Suspension Design - How To?

[Puk]

I've been iterating through different ways of attacking the design of the suspension for a 7eque car. I've read most of the reference books excepting Kimini one and the Miliken one (Santa proved to be a disappointment on that front - but thanks for the jumper mate). Rather than continue to lie awake in bed pondering this, can I ask for some feedback from anyone that has been through this already?

My thoughts are that you'd want the tyre to be as close to upright as possible under conditions of maximum load and it would be ok to compromise on this when the tyre was under little load. So if that is the starting point, does the following hold true?

1. When cornering the outside wheels would be upright, or leaning in slightly at the top like a motorbike type (but definitely not leaning out) AND it would be an acceptable compromise for the inside wheel to be less than upright because it is lightly loaded.

2. Under brakes the front tyres are most heavily loaded, so aim to keep them upright or leaning inwards at the top slightly (but avoid leaning out) AND it would be an acceptable compromise for the rear wheels to be less than upright - again because they're lightly loaded.

3. Under acceleration keep the rear wheels upright or leaning inwards at the top slightly (but avoid leaning out) AND it would be an acceptable compromise for the front wheels to be less than upright - again because they're lightly loaded.

So if all that hold true then it is just the small matter (gulp) of establishing the dip of the chassis at the loaded end under these 3 cases of maximum acceleration. If I know the target acceleration in any direction (ASSUMPTION: 1.5G in all directions is about the best I would except from a set of race tyres on car without downforce) then if I also know the spring rates, the location of the CofG, the tracks and wheel base, then it ought to be possible to calculate the roll of the chassis under acceleration (acceleration being positive when the car gets quicker, negative under brakes and lateral in a corner).

Knowing the dip of the chassis tells me the amount that the loaded wheel has been compressed by. For the sake of argument if the car dips 50mm braking at 1.5g then I want the front wheels to be upright when the suspension is compressed by 50mm.

Ideally I'd like the outside wheel in a bend to be compressed be the same amount when achieving 1.5g of lateral acceleration. Though quite how that is achieved is a bridge that is yet to arrive - maybe that is where rollbars come in?

Let me apologize in advance if I have not made myself clear - I can spend some time drawing up some diagrams if that would help clarify this approach. But if you've stuck with this post to this point - what do you think is it a sensible approach?

[davew]

PUK

Your assuming a 50mm dive under braking with racing spring rates may be a little high [out of a total race travel of 3"?]. But I would suggest that you look at adding about 16 to 18% anti-dive into the front end suspenssion geometry when you have a low CG.

Start your design with -.5* neg static camber and assuming low profile stiff racing side walls a camber gain of only about 2*, for the street, you can go 2.7 to 2.8* with performance side walls at max jounce and roll.

You did not mention roll centers, but try to reduce the difference between the two axles to get more equal wt transfer on the outer cornering tires.


Good luck on the suspension design Dave W

[KB58]

Think of it like an equal-sided triangle, where one corner gives you perfect camber when cornering, another corner gives perfect camber in braking, and the third corner gives zero scrub...

Now pick your operating point. You can do well at one, but only at the expense of the other two. It's all about compromise.

BTW, if you don't get my book, I recommend the Staniforth books, very good and easy to understand.

[KB58]

...try to reduce the difference between the two axles to get more equal wt transfer on the outer cornering tires.

Load transfer is related only to CG height and track width.

[mookie]

BTW, if you don't get my book, I recommend the Staniforth books, very good and easy to understand.

I recommend Staniforth as well..... if you run out of NyQuil.

Kurt's book is a very good read - Staniforth does cover the subject very very well. It's just kinda dry IMHO.

[chetcpo]

Here is step one. http://www.locostusa.com/forums/viewtopic.php?t=1438 If it doesn't make good sence to you then you may want to read a few more books. Therer is a good book list stickied around here somewhere.

[Puk]

Thanks everyone for your suggestions, I had to look NyQuil up and have ordered a case

DaveW I'm guessing at droop of 2", but I take your point - loosing 2/3 of the travel to weight transfer caused by acceleration doesn't leave much to handle bumps.

I could well be overlooking something but as far as I can recall the various books that I've read on the subject of suspension design all major on optimizing the motion of the wheel by playing with swing arm lengths and roll centers, but seem to skip over the question of finding the optimum compression under different load cases. If I knew that then I'd design the suspension geometry to produce -0.5* negative camber under that dynamic load, and then play with geometry to try and get the least amount of camber camber with only static load.

Maybe the books don't take this approach because experience has revealed that using a spring rate of around 200lb/inch plus a target ride frequency between 2 & 2.5 Hz then a camber gain of 2* does the job...

Kurt your book is on order as are the Staniforth books - I haven't read them since university years ago. They'll soon be as tatty as the Smith and Van Valkenburgh books on the window sill.

Cheers,
Puk

[davew]

PUK
Wrote " but seem to skip over the question of finding the optimum compression under different load cases"

The reason is simple that every tire is different. Unless you own a F1 team the odds are you are probably not going to get the info for the tires you plan to use.

Try searching for tire dynamics on the SAE site, I've got some 80's info but I'm sure there will be newer info on the low profile tires.

The Staniforth books are excellent, They should give a good baseline for the starting points and control arm design and options.

Good luck Dave W

[KB58]

... unless you own a F1 team the odds are you are probably not going to get the info for the tires you plan to use.

Exactly. The Millikin/Millikin book is that way, where they reduce everything to numbers, which is great, if you have the numbers. It was very frustrating to use all the "precise" equations, only to have to make guesses on some very crucial figures. I ended up pretty much ignoring the chapters on tire data, simply because I couldn't obtain any.

[mcteardrops]

PUK

I think the factor that most authors miss in this analysis is the amount the roll centers move during roll and pitch. Bob Bolles covers this area, and markets a computer program for roll center design. His bias is stock cars, circle track, so may not be optimized for us.

Almost everyone sets up the car to minimize bump steer, starting with the wheels dead straight, and at ride height. Where bump steer is going to spin us is with the suspension still at full dive after hard braking, rolled over hard on one front wheel, cornering near the limit, when we hit our bump. The Boles program addresses this situation.

Being locosters, seems like with the trial spring and suspension data in hand, and with target accelerations in mind, we could approximate the amount of dive and roll expected and place the car in that attitude and steering angle. Then we can see where steering and camber are going to move in this range of motion. Seems like Jack's PVC simulater would be ideal for this use.

Having said that, the angles that the tire likes, and the optimum for all the other turns on the track, will still have to be detiremined by the stop watch, pyrometer, and educated trial and error!

[KB58]

I think the factor that most authors miss in this analysis is the amount the roll centers move during roll and pitch.

I didn't!

Being locosters, seems like with the trial spring and suspension data in hand, and with target accelerations in mind, we could approximate the amount of dive and roll expected and place the car in that attitude and steering angle. Then we can see where steering and camber are going to move in this range of motion.

I used an early copy of Mitchell WinGeo sw. As long as you know wheelbase, track, and CG, all the numbers above are easy to find.

[violentblue]

I think the factor that most authors miss in this analysis is the amount the roll centers move during roll and pitch.

the suspension analysis program I used factored roll and bump, with minimal (.01 inch) movement of the rollcenters, but if you roll and bump at the same time, things go awry (only a little)

[Puk]

Thanks for the advice from everyone.

The various roll centre software sound useful I can see that using the roll center software (or a string computer, or a bunch of lines in a CAD program - pick your poison) will let you optimize camber gain with roll. But I keep coming back to the same question - how much has the chassis rolled at maximum cornering load (or similarly how much has the nose drooped under brakes, or the tail slumped under acceleration)?

To DaveW's point - you can't expect to know the optimal tyre camber to generate max grip and I agree but isn't it likely to be as close to 0* (so that the full width of the contact patch is bearing down on the road surface)?? But I hope that the camber adjustment designed into the top wishbones would allow for tuning - as long as the basic design has got the wheel to roughly the right attitude relative to the road.

It seems pretty straightforward to calculate the dip under brakes - its just a combination of weight transfer and spring rate. Spring rates fall out of the target ride frequency. [PooPoo] what am I talking myself into here!

So knowing the compression of the front end under max braking, the roll centre fun can be used to try and ensure that same compression is achieved under max cornering load. That part sounds more tricky - and I haven't thought about the steering angle that mcteardrops mentioned.

Holy Moly

[dhempy]

How much bump and droop does a typical seven have/see? I've been tinkering in Wishbone with up to 3" bump and 2" droop, and I'm starting to wonder if that's really looking for too wide of an envelope.

Also, how much bump steer is too much? Is +/- 0.5 degree from static toe acceptable? 1 degree? 0.1 degree?

-dave

[mcteardrops]

Puk

Figuring the roll uses the same calculation as the dive: weight. center of gravity, roll center and wheel rates. One of Bolles main points is balencing roll moments at the ends of the car. If you remember that both ends of the car will roll the same degree, it's easier to grasp.

Regarding the sad fact that the springs have to comfortably support the vehicle, as well as resist roll, and the occasional bump... I have to wonder about some of the ultra high or low panhard rods out there.