Anti-squat

[AMMO]

So that's what happens when you mention W*******d. Very clever. What about Locost. Robin Hood. Tiger. Dutton. Dax.

 

AMMO

[Blatman]

Yes W e s t f i e l d do have a double unequal length wishbone, fully independent arrangement, with rose joints and all sorts of adjustability. Mind you, it is not designed with anti squat, but then nobody's perfect. If W e s t f i e l d can do it, why not C******mquestion.gifblush.gif

[Mike Bees]

The question you have to ask is *why* do it.

 

The W*******d IRS gives horrible changes in camber as the body rolls (from some pictures that I've seen), the DD tube keeps the camber constant.

 

Mike

[AMMO]

Mike, thanks again for the De Dion tube / link arm explanation. That is now clear in my mind. Maybe you can tell me another thing. Do I require any negative camber at the rear and if so how much? Remember only occasional track use, the car will not be ultra low. Thanks.

 

Blatman, you don't own a W*******d by any chance?

 

AMMO

[Blatman]

With the rose joints in the rear W set-up, I'm sure such anomalies could be adjusted out........ As to why, how about better ride over bumps on the road, simpler adjustability for camber and toe-in/out, (ie no ear changing just to alter the camber) less weight, higher ultimate potential for handling tweaks, and it would seem that that at least two book authors I have read recently dismiss De dion as VERY outdated, and unworthy of consideration for serious applicationblush.gif I don't want to get into a debate about tradition or originality or any other "benefits" of C******m's current layout, just the fact that, as far as I am aware, given the choice, car designers would use a fully independent set up over a live axle or De dion every time.

Jusy my humble opinion and all thatwink.gif

[Peter Carmichael]

I was under the impression that Colin Chapman thought adjusting castor at the front was beyond the wit of most clubman drivers. Having myriad adjustments at the back as well puts it firmly in the domain of experts only.

 

I won't apply, thank you very much.

[Blatman]

Peter, you surprise me. I use the very methodical approach you have advocated in the past in order that I can simplify and understand the myriad adjustments possible, and I refer to a couple of books, or my (Lotus developement engineer) brother when I get lost. I don't believe for one second that this sort of set-up is beyond you, and I was hoping for a really good put downwink.gif or a really good counter argument at the very least.

 

[Peter Carmichael]

Imagine the Seven chassis stopping at the back bulkhead of the cockpit. The pickups for the rear suspension are all in front of this.

 

The rear chassis members only serve to mount the fuel tank.

 

An IRS would require the stiffness and strength of the chassis to be extended quite a lot further backwards. You lose some of the advantage of the double wishbones to the extra weight, although I concede that the de Dion tube is not the lightest thing in the world and is unsprung.

 

I am a big fan of the ready adjustability of the rear steer of the de Dion configuration. It is a quick and simple change which gives real handling results. You can change it in seconds with just a jack to take the loads off the springs. No measurements required.

 

I really do like it as it is.

[Graham Perry]

Peter is correct in terms of the weight . I seriously sat down and costed out a conversion to independand rear suspension a couple of years ago . The system would have been based on my own design,(I have built a couple of grass track buggies in the past from scratch) but what put me off was that unless I used very short wishbones the system came out at about the same weight, unless exotic (Read expensive ) materials were used also against this was that I didn't feel comfortable modifying the chassis and moving the loadpoints about, because the temptation is to put too much metal in the wrong places and then not enough where its needed most. Might try it in the future though.

[Mike Bees]

Ammo - rear camber requirement depends greatly on the tyres you use. Crossplies (ACB10) need only a very small amount of negative camber (0.25 is typical AFAIK). Radials need a lot more. The standard DD ears give 1.5degrees I think, mine are shimmed out to somewhere between 2 and 2.5 degrees.

 

Blatman - I was (perhaps wrongly) under the impression that the W*******d IRS was Sierra based. You've only got to look at the camber changes on a Sierra or a Granada when it's loaded up to see how poor it is. Surely this is a geometry problem not a bushing problem? Apologues if this isn't what W*******d use, although I've seen an IRS Westie exhibiting huge dollops of extra -ve under cornering roll.

 

Mike

[Blatman]

Peter, Graham. Thanks guy's for furhter enlightening me. Knowledge is power..................

 

Mike, W e s t f i e l d s use Sierra diffs. Older ones use Escort Mk1/2 diffs in a W e s t f i e l d carrier (which looks very similar to a Lotus Elan/7 and a half item), but all IRS variants use W designed unequal length wishbones and cast ally uprights. I stand by my contention that the handling/roll camber anomalies you describe can be sorted with careful attention to set up.

 

I'm just being overly defensive I thinkblush.gif

[AMMO]

Thanks Nigel for your experience of moving the link arms. Mike, thanks for the camber info. I was thinking of getting some Yoko 185/60/13 A021's and starting with 1.5 degrees all round as a starting point. What do you use to shim the back? Peter, the back end of the chassis as a support for the tank is an iteresting way of looking at it. Going the IRS route is probably a solution to a problem that doesn't exist. I don't understand the adjustable rear wheel steer bit. What do you adjust? Still looking into anti-squat and found an interesting quote from Carroll Smith: "Rear-wheel drive cars, like primates, need to squat in order to go". Elsewhere he does say that anti-squat is neccessary in softly sprung cars. The more power you have the more anti-squat you need.

 

AMMO

[Peter Carmichael]

Squat is just a suspension displacement.

 

Front goes up. Back goes down. Energy is stored (springs). Energy is dissipated (dampers).

 

What you want is for the transition to happen quite quickly, but for any shocks in the power delivery to be soaked up. This requires squat to some degree, otherwise you might lose traction because of driveline shunt and never get it back.

 

The more powerful the engine, the smaller the shocks are in relation to the overall power delivery.

 

(or that is my interpretation of Carroll Smith)

[Peter Carmichael]

Roll steer of the de dion axle is dependent on the ride height.

 

The higher the ride height the more it tends to give you oversteer. (ohgodnotthatagainlookiwrotetendsokletsleaveitatthat)

[Mike Bees]

AMMO - I just use a thin strip of ali sandwiched between the bottom of the ear (to add more neg.) and the DD tube. It doesn't need much!

 

Mike

[AMMO]

Mike, that's what I like. Nice easy camber mods. Once you get it right, you just leave it alone. So, Peter, when you jack up the back you are also changing the castor and trail at the front and the amount it squats at the back, right? Possibly the amount of rear anti roll, definately the spring pre-load (on bikes this is a critical tuning paramater, we have dampers which are adjustable for length to change ride height) and probably another half a dozen things I haven't even thought about. My point is that it would be wonderful if you could make one change that affected only one thing but one adjustment probably changes more things I can possibly think of. The discussion has made me realise that this is a very complex subject. The object for me is to learn. If it was easy it would be no fun. Thank you for your views.

 

AMMO

 

Edited by - AMMO on 23 Jan 2001 22:37:44

 

Edited by - AMMO on 24 Jan 2001 08:18:45

[Graham Perry]

Ammo, What Mike has done is pretty much a standard 'tweak' and its much cheaper than machining the ears to give more camber and easily reversable if you are not happy. I did mine just with a thin steel washer in the rearmost lower position. This, as well as giving a bit more negative also gives a small amount of toe in, which I prefer.

[Peter Carmichael]

Interesting point about independent adjustments, which is making me think.

 

The trail and castor issues may not be relevant when you are talking about ~3mm ride height change at the rear. This is significant to the rear steer - ask a sceptic, Keith Pickering - but the castor change is 3mm divided by the wheelbase (in radians, small angle arcsin approximation). The rear axle behaviour changes by a much larger amount because the angle is subtended over the much shorter length of the radius arms.

 

I would like to believe that the roll stiffness doesn't change because the spring load at equilibrium is unchanged. The length of a car's wheelbase means that squat and dive are less than with the short wheelbase of a bike and the modern Caterham rear end has a lot of sag travel so I doubt that preload comes into it as much as with a bike.

 

Caterham's efforts in developing the Watt's linkage were to make the car less load sensitive in its handling. The Black Bricks had massively longer radius arms (all four of them) - have a look at December's Low Flying.

 

I am trying to justify my assumption that the rear ride height is a largely independent change, but I don't have hard facts.

 

Edited by - Peter Carmichael on 24 Jan 2001 12:28:32

[AMMO]

Graham, thanks for the info on rear toe in. Even more complication! Peter, I didn't realise we were talking only 3mm ride height adjustment. I didn't consider the wheel base either which obviously make a big difference.

 

AMMO