Home  
Site Partners: SpotterGuides Veloce Books  
Related Sites: Your Link Here  

Go Back   TenTenths Motorsport Forum > Racing Talk > Racing Technology


Reply
 
Thread Tools Display Modes
Old 20 Nov 2005, 17:00 (Ref:1465705)   #1
Ilikeearlgrey
Rookie
 
Join Date: May 2005
Posts: 6
Ilikeearlgrey should be qualifying in the top 10 on the grid
Suspension & set up help

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
Ilikeearlgrey is offline  
Quote
Old 20 Nov 2005, 17:52 (Ref:1465745)   #2
graham bahr
Veteran
 
graham bahr's Avatar
 
Join Date: Jan 2003
England
cambs
Posts: 2,071
graham bahr should be qualifying in the top 10 on the gridgraham bahr should be qualifying in the top 10 on the grid
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
graham bahr is offline  
__________________
AKA Guru

its not speed thats dangerous, just the sudden lack of it!
Quote
Old 20 Nov 2005, 18:00 (Ref:1465752)   #3
Ilikeearlgrey
Rookie
 
Join Date: May 2005
Posts: 6
Ilikeearlgrey should be qualifying in the top 10 on the grid
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
Ilikeearlgrey is offline  
Quote
Old 20 Nov 2005, 18:18 (Ref:1465771)   #4
ubrben
Veteran
 
Join Date: Jun 2005
United Kingdom
Birmingham
Posts: 508
ubrben has a lot of promise if they can keep it on the circuit!
Quote:
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
ubrben is offline  
Quote
Old 20 Nov 2005, 18:35 (Ref:1465785)   #5
graham bahr
Veteran
 
graham bahr's Avatar
 
Join Date: Jan 2003
England
cambs
Posts: 2,071
graham bahr should be qualifying in the top 10 on the gridgraham bahr should be qualifying in the top 10 on the grid
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 is offline  
__________________
AKA Guru

its not speed thats dangerous, just the sudden lack of it!
Quote
Old 20 Nov 2005, 18:40 (Ref:1465790)   #6
graham bahr
Veteran
 
graham bahr's Avatar
 
Join Date: Jan 2003
England
cambs
Posts: 2,071
graham bahr should be qualifying in the top 10 on the gridgraham bahr should be qualifying in the top 10 on the grid
Quote:
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?
graham bahr is offline  
__________________
AKA Guru

its not speed thats dangerous, just the sudden lack of it!
Quote
Old 20 Nov 2005, 18:55 (Ref:1465798)   #7
greenamex2
Veteran
 
Join Date: Aug 2002
England
Hertfordshire
Posts: 1,686
greenamex2 should be qualifying in the top 10 on the grid
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.
greenamex2 is offline  
Quote
Old 20 Nov 2005, 20:06 (Ref:1465841)   #8
Goran Malmberg
Registered User
Racer
 
Join Date: Mar 2004
Sweden
Stockholm Sweden
Posts: 319
Goran Malmberg should be qualifying in the top 10 on the grid
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
Goran Malmberg is offline  
Quote
Old 21 Nov 2005, 00:47 (Ref:1466046)   #9
johnny yuma
Veteran
 
Join Date: Apr 2005
Posts: 626
johnny yuma should be qualifying in the top 10 on the grid
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 .
johnny yuma is offline  
Quote
Old 21 Nov 2005, 09:05 (Ref:1466233)   #10
Lukin
Racer
 
Join Date: May 2005
Australia
Perth
Posts: 137
Lukin should be qualifying in the top 10 on the grid
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?
Lukin is offline  
Quote
Old 21 Nov 2005, 10:46 (Ref:1466333)   #11
greenamex2
Veteran
 
Join Date: Aug 2002
England
Hertfordshire
Posts: 1,686
greenamex2 should be qualifying in the top 10 on the grid
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?
greenamex2 is offline  
Quote
Old 21 Nov 2005, 17:25 (Ref:1466653)   #12
Ilikeearlgrey
Rookie
 
Join Date: May 2005
Posts: 6
Ilikeearlgrey should be qualifying in the top 10 on the grid

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!
Ilikeearlgrey is offline  
Quote
Old 22 Nov 2005, 12:37 (Ref:1467399)   #13
ubrben
Veteran
 
Join Date: Jun 2005
United Kingdom
Birmingham
Posts: 508
ubrben has a lot of promise if they can keep it on the circuit!
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
ubrben is offline  
Quote
Old 22 Nov 2005, 15:44 (Ref:1467547)   #14
greenamex2
Veteran
 
Join Date: Aug 2002
England
Hertfordshire
Posts: 1,686
greenamex2 should be qualifying in the top 10 on the grid
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?
greenamex2 is offline  
Quote
Old 22 Nov 2005, 20:43 (Ref:1467821)   #15
Ilikeearlgrey
Rookie
 
Join Date: May 2005
Posts: 6
Ilikeearlgrey should be qualifying in the top 10 on the grid
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
Ilikeearlgrey is offline  
Quote
Reply



Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
effects of rear suspension on front suspension TEAM78 Racing Technology 11 6 May 2006 23:38
Suspension set-up. Adam43 Virtual Racers 1 15 Mar 2002 10:20
De Don suspension? H16 Racing Technology 2 20 Jul 2001 21:47


All times are GMT. The time now is 14:30.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2021, vBulletin Solutions Inc.
Original Website Copyright © 1998-2003 Craig Antil. All Rights Reserved.
Ten-Tenths Motorsport Forums Copyright © 2004-2021 Royalridge Computing. All Rights Reserved.