Semi-Trailing Rear Suspension
Turbo Mini Forums

You know how thinking about one modification leads to another and then another and before you know it you forget what you were trying to do in the first place :)

Well, I was thinking about improving traction and gearing by going to bigger diameter tyres. There are some very interesting looking Toyo Proxy R888 tyres in a 185/60/13 size. I thought that will do the business.

However, apart from looking a bit odd and raising the car approx. 25mm, they would most likley foul the radius arm on the rear suspension. They are 550mm diameter and the distance from the wheel centreline to the radius arm is 270mm.

So, how about a semi-trailing arm rear suspension setup on a custom subframe assembly? That way the arm could be moved away from the tyre.

Anyone ever tried it and had some success? I'm sure that I looked at a Mini somewhere this year that had something similar.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

I'm not sure if I 100% understand the definition of semi-trailing arm. Is that like the 'Dons' setup?

basically instead of the front mounting point of the arm being horizontal along the car (ie straight along the heelboard) they are angled slightly.

that would move the outside pivot point of the arm inwards and away from the wheel.

i'll try an find something on good with pics.

poifect and quick as lightening!!! woooooosh

http://www.autozine.org/technical_school/s...uspension21.htm

Edited by James_H on 29th Sep, 2008.

The Mini rear radius arm is a true trailing arm, I believe.

A semi-trailing arm is where the centre lines of the radius arms are angled so the the arm moves in toward the centre of the car.

It must have been the DON that I was looking at :$

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

http://turbominis.co.uk/forums/index.php?p=vt&tid=236601

been looking for that for ageeees!!
The DON isnt semi-trailing but could have been that topic that you were looking at.

The DON has trailing arms. In effect the geometry is similar to the standard setup, apart from having a longer swinging length, and weighing far far less. The springing is different as the coilover is angled in from the vertical, however a simple bit of maths means a direct comparision can be made spring rate wise if required.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

I was running 185/60/13 way back in 1994 and had no problems. Now ive got 195/40/16 and I have no issues with the radius arms but you have to be carefull with the back of the arch.

I dont see there being a problem.

Edited by Mr Joshua on 30th Sep, 2008.

Own the day

I'm pretty sure the Don's ABC10s are 185/50/13 as that's what I've got for Avon.

The BMW E-30 series use semi trailing rear arms. The benifit of this is that as the outside rear corner compressess in a bend that rear wheel toes in and steers the back of the car. It also alters the camber of the wheel making it lean into the bend (dig in) under load plus it allows the wheel to remain more vertical when the body starts to roll. My mate has this on his grass track mini handles pretty good.

Own the day

WWW.FORCE-RACING.CO.UK PLEASE CLICK HERE

Schrödinger's cat - so which one am I ???

its done via a very complicated system of angles and uhmm rubber..........when you turn in the electonomolies in the rubber sense the change and then make the diagonimalites go from one end of the rubber to the next. these then send a signal via the angles (remember those that i mentioned above? all starting to make sense now ey!) down to the tyre through the wheel nuts and the rim (what else did you think they were for?) then the electonomolies move in the tyre applying extra force to the outside edge under hard cornering making the back end turn in.

unfortunately when understeering the back will still want to turn in and because this system pulls 21g's when it is operating it has been known to rip the back end clean away from the car. but that is rare and only generally happens at ridiculous race cornering speeds of around 45mph...i know it sounds fast but with this system those speeds are possible!

Clever ey!

The main thing is that it induces a camber change on cornering that keeps the tyre in better contact with the tarmac.

The slight change in toe is insignificant, particularly as the slip angle of the tyres is far greater.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Schrödinger's cat - so which one am I ???

I am not sure (but I will look when It comes back) but I believe mercedes benz use the same set up on the rear.

Also if you look at the steering geometry on most modern cars you get an exagerated state of what I described earlier as you turn the wheels they lean into the turn.

Grass track racers ( the highly modified ones) deliberetly rake back the left hand front upright to increase the caster to cause that wheel to lean in when steering is applied to reduce under steer.

If I were to make a rear beam for my car I would design in this feature. A mini is light enough and agile enough to make use of such suspension geometry changes in a corner.

The only problem is you would need to produce the arms to suite.

Own the day

I've experimented considerably with rear tow and you really don't want any tow in at all. Having a rear anti roll bar increases the effect. As you transfer grip onto the towing wheel, tow in will chuck the weight/grip back onto the other wheel and the last thing you want through a fast bend is the resulting weave.

Rear tow out (slight) isn't so bad as you sort of get a bit of a lean and the back comes around gently.

Longman used to run + rear camber to degrip the back a bit and the older race cars ran slightly narrower tyres, again, to get a more predictable breakaway. People don't seem to think about this anymore but how the back behaves under harsh cornering is pretty important.

It's one of the most important parts of setting the car up in my book. Possibly more important than the front. Getting it wrong can make the back snappy/weavy if you get it wrong. Rear tow is about 200x more significant than rear camber IMO.

I don't think there is anything brong with the existing setup on an FWD car and wouldn't think about messing with it if I won the lottery.

It depends though, are you looking to go safely at very high speed around corners or just look kool at maccy d's?

Most modern cars have McWobble struts, which in my estimation are not suited for FWD as you don't want a front ARB on a nose heavy car. Ie, you don't want to degrip the front.

Anyway, all IMO

Bugger off, I'm getting there.

With the semi-trailing arm setup, you would start of level/straight with zero toe.

The toe would change progressively as the tyre was loaded, so you would not get the kick the same as if you had set it with rear toe in.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

What about playing with a single wishbone, or arm setup?

It depends on the rate of weight/grip transfer from one side to the other, if you increase your roll stiffness, it becomes more snappy. Rather than slidey, if you know what I mean.

I run my rear wheels with the slightest tow out and it seems nice. I get a nice progressive "back comes around" without that shit yourself feeling you get with lots of tow out. Also the wanderyness.

With it dead ahead it seems a tiny bit loose (if you know what I mean) and a bit of rear tow in seems to make it feel more sure footed, as you say with it staying constant, when you unload the inner wheel, it tries to correct itself which I cant stand. You have to be so careful to stop the weave and all it takes is a bit of a lump in the road to have a brown trousers moment.

Having a progression from about 1/8" tow in to about 1/8" tow out would be nice.

I run the wheels concentric in the arches

Front -3 deg camber, enough caster, not sure how much, tow out by about 1/8". Shocks set dead hard

Rear -1.25 cam tow out 1/16" 14mm ARB shocks set about 2/3 hard.

I like the idea of this variable tow business. :)

Bugger off, I'm getting there.

Own the day

Have a look at this:



Tyre slip angles are far greater than any toe setting under hard cornering, so the effects of toe are insignificant.

Have a look at the many photos and videos from Combe at the weekend in various recent posts.

As the Mini corners, the body roll due to the side forces cause the rear tyres to move away from vertical and thereby reducing their potential grip. This is because the rear trailing arms have zero camber change due to the suspension movement.

A semi-trailing arm could go some way to improve the rear grip by keeping the tyre vertical as it gets loaded.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Ok, this something I've thought about a bit/buggered around on when I had my silly engine and 5.20 crossplies on 3.5" rims. It had a really nice break away and drifted really nicely and perhaps arguably more safe feeling at high speed than 165's. Snag is I'd get through a set of fronts in a very short time.

I was hellbent on getting understeer to a total minimum and despite putting on a fair degree of -ve camber, it still slid a bit. I decided to lower it more to stop the lean a bit and in order to stop the back tyre binding up, I fitted an antiroll bar. Just to keep the back flatter. You can argue it degrips the back or grips the front, I'd say the former. It corners flat with it's inside wheel cocked up.

I know minis do this when pushed hard, how about what it does when it's doing that. Look at tyre compound/pressure and how the contact patch changes under mega side forces. I run sticky rear tyres and still run -ve camber so it holds well. If I chuck it, I can get the arse to come around for very sharp uphill corners.

By degripping the back, whether it be with a rear ARB, narrower tyres or more +ve rear camber, you make it more neutral.

One thing I did do for a short time was run a set of 7" force rims with 032's. They keep the contact patch absolutely fixed and you get the grip with the compound. When it goes, it goes though.

I hardly have any body roll at all. Old pre 64 hard rubber cones and hilos with a flatter land to give more of an exponential spring rate.

Out of all the buggering about and I know that I am usually more theory than practise here is where I'm properly well researched (for very fast road) and the best thing you can do to a well set up mini is put a rear anti roll bar on it.

It turns it from being a bit nosey to just right.

As the transfer is a bit more sudden, you need the rear tow to be either dead ahead or for windy stuff, a tiny bit out. Never a lot out......

My mate was demonstrating how you could chuck it around corners to someone at his work (it lived there) and hit the rear wheel against a curb (private estate) and bent a rear radius arm. I couldn't adjust it out and as a temporary measure shimmed the other camber bracket out to get equal tow out. Probably what you'd call quite moderate to high tow out.

It felt very unstable at high speeds in a straight line and you had to nail it everywhere and be very careful on long corners as well as chucking it around sharper ones. It was truly horrible.....

Bugger off, I'm getting there.