Rear Upright Design

[StuartBGT]

After looking at god knows how many rear uprights in an attempt to understand why they are the shape they are (in order to fabricate my own and get it right) I`ve come to no conclusions! I thought this would be a simple bit of mental reverse engineering and all would become clear, but other than some reasonable theories, I`m either missing something here or there are a good few people using uprights, that while looking the part, are totaly wrong for the car.
For example, why do the upper mounts tend to be inclinded toward the chassis? For the correct upper arm length ratio in relation to the lower arm length? or to clear the wheel while staying long enough to give good torsional strength? I`ve seen examples where both or neither would be relavant to the vehicle they are fitted to, so am I missing some thing?
Why is the lower portion of the upright, when measured from the axle line centre to the lower arm outer mounting point, often around a three to two ratio in length longer, in comparison with the upper portion? Again, is it greater strength in length? Or purely dictated by the chassis pick up location? (my favorite) and if so, why are they some times also angled in toward the chassis, surely having the pivot point closer in to the wheel is better? Or some thing else?
Having read countless `Suspension/Chassis read this and you will be a genius Tech` books I`m still none the wiser and the pub guessing ideas have now run dry.
Can anyone point me to the right book? or better still, as this is a forum, share some probably had earned knowledge with the rest of us, I can`t be the only one in the dark here, can I?
Oh, by the way, the chassis I`m chopping about is a twenty year old KVA GT40 replica, it came with Granada rear swinging arms and a Cortina front suspension `K` frame so you`ll understand why I`m so keen to change it to something closer to a true GT40 (twin arms and twin tow links each side), although I`m using 18" rear and 17" front wheels so copying an origional upright doesn`t realy make it the right design for the car.
The front end I`ve redesigned and am now happy with, but these rear uprights are

[graham blackwell]

contacting John Beardmore at www.beardmorebros.co.uk
His new scratch built Morris Minor spaceframed hill climb car is quite the work of art, and he's in to suspension design etc, having spent many years studying books on the subject.

www.beardmorebros.co.uk

[Dave Brand]

Originally posted by StuartBGT
For example, why do the upper mounts tend to be inclinded toward the chassis? For the correct upper arm length ratio in relation to the lower arm length? or to clear the wheel while staying long enough to give good torsional strength?

My guess, & it's no more than that, is that it may be done to line up the c/l's of the upper & lower suspension joints & the driveshaft joint.

Why is the lower portion of the upright, when measured from the axle line centre to the lower arm outer mounting point, often around a three to two ratio in length longer, in comparison with the upper portion?

It seems to me that, within reason, the bigger the centre distance between suspension joints the better; make the upper & lower portions of the upright equal length & you run into ground clearance problems! There's also the length of the spring/damper unit to consider - again, I'd think that as long as possible makes it easier to get the desired performance.

I should point out that I have no experience in chassis design - I'm just looking at it from an engineer's perspective

[greenamex2]

The rear uprights that are on the John Beardmore web site bare a remarkable similarity to the ones on my kit car (AM Sports Cars EX2 - www.amsportscars.co.uk).

These are now manufactured by Gardner Douglas Cars (www.gdcars.com) and used on their very highly rated Lola T70 replica. I know that GD, like you, use Granada bearings and hubs so they are probably the ideal people to talk to.

Another alternative is NF Auto Developments who are now selling the uprights to their highly commend P4 replica.

As for the position of the pickup points and wheel bearing, these are interdependant on the options you have with pickup points and driveshaft location on the car. Current thinking seems to be when the cars is att rest have a straight or slightly downward sloping lower arm then the top arm at an angle to get the correct camber compensation in roll to keep the tyre at 1/4 degree negative camber. Also longer arms the better. Obviously the driveshafts should be parallel to the ground at rest and, ideally, meet the upright at a right angle (damn near impossible with a Renault box in most mid engined cars I have ever see!).

Having said that, Ultima's break a lot of the 'rules' of suspension and they seem to handle pretty well.

[kudosdude]

On my first post to this site I feel slightly rude posting a link to another site; but bearing in mind you intend to fabricate your own (hardcore!), might I suggest www.eng-tips.com.

Just so you know, I too have read quite a few books on suspension (about 4) and have come to the conclusion that as long as the upright matches the wheel, you can more or less sort out handling issues by playing with inboard suspension points and arm lengths (but don't quote me).

[StuartBGT]

Thanks for taking the time to reply guys, much apriciated.
Firstly,
Graham, I`ll have to take a look at the Beardmorebros site, I`d been told good things and meant to take a look at this site some time ago, highly recomended, and now would seem a pretty good time, cheers.
Dave, getting the rear outer joints, including drive shaft pivot, to line up (in a simsilar fashion to the setting the rack up at the front (if I read it right)) is an interesting theory, admitedly, I`m not sure as to it`s worth, but one I`d never thought of, nice one, any comments invited?
Although getting the lower outer mounting portion of the upright low to the ground should never get you into ground clearance problems as long as it`s nice and close into the wheel (where for strength, gut feeling says it should be) as the wheel and tyre asm` itself will always keep it a given distance off the ground. This being a `road race` design car it`s never going to see the kind of roads that will cause anything high enough to hit the lower mounts.
Denis, another one of the reasons why a standard `replica GT40` type upright wouldn`t work for me is the bearing size, I found the rear bearings and shafts from a late Ford Sierra 4x4 (Cosworth type) are larger than the old Granada stuff so descided to go with these (plenty of big BHP GT40s are busting the old Granada shafts) and am still not sure of the `use the upright from this onto that` kind of thing, I`m certain I`ve seen some real **** ups due to this and I guess that`s half the reason for this thead, I need to learn it so lets put a reasource up that others can take a look at. I`m certain that a correctly designed component from one car will not suit another unless by some fluke they share remarkably similar (read as almost indentical) chassis dimentions. Yeah, I agree with what you are saying in regard to suspension arm lay out but there must be some hard and fast maths involved here somewhere, and this has to be in relation to wheel size and depth of hub mounting, and then in relation to what you can do with the given placement of your chassis mounts, regardless of suspension points.
Kudosdude, make no apologies for your first post on this site mate, this is mine too, I`d trawled through too many others and not found a site where people knew any more than how to `chip` there engines and lower the ride hight in order to make a realy `interesting` ride. And thanks for the link also, looks like I`ve got some more bloody reading to do! With regard to the upright just matching the wheel and working from there, come on, there has to be more to it than that. Don`t get me wrong, I`m good for all advise and links etc but there are so many little compromises that HAVE to be made when designing chassis/suspension that any you can knock out from the offset has to make the final set up/product that much better.
Hope I`ve not upset anyone and am still making sence (after seven Stellas) and yeah, I know my spelling is questionable, but please, keep `em coming.
Cheers guys.

[StephenRae]

Allan Staniforth wrote a book years ago about the building of his Terrapin single seater with a Mini engine and box. He explains all.

[Goran Malmberg]

Stuart,
There are not many books discussing the effects of suspension layouts as you point out.
I have been writing a book myself, and been thinking about how to explain this matter without making the whole story to mixed up. In a way the question is not to difficult, but the combinations are endless and there are no supreme solutions.

Rear A-arm geometry is to be in harmony with the front suspension and here we are dealing with KPI, that position the upper ball joint more to the inside of the car. But the exact position at the rear, is not to important. Very much depend on practical solutions for the car in question, but upright manufacturer produce what’s most common.

We want the lower A-arm not to be to fare of the ground in order to get a force line that is not to steep angle, or that must use an Ic that is a product of not so good A-arm angles. We should see to that the Rc is kept at constant level during wheel travel and roll. It is important that the jacking is not altered during roll or else the geometrical weight transfer will alter between front and rear axle, creating a change in over – understeer.

You may throw an eye on my site http://hem.passagen.se/hemipanter/ where I got some images of my own fabricated rear uprights. There are also drawings of the chassis layout
concerning A-arm geometry.
Regards
Goran Malmberg

[kudosdude]

Just thought you might like an excerpt out of "Competition Car suspension" by Alan Staniforth (very basic but good book);

Uprights
The geometric positions of the top and bottom pivots at the outboard ends are not so vital as those on the chassis, mainly because they can be integrated with the inboard ones which normally take first priority. The two main criteria are that a. they do not contact the rim . . . , b. the further outboard they can be contrived the more the leverage of the wheel agains the links can be reduced.

I knew I read it somewhere

[avsfan733]

if you want real math there really isn't any substitute for this book: http://www.amazon.com/exec/obidos/tg...glance&s=books

its called "race car vehicle dynamics" and in my opinion is simply the best one out there but you will be doing a lot a lot a lot of math

[StuartBGT]

Guys, many thanks, I think I can start to see how it interacts with the rest of the chassis now. Time to go book shopping.
Goran, one of the things I was wondering was just how the dynamics of the rear uprights should tie in with the fronts, and to what degree, thanks for confirming this theory. By the way, I had already had a look at your site, that`s some nice engineering on the Pantera mate, well impressed.
Cheers.

[Kid Prozac]

Originally posted by Denis Bassom

Having said that, Ultima's break a lot of the 'rules' of suspension and they seem to handle pretty well.

Care to elaborate on what you mean by 'rules'????

[greenamex2]

From all the books and articles the current flavours of the month seem to be -

Long wide based arms (although longer ones were/are now an option).
Damper as upright as possible.
Driveshaft dead square at rest (seem to depend on the box and engine height chosen).
Anti roll bars (again, now an option and also very debatable whether they are a good thing or not).

If you have ever looked over the ex Colin Blower Ultima that competed in the British GT Championship you wouldn't recognise the suspension.

But as I said before, I have driven an Ultima and thought it handled fantastically.

The point I was trying to make was that you can read all the theory you like but as long as the main elements are correct (camber control, castor, bump steer and controlling roll centre) then it doesn't really matter how you do it.

[avsfan733]

see the "rules" change based on the kind of flavor of the month thign you talk about also different research and computer modeling leads different people down different ideological paths. not long ago leaf sprigns front and rear where not uncommon on racecars

[KevinS]

Originally posted by StuartBGT
For example, why do the upper mounts tend to be inclinded toward the chassis? For the correct upper arm length ratio in relation to the lower arm length?

Close! Dynamic camber... If the upper arm is shorter than the lower arm, change of (horizontally measured) length will be greater when the suspension is compressed. This is due to the fact that the shorter arm will rotate over a bigger angle, so it 'pulls' the upright inward more than the lower arm. This allows you to drive with less static camber so the tyres wear more even and give more grip when accelerating or braking. Due to roll of the car, the outside wheel will gain camber when cornering.

Originally posted by StuartBGT
Why is the lower portion of the upright, when measured from the axle line centre to the lower arm outer mounting point, often around a three to two ratio in length longer, in comparison with the upper portion?

Rollcenter and chassis dimensions. The rollcenter is mainly determined by the angle of both upper and lower suspension arms. With the driveshafts horizontal it may result in your 1:2 or 1:3 proportions, depending on where the chassis pickups are. Moving the upper arm + pickup + pivot down (to 1:1 proportion for instance) will increase the dynamic camber effect mention above.

Most of the time the shape of the upright is a result of the shape of the chassis and prefered geometry.