (feet only just behind the front axle line) limits minimum distance between lower pivot points, I’ve gone for 600mm as a starting point, could possibly go a little less?
Track width around 56-57” – So 1210mm – 1240mm
Ride Height (front) between 100mm for road & 75mm for track (suspension optimised at around 85mm)
Suspension operating range about 50mm in bounce and droop and I guess up to about 3 degrees of roll

I’ve really just started playing around with the model, in a real trial and error fashion. The things I would be interested in would be a second (or more) pair of eyes over the model to see if I’ve missed anything. If anyone has a proper suspension calculator it would be nice to input the figures to see if it agrees. What would be fantastic is some pointers on directions to work in to improve results i.e. short cut my trial and error or at least provide some ideas. If anyone has solid works I’ll happily share the model.


MikeCapon - 1/1/11 at 12:19 PM

Hi. There's a couple of useful links on this thread....

Linky

And I'd love a copy of your SW model providing it's in 08 or earlier. Looks really useful. My mail is below.

Cheers,

Mike


orton1966 - 1/1/11 at 02:20 PM

Thanks for the links, some intresting stuff there.

I have the latest version of solid works but i'll see if there is a way to save it as an earlier version


Sam_68 - 1/1/11 at 03:44 PM

Just to muddy the water a little more for you, one thing that most of the textbooks don't do a very good job of pointing out is that cars do not roll around their geometric roll centres/roll axis, so whatever camber angles you're calculating for your notional 3 degrees of body roll will only vaguely resemble what actually happens on the road.

Also, whilst Staniforth et al seem to have grasped the fact that a fixed geometric roll centre is a good thing, they don't explain why: the roll centre is one of the factors that governs diagonal weight transfer, so if it is moving about whilst your car is negotiating a corner, the loads on the tyre contact patches will be shifting about... think of it as being a bit like having an anti-roll bar whose rate is constantly varying through the bend.

Unfortunately, to get any meaningful understanding of what's happening, you need to start from an analysis of the car's weight transfer characteristics (which means making some assumptions/educated guesses about weights and spring resistances; see the chapter on weight transfer in Staniforth for the basics).


ettore bugatti - 3/1/11 at 01:57 PM

It may not be that important, but I wondered how much the SAL is in your design.

Otherwise it looks like most starting points in suspension design have been covered.


orton1966 - 5/1/11 at 06:20 AM

Hi Again,

Been playing around with my model and think I‘ve made some progress. The following pictures show the effects of static (85mm ride height), 2 degrees of roll, and 2 degrees of roll with 1” (25mm) of bounce.

The improvements have come through a shorter top wishbone, with more angle resulting in shorter swing axle length.

Anyone have any thoughts about the layout, figure etc.






jonny007 - 17/5/11 at 07:40 PM

Hi, I appreciate this is a fairly old thread, however would you still be happy to share this model. I am about to part cash for analysis software and I think this will be a slower but cheaper method to fix my upper w/b mount.

cheers Jon


designer - 17/5/11 at 09:28 PM

I think it is always a good to start any suspension design, to have a horizontal lower wishbone.