Suspension (Technical)

[Guest Mycroft]

In this thread I will post a run through of the Susp and how to modify it for English roads, when it has run its course and all the little bits of info that are pertinent have been revealed, I will collate the entire thing and let JB put it in your reference section.

 

Now I'll go and find all my old B.Sc papers.

 

(Edited by Mycroft at 11:29 am on Mar. 16, 2002)

[Guest Mycroft]

So the simplest thing is to answer the question posed in the Braided hose thread in General.

 

WTF is 'rack back'???, it is simply the description of how the wishbones behave under severe braking and extreme cornering, now if you look at the 2 prongs they have holes in them and they affixed to the subframe with a single steel bar that passes thru these holes, to ensure a quiet and comfy journey, we 'buffer' the whole thing with bushing, this bushing will and is intended to flex.

 

The Susp. under severe braking is designed to minimise 'squat' (slapping its nose on the tarmac) this puts a lot of pressures on the wishbone this is imparted to the bushes.

 

The forward movement of the car is resisted by the wheel under braking, this is in effect trying to push the wheels back on themselves, this is 'rack-back' if you have lots of Alloy wheel and not a lot of rubber (height wise) then the force encountered by the bushings is greater, (in fact so great that all that wonderful care we all take to ensure we have a good toe-in set to tenths of a millimetre are negated and most cars with inappropriate Tyre/rubber combos will actually become toe-out) making the car less controllable, some of the toe-in is there to overcome this natural occurence, but not very much so if you have bigger wheels you need more toe-in but as you increase the toe-in the car beomes more prone to tramling with those big wheels, so a compromise has to be sought, you drive around on that compromise.

 

Companies pay a lot of money to have an Engineer work out all the numbers, and then test it on real cars with real drivers, when you change things you are playing with things that cost a fortune to 'get right' so you must be armed with knowledge at least equal to theirs.

 

The Maths are simple Calculus so that part is easy, what is harder is the knowledge of materials under stress, many of these calcs are fiendishly complex, the best engineers are the 'hands on' guys they intuitively 'see' how the material will cope with the pressures put upon them.

 

For most this is beyond what their minds can grasp, the 'idiot' in the Pub who says 'I got bigger wheels and they look great and I have loads of grip' has as much intuitive grasp of the game he is playing as the Musk Deer understands that its Anal Gland is somehow an important part of Aftershave.

 

Before we move on do you have any thing to ask/contest?

 

 

 

(Edited by Mycroft at 3:22 pm on Mar. 16, 2002)

[Phil Irwin]

With you so far, but with regard to wheel/tyres, I presume the calculations give an optimal size for each. That being the case, how wide is the "acceptable" band?

 

I presume that another factor considered has to be styling at the expense of handling possibly?

 

With regard to the Supra, why did they go to 16" for the SZ when everything else was running on 17"?

 

 

[Steve Cargill]

One question regarding less tyre height = more strain resulting in more toe-out.

 

If you are breaking at 1G then as I see it the force through the hubs and so through the suspension will be the same no matter how much rubber is between the road and rim.  All the bushes see is the car is being pulled back at 1G.

 

I can understand the anti-squat putting lots of strain on the suspension, and 'taller' tyres providing more of a shock absorber so perhaps the bushes don't see spikes as big.

 

(It's not quite phrased right, but it's just an enquiring question)

[Adam W]

But surely if you have much wider grippier tyres, you can now brake at . . . 1.4G or something before the wheels start to lock and the ABS kicks in, and the bushes are out of their design range.

[Guest Mycroft]

Good questions.

 

Width of wheel only has a marginal effect on the loads imposed, at first this seems counter intuitive, but get under the skin of the problem and the tyre can be seen as a 'shock absorber' this allows some of the energy to be taken up in the 'face' of the carcass (the bit that touches the road) so a 15×6 and a 15×8 will transmit about the same amount of this type of load to the bushing, the real problem is Aspect ratio, the height of that rubber from tarmac to rim. So as long as you don't go right over the top the width won't effect the bushings, it will effect everything else but not them!

 

Styling is your choice. Performance that is also, you can have both but it is difficult to achieve.

 

This one is a good'n all the 'other' manufacturers were hitting the 17/18" when the Supe and Soarer were on 16's and then 19/20" when our lovelies were just hitting the 17's. Why? Because of design quality, our lovelies have just about the best suspension design you can get on a road car the Bumbandits, Poorshits, and Smerc have to cope with swing axles weissach modulated but still the same in principle as fitted to a VW Beetle, look under the Ferrari F550 it has an IDENTICAL setup to ours. (transaxles don't count here) They didn't need the size/width thing to keep up, the integrity of the design was enough to keep them in contention even with the smaller wheels!

 

It was drizzling earlier and I went for a spin, found a Volvo T5 coupe, he was game so I played, his car has a compromised design, when we hit 130 on a slow bend and he started to 'wash-out badly, I slipped past at 145 with a delicious amount of controlled slip, no tailout, just increased slip angles and holding there that is just beautiful, you cant do that on anything but the finest chassis, 145 and just a beautiful controlled precise drive, that makes these cars the most serene motors to grace the roads along with the Ferrari's and Lambo's.

Due to the fragility of the Wop mobiles I would never do what I just did without a DAILY inspection of all the moving parts, weekly is enough for the Soarer. (and probably the Supra)

[Guest Ash]

 

 

What's a Musk Deer?

 

Yours,

J

[Guest Mycroft]

Nice!

 

Just noticed 5 'blobs' and Supra star, can I ask a favour? (yes) (thanks) Can I be a Soarerstar and stay that way, please...pretty please...go on.

 

(Edited by Mycroft at 6:59 pm on Mar. 16, 2002)

 

Thanks!

 

 

(Edited by Mycroft at 9:10 pm on Mar. 16, 2002)

[eyefi]

ok, move on.

[Doughie]

If I didn't know any better, i'd swear people were talking to themselves.

[Gordon F]

Quote: from Mycroft on 2:46 pm on Mar. 16, 2002[br]

The Susp. under severe braking is designed to minimise 'squat' (slapping its nose on the tarmac) this puts a lot of pressures on the wishbone this is imparted to the bushes.

 

This is actually 'dive' or pitch. Squat is when the back of the car 'sits down' under hard acceleration

 

Quote: from Mycroft on 2:46 pm on Mar. 16, 2002[br]

most cars with inappropriate Tyre/rubber combos will actually become toe-out) making the car less controllable,

 

Surely more toe out under braking will give a larger tyre footprint on the road? If a car gains toe in , combined with the camber, The car will be riding up on the inside wall of the tyre?

[Guest Mycroft]

You are partially correct, dive is the correct term for braking alone but we are also 'qualifying' this with hard cornering and that is 'roll'. I used squat with the 'qualifier' to cover all the 'whole thing' as this happens at the rear also. I am gratified though that you managed to understand what I meant.

 

Toe-in or toe-out, the contact patch remains the same. I don't understand the last sentence.

 

(Edited by Mycroft at 6:14 pm on Mar. 17, 2002)