Suspension & set up help

[Ilikeearlgrey]

Guys, I need some help and seem to be getting conflicting advice and it's confusing the hell out of me. I have 2 way shock absorbers, Bump and rebound and need to know what each adjustment does to the handling in layman's terms and what adjustments to make in what circumstances

Rebound - what exactly is this, is this the strength in which the shock goes upwards from hitting the bump, ie, the more rebound the stiffer the shock. What does changing this affect

Bump - What exactly is this, is this how fast it comes back down and how does changing this affect handling.

Also, on longish progressive fast corners the car sort of bounces across rather than turning progressively, it doesn't feel as if the weight distribution is bad, but it's clear from the onboard camera that it's not a smooth transition, what needs adjusting.

And, I have understeer going in and a tad of oversteer coming out, i suspect it's me too early on the power but am I right in thinking that I should go stiffer on the rear springs?

One more question as well regarding tyre pressures, I still can't get to the bottom of this. If it's cold or wet I have always increased pressures and reduced them when it's hot, but i've been advised that I should be doing it the other way around, any views?
The car is a 1100 kg saloon

[graham bahr]

bump damping is the damping on the compression stroke of th shock, rebound is the otherway as the shock extends.

tyre pressures, most run lower pressure in the wet, but that wont work for every different tyre/car/driver combination.

you dont say what your car is fwd/rwd/4wd, but stiffening the rear springs will make it more oversteery.

re your "bouncing" difficult to say without much more info, but common causes would be, too lower tyre pressure, tyres too wide for the wheels, so you have excessive sidewall flex, insufficent rebound damping

[Ilikeearlgrey]

Car is RWD and running slicks, so, Rebound is the shock coming back down and increasing this makes it stiffer and thus bounces back quicker, and Bump is the speed of the shock going up ?

The rebound is already set quite stiff and it still bounces, part of the reason why i was thinking of increase rear spring rates and to reduce some of the understeer at the same time, not too bothered with the oversteer as it's mild

[ubrben]

Originally Posted by Ilikeearlgrey

Car is RWD and running slicks, so, Rebound is the shock coming back down and increasing this makes it stiffer and thus bounces back quicker, and Bump is the speed of the shock going up ?

The rebound is already set quite stiff and it still bounces, part of the reason why i was thinking of increase rear spring rates and to reduce some of the understeer at the same time, not too bothered with the oversteer as it's mild

Bump is compression of the damper and rebound is extension of the damper. Dampers develop force as a function of velocity and so the bump and rebound velocities are the important thing.

As for the effect on handling, any damper force will stiffen the particular end of the car in roll and therefore shift the load transfer distribution towards that end. Bump and rebound damping both contribute to this.

Do you know what your bump and rebound ratio is in the damper and roughly what the ratio of sprung to unsprung mass is?

The best laymans advice on tuning is the Koni notes given in Race Cae Vehicle Dynamics and Carrol Smith's Tune To Win. You should be able to find them on the net.

A more scientific approach involves the use of damper velocity histograms and is presented by Claude Rouelle in his Motec Seminars.

Ben

[graham bahr]

if the car understeers going into the corner i would soften the front, stiffening the rear will make your mild exit overteer much more dramatic

[graham bahr]

Originally Posted by Ilikeearlgrey

Car is RWD and running slicks, so, Rebound is the shock coming back down and increasing this makes it stiffer and thus bounces back quicker, and Bump is the speed of the shock going up ?

its the other way round!

what is the car?

[greenamex2]

Bump is the easiest one to explain. It is a bit like increasing/decreasing your springs rates.

So increasing bump damping will be like increasing the spring rate. The stiffer you go the more the car will tend to slide more on that corner. Decreasing it tends to give more grip on that corner. HOWEVER, if stiffening the bump damping is stopping something horrible happening with the suspension (such as bottoming out, poor camber control, bump steer, hopelessly mismatched springs etc) or your sliding is caused by cold tyres then it will produce LESS sliding on that corner.

Rebound damping is far more subtle in it's impact on the cars handling. It has more to do with how a lesser loaded part of a car in a corner affects the rest of the car and is much more of a driver preference. Generally less rebound will make the car softer and easier to drive but won't have nearly such an effect on lap times as bump.

If the car bounces round corners then the problem is usually insufficient damping for the spring rate (or just a very bumpy surface!). HOWEVER, the first thing to check is whether the spring is working at all. Is there any possibility of the 'bouncy' end going solid? Remember you need to whether the bump stop is being hit, the spring is going coil bound, the damper is going solid internally or any suspension component is hitting the chassis. I have had ALL of those at one time or another.

On the subject of wet setup. Road tyres would NORMALLY be lowered from their dry cold setting, race wets would either be kept the same or increased a little. Some people advocate raising the temperature to 'open up' the grooves in the tyre, these people usually aren't the poor sod who has got to get round a slippery wet corum curve on one season old dried out road tyres!

If you have proper gas mono tube dampers (ie not the cheaper twin tube dampers with some gas in a crisp packet type) then you should also check whether they have enough gas pressure in them to work properly. Not having it can cause some really odd things to happen!

As for which end to stiffen/soften. What are your tyre temperatures like? If the answer is "they feel warm when I get out of the car" then you need to beg/steal/borrow a tyre temperature probe and start using it, especially with slicks! I guarantee that the probe will improve your lap times more per pound spent than ANY other purchase you could possibly make.

[Goran Malmberg]

For those guys at the forum that knows me, you might mail me in privat,
and I should send you an article about shock absorbers I wrote for a club magazine last spring. About their function and settings etc. In exchange you may give me some opinion about the article.It should tell a lot basic stuff I guess.
Goran Malmberg

[johnny yuma]

If the rear shocks are too stiff,particularly on live rear axle,even more if leaf springs,you can end up with almost no "suspension" at all and skip and dance on mild bumps.Dry track,increasing tyre pressures up to around 40 psi usually improves grip on the increased end of the vehicle .

[Lukin]

With regard to tyre pressures I would increase them in the wet for the same reason advocated before. Get more water out or meet the wall.

With regard to entry US and exit OS. If your driving a front engined RWD that's what way the cookie crumbles.

However, where abouts in the entry phase is it? Is it in your first initial steering input? If so bars won't have a great effect because there is very little lateral load transform. In the first few metres of the corner, camber, castor and toe have a much greater effect. In general, entry US is different to steady state US.

One of the best things anyone told me about driving technique (Hail Fat Boy) and it's effect on setup is to make sure the thing is stuck on entry first and foremost, then work on exit.

One of the big things that happens is people have an unstable car on corner entry (brakes might get rearish, or damper settings may unload the car evenly) so they slow their entrace a little. To make up for it, they get on the throttle earlier trying to gain exit speed, and the car just pushes wide. So to fix that they stiffen the rear bar (or something similar) and what does it do? After you load the car up a little it gets more taily on entry. So you slow down more at the mid corner and get on the throttle earlier and push even more.

Before you know it, you've lost 5% of your mid corner speed and have the car setup so it will murder your tyres in the race.

I can't really give you must advice on whether or not to change springs and/or bars. But trying them at a test day wouldn't be at all harmful. Either would playing with your dampers.

With dampers (the way I see it) they are used for controlling the attitude of the body and the tranmission of road inputs. The body motions (roll, pitch, warp and heave) are low frequency (2 Hz-7/8 Hz) while bumps can be anywhere from 10-40 Hz.

To increase the speed at which load is transferred during lateral and longitudinal acceleration, you stiffen the dampers. It gives the driver the feeling of increased response and sometimes better feedback. Only problem with that is it is killer over bumps. If you look at the RMS load (excellent expression I got from Ben, makes me look twice as smart as I actually am) you will see stiff dampers vary the vertical load (and hence tractive loads) at the contact patch a lot more than if they were set softer.

EDIT: Can drivers feel inputs at 25 Hz and above? What's everyone's opinions? I would think they can be 'aliased' (no not attacked by Jennifer Garner) and wouldnt pick up a difference between 60 Hz and 30 Hz.

Thats the problems with dampers. Your effectively a wolf chasing two rabbits in a field.

Advice on that part.... Play with it again. See how you go. You got any logging equipment?

[greenamex2]

There are very few circuits, even in the UK, where bumps are a real big problem. And most of these can be driven round. So, no I don't believe you should worry too much about the effect of bumps on damping, even if you can 'feel it'. The only exception is if you are riding the kerbs a lot, in which case will probably now get black flagged anyway.

What corners (both circuit and on the car) are you experiencing the bouncing on?

[Ilikeearlgrey]

The corner I was having problems with was Woodcote at S'Stone, the car is very well balanced generally and is a Front engined RWD saloon and although it was ok, this buncing has got me thinking that it could be alot better, hence this post

I've upgraded from single adjustable to double adjustable and it's added to the confusion!

[ubrben]

To the OP: do you have a setup sheet? What is you mass distribution and spring and bar setting?

If you don't have a decent setup sheet that describes what the car is chances are you're Lukin's wolf chasing two rabbits but wearing a blindfold as well.

As for the bouncing I can think of two things off the top of my head that aren't even related to dampers. Firstly could it be a roll oscillation due to having a stiff ARB, as I said it would be nice to know what your percentage roll stiffness from springs and bars is.

The second is related to roll centre heights, would you say they're high? Because if you run too high roll centres you can cause the car to skip sideways during a corner. This is another thing to do with contact patch load variation but not damping. Many people would call it jacking but the explanations in all the books I've read miss the point.

Ben

[greenamex2]

Woodcote's is certainly not a bumpy corner so you can rule that out. How much power has your car got? Did it do on the single adjustables and how many races did you have the doubles on for?

[Ilikeearlgrey]

The Rear Anti roll bars were set stiff as were the fronts, might have some bearing on the Entry US, and as pointed out, i do think alof of the understeer is me getting on the power too early trying to make up for the initial understeer. The ARB's are 24mm front and 27mm rear I believe. Haven't a clue about mass settings and i'd say the CG is quite low, the rebound was set to aorund 18 rear(out of 24 settings) and Bump to around 14 (again 24 settings), the front to 14 and 12 clicks respectively if that makes sense. The car has around the 300 mark and I can't say that I noticed it on the single adjustables.

Still a bit confused here with the bump and rebound, i think i'm looking at this the wrong way. I always though that the rebound was the rapid downward movement of the shock after it has impacted and Bump being the speed the shock compresses on impact ie goes up, like a spring compresses and then rebounds. Or is Rebound the stiffness of the resistance as it travels upwards and the stiffer this is the faster it rebounds? Same thing but different way of looking at it

[greenamex2]

You have it the right way round, bump is the shock compressing, rebound is the shock expanding. Your adjustment affects the resistance to these movements.

Can you confirm what the single adjustable dampers were, and what the double adjustables now are? What I am really interested in is whether the open and closed lengths are comparable, and whether the adjustment ranges would be similar.

Without the about I would still be looking at a possible bottoming out issue. Your power to weight ratio and slicks means you would be taking Woodcote pretty much flat out AND close to maximum cornering G, that is the car should be somewhere near maximum roll.

I am a little surprised at the ARB's. Is there much difference in arm lengths and suspension arm mounting points between front and rear?

[ubrben]

Originally Posted by Denis Bassom

I am a little surprised at the ARB's. Is there much difference in arm lengths and suspension arm mounting points between front and rear?

My point would be that you can't be surprised about the ARB sizes without knowing what the material, wall thickness, bar length, lever arm length, and ever arm installation ratio are.

The only thing that matters is what the roll resistance is at the ground in Nm/deg or lb-ft/deg

If you haven't got a full set up sheet with this basic information for your car, the subtleties of damper adjustments are the least of your worries.

Ben

[Al Weyman]

The rebound is as the spring/shocker opens back up after compressing surely and the bump is as it compresses (after hitting a bump!). I would have thought to stiff on the rebound could cause a wheel to hang in the air and induce under/oversteer on the end it was too stiff.

[Lukin]

I think Ben and Denis are on the same track. For a VD F Ford (well a 94 at least) the most convenient way of describing ARB settings is in mm and often front and rear are the same. But the bars themselves are greatly different. For the type of car I wouldn't be suprised to find the FARB contributing 70% of the total roll stiffness coming from the ARB's.

Remind me and I will send around Claude's basic setup sheet when I get back from work next week. It's useless without N/deg values for your ARB's but it will let you know what your looking for to properly quantify mechanical balance.