INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

Annoying suspension geometry questions

Annoying suspension geometry questions

(OP)
Hi Everyone. Firstly let me apologise for asking the same questions as many others will have done before me. I'm struggling a bit with lack of experience.

I'm designing the suspension for a light weight sports car:

700kg
300 bhp
front engine
rear drive
twin wishbone front
twin wishbone rear
14" wheels

It will be used both on the road and track but never for racing. I intend to use a performance (road legal track tire) such as Toyo R888/Yokohama A048 etc..
It will be possible to use a slightly wider tire on the rear (but the wheels will have the same rim width)

I have design freedom for all inboard suspension points front and rear, and so roll centre positions etc.

I have a CAD assembly of a front and rear corner and a Suspension kinematic model in WinGeo.

Where I'm struggling is to decide on inboard suspension positions without tire data. I understand that the influence of springs bars and tires are far more influential than subtle changes in my geometry, I'm just trying to make sure I have a good starting point. For now I've just included roll and ride iterations below:

The front suspension is a work in progress but at present gives very similar results to the rear in terms of camber gain and roll centre migration (ignoring the affect of caster and steer angle for now)




Shot at 2012-08-14


Questions:
Does this look sensible?

Should I try and decrease the positive camber of the outside tyre at full roll?

How much anti squat should I aim for? (at the moment there is none, I have a SAE paper suggesting 80-90% for a passenger car... )

How much roll axis inclination is sensible, at present my static roll centres are:
front Z - 38
rear Z - 50
I could probably get the front on the ground with some more work.


Thanks all for your help. I'd love to be about to move on from this stage soon and start building, just want to avoid dropping a clanger..

tom

RE: Annoying suspension geometry questions

(OP)
Hi Greg,

I really don't, but is it actually possible to prevent that on the rear without using a lot of static camber? (this is my first suspension design, still not totally clear what is achievable)

I read at some point that ball park camber gain for a performance car is 0.6-0.9 deg/inch bump, which I'm achieving.

Interestingly I just found this data for the rear of a Caterham SV


Shot at 2012-08-14

Their camber change per degree of roll is higher than mine!

RE: Annoying suspension geometry questions

(OP)
Oops, wrong link, try this:


Shot at 2012-08-14

RE: Annoying suspension geometry questions

(OP)
I'm measuring relative to the ground, I assume they did the same..

RE: Annoying suspension geometry questions

(OP)
I misread their graph, camber change per degree of roll isn't as bad as mine, but its still potisive after ~2.5deg body roll.

RE: Annoying suspension geometry questions

OK, that makes more sense. I measure relative to body hence the confusion.

I don't see anything especially alarming there.

I see no point in dropping front roll centre to ground level, it should either be below ground level and stay there or above, and stay there.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Annoying suspension geometry questions

(OP)
Thanks Greg.

I tweaked a little more and the rear RC is now ~60 with front ~38.

Whilst I have your attention, can you comment on anti squat? I'd like some, perhaps 15% but this has introduced significant bump steer. Is the game now to move the trackrod out of plane with the lower wishbone, and use its arc to try and cancel the bump steer?

RE: Annoying suspension geometry questions

I'm not, particularly, a huge fan of antisquat. If you are using stiff springs and your cg is low, you don't need it. I've never been very comfortable with the %age definition for a/s, I'd rather work back from a desired maximum pitch angle (or suspension deflection) at maximum acceleration, than a one size fits all %age.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Annoying suspension geometry questions

Surely the Caterham has a DeDion rear Axle?

I would not be enthusiastic about anti-dive or anti squat on a race car.

RE: Annoying suspension geometry questions

FennLane, The last DeDion axle I am familiar with in first class racing cars was tried by Chaparral circa 1969, but it was an articulated DeDion with a vertical hinge in the middle. In F1, the last one was in the '50s. F1 cars are fairly uniform in having upper and lower wishbones front and rear. It's a matter of packaging as well as geometry. Look at the fronts, bad geometry, good packaging.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close