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Suspension And How It Works


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#1
red

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This is probably a bit cheaky but as we have sombody on the forum who knows about suspension and how to set things up, or what works and what doesn't I was wondering if we could have a pinned topic for the above where you could post a question and ADI ( sorry I don't know your name) could answer them if it's possible, some of the ideas I had I've listed but then ADI may have a better idea of how to go about this.

Front ARB and adjustability, how to.
The effects castor/camber will have on turn in handling etc.
Wheel off sets how critical are these to the suspension.
How tyre profiles and sizes work with suspension and changes in suspension.

I'm interested as for me it's the best way of getting my times down other than more power through the wheels,I'll send ADI a PM and see what he says as I don't want to inundate him with thousands of questions :(

Regards Russ.......

#2
Guest_jonnie205_*

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Why don't you get yourself a book on suspension set-up? There are loads out there crammed full of useful info. Im sure someone who really knew their stuff could write pages and pages on setting up a car for certain applications. (hence all the books i spose!)

That or just get out there and try everything yourself and see what happens! Trouble is, no two laps are ever the same. There are so many variables that you can't control you just make the the best of it at the time. Unless you're setting up an F1 car that is and you have control over a few more of these variables...

#3
Adi

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Front ARB and adjustability, how to.



The front anti roll bar............the most effective way that I know of for an adjustable front roll bar is.......to mount one straight through the front chassis legs.

On some of the factory rally cars.....there was a hole drilled in the chassis leg, then a tube fitted and welded in place. The correctly sized bar was fitted through the tubes with poly bushes holding it in place.
The bar is machined with arms brought forward from the roll bar.....and then rose jointed drop links fitted to the lower wishbones. The lever arms....have different mounting holes to adjust the lever length......and this enables the different rates of the front roll bar.

If a rear roll bar is fitted in a similar fashion to the front......this allows full adjustment of roll rates front and rear.

I have designed a rear roll bar using the above method to fit on my 206. Unfortunatly there isn't the selection of torsion bars or anti roll bar sizes for the 206 that there is for the 205.

The other geometry descriptions and effects I will include a bit later 2day or 2moro.

#4
Adi

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The effects castor will have on turn in handling etc.


Castor angle effects both steering weight and feel/feedback.

Positive castor is when the top mount is leaning towards the rear of the car. So, if a line intersects the top mount and lower ball joint and continues to the ground, it will be ahead of the tyre contact patch.

The greater the positive castor angle, the heavier the steering will be at low speed, but the greater the stability of the car at higher speeds. With +ve castor, the effect of self centre steering when accelerating out of corners, is greater.

With 0 castor, the turn in will be sharper....but when existing the corner, greater steering effort will be needed. So, a tendancy for the car to run wide at mid corner and on the exit.
With a greater degree of +ve castor, the turn in won't be as sharp, but at mid corner and the exit of a corner, there is less amount of steering effort needed.

There is a modern trend to have a larger degree of castor, so when steering lock is applied, there is a greater degree of -ve camber on the outside wheel. There will also be a larger degree of +ve castor on the inside wheel......which aids front end grip.
Even some hot hatches are being produced with 5-6 degrees of + castor. This gives the benefit of the tyres running little, if any, -ve camber whilst driving in a straight line. So, meaning less drag.

#5
Adi

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The effects camber will have on turn in handling etc.


One of the functions of any suspension set up is to keep the tyre contact patch as flat as possible to the ground at all times.
Suspension set ups like double wishbone and multi link allow camber gain. So, when the chassis rolls whilst cornering, the wheel/tyre will gain camber to ensure the tyre contact patch is kept flat.

With the McPherson strut set up, camber gain is minimal. So, either castor has to be increased...or static -ve camber has to be increased.

The amount of static -ve camber needed is really a case of trial and error. To help achieve the correct setting....ideally a tyre pyrometer is needed. After driving the car around a typical set of corners......test the tyre temperature across the tread and see where the hotest and coolest parts are. Ideally the tyre temperature wants to be even across the whole tyre. If it is warmer on the outside, then more -ve camber is needed. If it is warmer on the inside of the tyre, -ve camber wants decreasing slightly.

There are only really a couple of down sides with using any amount of -ve camber.

1. There will be slightly more steering force needed at low speeds compared to if the wheels were set up with positive camber.

2. The car/steering will be less stable at speed with static -ve camber dialled in. This is because the tyres will tend to follow any ruts and bumps etc in the road surface. As the wheels/tyre are leaning inwards at the top, they will be wanting to pull towards each other all the time. A way of improving this is to ensure the toe of the front wheels/tyres are set up, toeing out.

The amount of camber needed for a wet set up would be less than for the dry. This is obviously as the cornering forces are not as high in the wet. Also too much -ve camber in the wet, can make the steering slightly less stable as the effect ot the tyre following any bumps etc....will be magnified.

The amount of camber needed for a loose surface set up, is minimal compared to that for an asphalt set up.

#6
TT205

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Since speedlines are a 6 inch rim and (as I understand it) are deigned for a 185 55 or a 175 tyre, yet most people both for road or track use a 195 50 - should we be using 6.5 or 7 inch rims to maintain the footprint and reduce roll off on the tyre wall?

Or maybe we should be using 185's ?

Cheers

Dave

Edited by TT205, 23 September 2003 - 10:29 AM.


#7
Adi

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Wheel off sets how critical are these to the suspension


Changing wheel offsets will change the Scrub radius. The scrub radius is the distance between where the tyre centre line (vertical...from front view of car) meets the ground.......and the king pin inclination or steering axis inclination line meets the ground.

If the KPI line hits the ground inside the tyre centre line, that is negative scrub radius.
If the KPI hits the ground outside the tyre centre line, this is positive scrub radius.

Negative scrub radius is usually found on FWD cars......and the amount effects the steering weight. The more negative scrub radius there is.....the heavier the steering. This would be the same if wheels with no offset was fitted.

But if there is too much negative scrub radius on FWD cars.....torque steer and bump steer become apparent. This again would happen if wheels with less offset than standard are fitted.

Negative scrub radius is good on FWD cars with split diagonal braking systems. If one side is to fail.....thus leaving only one front brake and the opposite rear brake then under braking, the steering is stable.
If positive scrub radius was used in the above situation......the steering would pull sharply in the direction of the braking front wheel.

#8
Adi

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should we be using 6.5 or 7 inch rims to maintain the footprint and reduce roll off on the tyre wall


For a 6J rim......to keep the sidewalls a bit tighter.....so quicker responding, a 175 tyre would be an advantage......though 185 is OK but will be SLIGHTLY less responsive.
On the 106Rallye S2 a 6J rim was used and Peugeot fitted a 175/60 tyre.

For a 195 tyre a 6.5J used to be the best for steering response. And depending on the width of the particular 195 tyre it may still be. But all tyres seem to be getting wider than their marked size would suggest.

Again a 7J rim for a 205 tyre.

Remember a tyre with better spread sidewalls will have better steering responce. But will also break away quicker and sharper on the limit of adhesion.

This is why most vehicle manafacturers will use slightly narrower rims for a given tyre size......than what is actually ideal...depending on what they are trying to achieve.
Quite often, a 6J rim will be used for a 195 tyre and a 205 on a 6.5J rim. This would slow the steering responce....but also make the car less agressive on the limit.

Also it will be an advantage to try and find out information on a particular type of tyre as they will all have different strengths of sidewall.
The Toyo Proxes tyres have strong sidewalls where as the Goodyear Eagle F1 GSD2 had a soft sidewall......so the width of tyre/rim can be altered accordingly.

#9
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How tyre profiles and sizes work with suspension and changes in suspension


This is really an open ended topic. And one that could go on for ever.

A lot really depends on use of the car.....as to how the tyre size and profile are matched to the springing/damping.

On modern road cars.....the wheels are getting larger and the tyre profiles smaller. This is mostly for the look of the car rather than performance. With the improvement in damping technology the ride can still be compliant. However, the tyre is also used as undamped shock absorber and the less profile on the tyre, the less there is to absorb large or sharp bumps. When the tyre or suspension can't absorb the energy, the chassis has to absorb the excess. This is can often disturb the chassis and end up with the tyre losing grip.

On a track however, the bumps are small and only really felt when the spring rates are high. As a result, tyre profiles can be smaller.

A lot relating to tyre profile relates to the damping rate used as well. So to a certain extent....the 2 go hand in hand.

As long as people remember that handling and grip are 2 different things even though they go tegether. People try and fit the widest tyre physically possible and even though the extra grip will be beneficial......an unbalanced car will still be slower.....even with wider tyres. A well balanced car will be quick regardless.


If anything else needs to be added........just ask and I'll try to expand on this subject.

#10
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Thanks for those very informative replies ADI, the castor camber one is very infomative, I think my first purchase will be adjustable top mounts.

Regards Russ......

#11
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Changing wheel offsets will change the Scrub radius. The scrub radius is the distance between where the tyre centre line (vertical...from front view of car) meets the ground.......and the king pin inclination or steering axis inclination line meets the ground.

If the KPI line hits the ground inside the tyre centre line, that is negative scrub radius.
If the KPI hits the ground outside the tyre centre line, this is positive scrub radius.

Negative scrub radius is usually found on FWD cars......and the amount effects the steering weight. The more negative scrub radius there is.....the heavier the steering. This would be the same if wheels with no offset was fitted.

But if there is too much negative scrub radius on FWD cars.....torque steer and bump steer become apparent. This again would happen if wheels with less offset than standard are fitted.

Negative scrub radius is good on FWD cars with split diagonal braking systems. If one side is to fail.....thus leaving only one front brake and the opposite rear brake then under braking, the steering is stable.
If positive scrub radius was used in the above situation......the steering would pull sharply in the direction of the braking front wheel.


Just a quick question , but would wheel spacers change things with the offset or do they just give you a wider track, or am I getting hold of the wrong end of the stick ?

Regards Russ......

#12
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Where can you buy those adjustable top mounts?

I think the top mount on my car will have to be replaced anyway, so maybe I should go for adjustable ones.

#13
Adi

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but would wheel spacers change things with the offset


Yes spacers change the offset. But also widen the track. On the rear suspension where there is no real geometry to speak of......spacers don't really matter. In fact they have the effect of rasing the spring rate.....so can be useful.
But on the front, it can be a bad thing as it can cause torque steer and tugging of the steering over bumps etc.

#14
TT205

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I got mine from Transautosport to run with Leda's on small diameter springs

See
Posted Image

Sorry, no pic of the eccentric top mounts yet

~ 115 + VAT + Delivery

Speak to Martin

Cheers
Dave

Edited by TT205, 23 September 2003 - 08:05 PM.


#15
Adi

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I think my first purchase will be adjustable top mounts.


If we are talking about the eccentric top mounts.....they will only work with smaller diameter springs. Usually with coilovers using 2 1/2 diameter springs. But I have heard of an eccentric top mount being used with a 100mm diameter spring. So you will have to check it out.

The standard Peugeot diameter springs are 6".

#16
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If we are talking about the eccentric top mounts.....they will only work with smaller diameter springs. Usually with coilovers using 2 1/2 diameter springs. But I have heard of an eccentric top mount being used with a 100mm diameter spring. So you will have to check it out.

The standard Peugeot diameter springs are 6".


Thanks for that saved me 150-00 already :huh: , as I bet they would have sold them to me without that bit of info, the ones Dave has fitted are top quality so I'll stick with Peugeot items, I'll just have to admire yours Dave and dream on hey :) Dave thats the picture I took isn't that copyrite infringment :)

Regards Russ.....

#17
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There are a couple of different eccentric top mounts for the 205 GTI, if any of them fit the std dia spring/strut I don't know. Would be useful, as I only want to dial in a touch of extra camber.

Adi, I think it would be useful to go through the different aspects of spring and damper rates, eg compression and rebound. How over stiff handles differently to over soft, and front rear balance.

Also ride height and side effects of lower (less travel).

Finally how adjusting tyre pressure can make a huge different, infact using s*ite tyres on good suspension can be such a waste of good suspension.


There will also be a larger degree of +ve castor on the inside wheel......which aids front end grip.


Ackerman linkage, where the inside of the car follows a tighter radius than the outside, so the inside wheel turns tighter. This is easily achived by not have the steering linkage arms from the hubs running parralel backwards, but pointing them to the centre link of the rear axle.

#18
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This is easily achived by not have the steering linkage arms from the hubs running parralel backwards, but pointing them to the centre link of the rear axle.


That will change the horizontal path....but not the vertical one.

When the car rolls......the outside wheel angle is changed by camber/castor. But this also effects the angle of the inside wheel. Quite often you will see the inside wheel of a normal road car being in negative camber....whilst outside wheel is in +ve camber. This is obviously the wrong way round.

By increasing the castor angle......not only does the outside wheel gain -ve camber, but the inside wheel gains +ve camber. This so the inside wheel is not running on the inside of the tyre...thus providing more grip.

#19
TT205

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Top mounts picture - finally

Posted Image

Dave

#20
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ADI with respect to strut braces, I currently don't run one and was wondering if they are of use on a car with standard bushes etc and mostly run on tarmac circuits, my thought was they only helped rally cars when they left the ground over humps etc to help with the impact on landing and holding the tops in place, also what about a lower strut brace and it's mounting position is this of use as I've been told they are only required if you run slick tyres?

Regards Russ........