Theoretical engineering question about front suspensions: Grassroots Motorsports forum: Grassroots Motorsports Magazine

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Nov. 15, 2008 2:45 p.m. Helterskelter Reader

Suppose I'm designing a double wishbone front suspension for a mid-engined go-kart in ADAMS & Pro E.

Are there some general guidelines for the amount of camber & toe I want in the front suspension? It is geared towards autocross type motorsport.

In case it helps, there's ~3 inches of suspension travel, C.G is 9.5inches, wheelbase is 60 inches, track is 54 inches, tires are 7 inches wide with a 20inch diameter and will be modeled as racing slicks.

I'm looking for general rules and some background theory explaining why I should follow these rules. Basically I'm trying to improve my cornering forces once I model the entire car. I can go back and change the suspension later, but would like to have a solid baseline. Thanks in advance.
Nov. 15, 2008 3:01 p.m. slantvaliant Reader

You might poke around over at Locostusa
Nov. 15, 2008 3:47 p.m. Helterskelter Reader

slantvaliant wrote:

You might poke around over at Locostusa

I did poke around for a while before posting here, but the problem I had was many of them are using donor cars, so it's not nearly as flexible as a completely scratch-built go-kart. I tried looking for some rough & general rules, but I must be using the wrong search strings, because I was unsuccessful.
Nov. 15, 2008 8:04 p.m. iceracer Reader

One of the things is designing the control arms so that they locate the roll center where you want it. Also the instant center must be toward the inside of the car. Of course the arms should be unequal length so as to achieve some camber gain. The roll center center should be at ground level or no more than 3" above. I took most of this from "Steve Smith's Advanced Race Car Suspension Development " book.
Nov. 15, 2008 10:13 p.m. Keith UltraDork

The Locost guys aren't really constrained that much by their donor parts, as they still get to specify the arm length and inner mounting points.

I think what you're really asking is what you should be shooting for. How much camber gain is ideal? Toe is irrelevant, really. Set the car up for minimal bump steer and use the tie rods to set the toe. The amount of static camber is going to depend on your tires - race tires tend to prefer more camber than street tires, and IIRC radial race tires want more than bias plies. Although I reserve the right to be incorrect on that last one :) The amount of desired gain is a matter of choosing your compromises. Too much camber gain and you'll lose braking traction when the suspension compresses. Too little and you're going to lose cornering force. On a light car, I'd be tempted to bias towards the latter.

The most important thing based on my reading is to keep the roll centers under good control, and I believe that's one reason the Miata handles so well. I suppose you could do worse than to start with the Miata geometry and see what that does for you.

Either we're already having this conversation via email or it's a popular question
Nov. 16, 2008 11:36 a.m. iceracer Reader

A good way is to draw it out on a graph paper.
Nov. 16, 2008 12:06 p.m. curtis73 Reader

I suggest a book; Race Car Vehicle Dynamics by Milliken. Its not cheap, not in print, and 865 pages of calculus, but you don't need to understand the calculus to understand the facts of how different geometries will react.
Nov. 16, 2008 1:23 p.m. DMSentra New Reader

"Too much camber gain and you'll lose braking traction when the suspension compresses."

I may be going about it all wrong but I designed mine with sufficient gain while turning and mild dive(1.5deg. - gain), but considerably less under straight dive. By doing this I created a bit of jacking effect though. Thoughts anyone?

I thought maybe I'd behave and not thread jack so started one here: http://grassrootsmotorsports.com/forum/grm/trading-high-static-camber-for-a-jackin...