Suspension and Torsion Bars - Peugeots/Citroen/Renault - How it works

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Suspension and Torsion Bars - Peugeots/Citroen/Renault - How it works

Post  decoc182 on Fri Feb 12, 2010 11:03 am

Castor and Static Camber

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.


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.

Wheel Offsets

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 becomes 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.

Tyre Profiles

For a 6J rim in order 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 106 Rallye steely for example, 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.

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 two 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.

Strut Brace

Strut braces are useful for keeping the front suspension geometry stable. When the front suspension is under load whilst cornering the inner wings can flex allowing the tyre angle to change. This will usually mean the outside tyre tucking under more,thus losing grip. A brace is there to increase the torsional rigidity of the shell and as such, allows the suspension to perform better.

But as always the benefits depend on the rest of the suspension set up and more importantly how strong the shell is. The stronger the shell the less twist there will be. A brace will be less needed or benefitial in this situation.

Spring Ratings and Damper Ratings
Too soft a spring rate will provide a good ride quality. But, the suspension takes too long to absorb the energy, thus the tyre isn't in full contact with the ground for as long as it could be. So not providing the maximum grip.
Also when cornering, especially with a McPherson strut set up, the tyre rolls to a greater angle and doesn't stay square to the ground when under full load. Again, not providing the full grip.

If the spring rate is too high, then when the tyre hits a bump, the spring will not absorb enough of the energy. That is passed onto the chassis. The chassis is sent in the upward direction thus the tyre is not in full contact with the ground. Again not providing full grip. If this happens whilst cornering, it could spell trouble.

Damper rates work hand in hand with spring rates and can give similar effects if the rates are wrong.

In general, the compression side of damping will influence the cars stability and response. The rebound side should influence comfort and traction.
But, for example if the compression side of damping is too high, then the ride will be harsh and crashy. This is why, setting up and diagnosing problems can be very difficult.

Dampers can provide a very useful tool for tuning handling balance especially on aspects of cornering such as entry and exit. This is because the diagonally opposed dampers are usually working in the opposite direction. Working out exactly which direction each damper is moving, at the different stages whilst cornering, you can learn to change the damping accordingly to alter the handling balance.

Anyone looking to improve handling in general, should spend as much as possible on dampers. Modern dampers have improved so much since Active Suspension was outlawed in F1 in the early '90s.
Racing dampers using Digressive technology have so much more control over suspension movements than either progressive or linear dampers.

Can I just point out that the different types of damping effect the shaft speed in damping, rather than the position of the shaft within the stroke. I have heard people mention that progressive dampers have a higher rate of damping the shorter the shaft becomes. This is not true it is the speed of the shaft movements that effect the rate of damping.

This is the reason that Digressive dampers are so effective. These dampers can control slow shaft speed movements such as cornering where other types of dampers would have little influence. Then when the shaft speed increases on bumps etc digressive dampers are more comfortable where the other dampers are harsh and crashy.

As far as balance is concerned,in general the end of the car that has the most roll stiffness (provided by springs and roll bars) will have the most lateral movement and will lose grip first.

This is why on cars like Peugeots (that have struts and coils at the front and torsion bars a the rear) it is OK to raise the front spring rate to control roll etc, but if the rear rate isn't increased accordingley then the car's balance will be very much towards the front,and understeer.

Another "general rule" is that the opposite end of the car to the drive wheels will have the higher roll resistance. This is so the drive wheels are kept flat and the suspension is more complaint to enable the tyre to follow any undulations.

Ride Height

Ride height and centre of gravity height, seem to be the main points that rule general suspension tuning.

Whilst these can be important, suspension travel is also very important, and in certain cases, more important than ride height.
If the chassis is lowered a large amount for general road use, then this is going to leave potentially dangerous problems. When there is a minimal amount of suspension travel, and the wheel/tyre comes into contact with bumps if there is too much energy for the short spring (or high rate spring) to absorb, the excess will be passed into the chassis. This will send the chassis in the upward direction and reduce the tyre's grip with the road surface. Again,if this happens whilst cornering at speed then a trip to the nearest hedge or wall, will be the usual outcome.
Another major bad point with excessive lowering is, when the spring hasn't enough length to absorb the energy from larger bumps the damper will constantly be slammed shut at the end of its stroke. This will lead to the internal seals blowing and the oil leaking from inside. It will only be the more expensive dampers that can be rebuilt in this situation. The dampers that can't be rebuilt, will have to be thrown away.

One point that seems to escape most people when altering suspension height, is BUMP STEER and the angle of the lower wishbone.

If the steering arm and lower wishbone aren't working together on similar planes, then when the suspension goes in the bump direction, the wheel will change it's intended path. This means the driver will constantly have to make steering alterations to keep the car travelling in the same direction.
To avoid bump steer the lower wishbone should generally be kept inclined downwards to the wheel. That is, the inside mounting point be higher than the outer balljoint.
Bumpsteer and/or the lower wishbone inclination is less of a consideration on smooth surfaces like a track.

There is however a positive reason for lowering Peugeots with beam axles, about 25mm all round.
Every suspension has a roll centre. So a "normal" car will have a front and rear roll centre. For the best ride/handling comprimise, the rear roll centre needs to be HIGHER than the front. This is so the rear weight transfer will be quicker than that of the front so aiding corner entry and exit. But at the same time having softer rear spring rates to aid comfort.

With the rear beam axle on the Peugeots,the rear roll centre is LOWER than the front. In fact the rear roll centre is at ground level at all times. This means that if softer springs are used for a smoother ride then the rear end of the car will roll more than the front so producing understeer.
Now if the rear roll centre cannot be raised,the next best thing is to LOWER the front roll centre. So by lowering the car approximately 25mm, the best comprimise will be achieved again though remembering the angle of the front wishbones.

Torsion Bars

I thought id write about my experience of torsion bars in case any of you were a bit confused how they worked.

The rear axle on most french cars consists of a solid beam tube, with two independent suspension arms pressed into a bearing in the inner tube. An anti-roll bar ( thick long bar-splined at the end) sits inside the tube, and connects to either side of the arms on the endcaps.

Each arm has its own torsion bar, which connects to the swing arm, and then connects at the other side of the beam at a fixed point which doesnt move. As swing the arm moves up or down, it twists on the torsion bar at its fixed point, but will move back to its original position once the load is removed.

A common confusion with torsion bar lowering is the myth that the back end is stiffened in relation to the front, this is NOT the case. Think of a torsion bar as a coilover spring. When you lower the spring platform on your coilover spring, you are not stiffening your suspension at all, as your spring rating is unchanged, instead you are moving the platform up or down, shortening the suspension travel or lengthening it.

The torsion bar works in a similar way to this, when lowering the rear end on a torsion bar beam, you remove torsion bar from its fixed point, and rotate it clockwise or anticlockwise (lower or raise) to adjust the spring platform. The torsion bar locates on splines, so you simply remove the bar out of the fixed side on the axle, rotate it one spline..or two..and re-locate it back into its slot on the fixed side. This has lowered the spring platform. A more acurate way is to use a dummy shock, or measure from the hub to the floor, and reduce this measurement 10mm at a time until perfect ride height is achieved.

It is reccomended, that if you run a greater weight spring at the front, that you should match the rear end to the same poundage, which involves buying thicker torsion bars, or finding ones off a sportier model to fit. A common mistake made with french hatches, people fit lowering springs/coilovers to the front, and often leave the back untouched. This can lead to really bad understeer issues. Running to much on the back can lead to oversteer issues.

Tyre Pressure

Tyre pressures can be a useful tool in changing handling balance.

Generally lower tyre pressure will cause a greater slip angle. That is, the tyre will change direction slower and run wider whilst cornering. So as Skip Brown regularly set cars up the front tyres would have 28psi and the rears 24psi. This may not be a favourable set up for all but can be used to fine tune and tweak the balance of the car.

Tyres are the probably the largest single part of a car that can change how the car feels and handles corners.
There is very little or no use in wanting to improve a cars performance and not spending a decent amount of time or money finding a good quality tyre.

A tyre will not only provide grip for a car but also steering feel and response. By fitting a good quality tyre the car will not only be safer in more testing conditions, but generally perform/handle better in all conditions.

Even though the general quality of all tyres has improved in recent years, there is still a large gap even between the premium brands.
So it would be a waste of anyones time and money if money should be spent on improving suspension and the same time and money isn't spent on fitting good tyres.

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Re: Suspension and Torsion Bars - Peugeots/Citroen/Renault - How it works

Post  Gilmour on Fri Feb 12, 2010 9:49 pm

some quality information right there mr cochran cheers
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Re: Suspension and Torsion Bars - Peugeots/Citroen/Renault - How it works

Post  wideboi on Sat Feb 13, 2010 7:59 am

im loving this thread rich Smile

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Re: Suspension and Torsion Bars - Peugeots/Citroen/Renault - How it works

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