Having now made my chassis suitable for rocker arm top suspension so I can have inboard sping/shock units I am considering options for the units.

Basically the options are to have the inboard part of the rocker the same length as the outer and use a standard spring/shock unit with standard rating, or I can make it shorter and use a higher spring rate. The advantage of this is that I get less unsprung weight and can use a bike unit (Yamaha R1 looks good) which are quite cheap and have plenty of adjustment. My concerm, however, is that the higher spring rate will be putting extra loading into the chassis through the wishbone mounts.

What are your views on the best way to go?

Keep it simple and go for 1 to 1 ratios.

but if you decide not to, i have a selection of r1/r6 shockers

What are you using for the ball joint on the upper wishbone/rocker out of interest?

Make it 1:1.

It makes spring choice so much easier, you can compare spring rate to the much more important wheel rate, the springs can be lighter and much more importantly you get lots of damper movement. Dampers are displacement devices, they need movemement to work. So the more the dampers move the better.

John

Thanks - looks like 1:1 is the way to go. Almost a shame as I like the look of the bike units, and they are cheap! Quite happy to take the advice though.

I dont agree...

For starters, assuming you're using sierra track and a standard width chassis, you're never going to be able to get the inner parts of the bones as long as the outer parts to achieve a 1:1 ratio.

Secondly I just dont think you want a 1:1 ratio anyway. Bear in mind that with the shock inclined at, say, 40 degrees, and the shock mounted, say, 0.8 of the way along the bottom wishbone, a book locost has something like a 1.6:1 leverage. If you use a 1:1 ratio, and assuming an average locost weight of something like 600kg, then you'll be looking at something like 60lb springs to get a sensible wheel rate of 80-90 cycles per minute, which I imagine would have to be custom made. Any higher and it'll be massively oversprung.

You'll also need dampers with as much travel as you want total wheel travel which will mean fitting in some huge shocks (a look at the avo catalogue would suggest something like a 17" open shock to give 6" travel, minus 1" for the bump stop, giving 5" total wheel travel for bump and droop). Probably aint gonna fit!

Much better to design for a ratio around 1.6 imho, then you can use standard length shocks and standard rate springs, plus you can physically fit the shocks and rockers in the car. No problem going for an even higher ratio and harder springs with shorter travel dampers (if that's what bike ones are like - I dunno). You'll still be using them how they're designed to be used in the bike, and can presumably change the springs if you need too.

You're right about the wishbone pivot being highly loaded if the leverage is high, but that's nothing that can't be overcome. My design has various mounting points for the shocks giving leverage ranging from 1.6 to 2.5, corresponding to adjustable wheel rates of about 80-120 for the front, and about 85 to 130 for the rear, with 350 and 400lb springs respectively. My pivot brackets are 6mm rather than 3mm with M12 bolts so plenty strong enough. The chassis is also well braced in this area.

You really need to sit down, decide what wheel rate you want to aim for and do the sums and some drawings, bearing in mind what bits are available and the space you have to play with. I did a spreadsheet to help me work out what was going on (you're welcome to it if it would help) as well as drawings on autocad. Oh and I take it you at least have the Race And Rally Car Source Book if you're trying to design your own suspension??

Hope that helps,

Liam

[Edited on 16/5/06 by Liam]

I agree with Liam on this one. If using inboard suspension you will no doubt also be using your uprights vertical. Bear in mind that most outboard suspensions will use inclined uprights which will reduce the effective spring rates.
My Formula 27 uses leverages of around 1.6:1.

I have the Race And Rally Car Source Book and am in South Yorks if you want to come over and borrow it for a while.

I'm using r6 shockers on mine, pics in archive.

quote:

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Originally posted by Liam
Secondly I just dont think you want a 1:1 ratio anyway. Bear in mind that with the shock inclined at, say, 40 degrees, and the shock mounted, say, 0.8 of the way along the bottom wishbone, a book locost has something like a 1.6:1 leverage.

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Thanks for the replies.

Gazza - your setup is along the lines of what I was thinking of.

Tangoman - I have Allan Staniforths Competition Car suspension book which covers much of the theory, but thanks for the offer.

I am now thinking the best bet is to get the steering rack in place and then see what I can fit where. Certainly thinking that the shocks can be more upright than if they were outboard, and as I am using MK Indy wishbone dimensions I am concerned about getting a 1:1 ratio.