Suspension And How It Works

[red 0]

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.......

[Guest jonnie205]

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...

[Adi 0]

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.

[Adi 0]

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.

[Adi 0]

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.

[TT205 42]

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 September 23, 2003 by TT205

[Adi 0]

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.

[Adi 0]

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.

[Adi 0]

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.

[red 0]

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......

[red 0]

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......

[Guest B1ack_Mi16]

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.

[Adi 0]

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.

[TT205 42]

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

 

See

 

Sorry, no pic of the eccentric top mounts yet

 

~ £115 + VAT + Delivery

 

Speak to Martin

 

Cheers

Dave

Edited September 23, 2003 by TT205

[Adi 0]

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".

[red 0]

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 , 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.....

[NickR 10]

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.

[Adi 0]

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.

[TT205 42]

Top mounts picture - finally

 

 

Dave

[red 0]

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........

[Adi 0]

strut braces, I currently don't run one and was wondering if they are of use on a car with standard bushes etc

 

 

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.

 

The best thing to do, is try a brace, or both braces and see how the car feels.

[Adi 0]

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.

 

 

There are many ins and outs on this subject.....so I will try and keep it brief.

 

 

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.

[Adi 0]

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

 

 

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.

[NickR 10]

I have made this into an article on my website:

 

http://www.lbft.co.uk/articles.asp?id=2

[Adi 0]

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

 

 

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.