Damping and Springing

[Julian Thompson]

I read up a fair bit recently on handling and setup. There are some interesting threads in the archives about it - certainly enough to put me in to the category of "a little bit of knowledge is dangerous!".

 

Therefore, having played about, I have made some observations and find myself at a bit of a crossroads.

 

I have noted the front spring rate of the car as 200lbs. The rear spring rate is unknown but best guessed as 150lbs.

 

 

The dampers, I think, are ProSport. The car has been run on front 185 A048 and rear 205 A048, but is presently on 175 front CR500 and rear 205 CR500. The car is not running any anti roll bars since I have enough to appraise without this complexity added in.

 

Initially, the car was just too soft, and would ground its sump constantly on bumpy roads. Winding the front collars up obviously wasn't an option because the car would have ended up far too high.

 

Therefore, a coil and a bit was removed from the front springs. This increased the stiffness appreciably at the front, and left the ride height unchanged. The rear ride height was experimented with by winding the collars up and down, and I noted that with the rear higher the car oversteered more - and with the rear lower the car understeered more. A balance was set to taste.

 

Next up, I read and tried to digest one of the excellent threads penned by Peter C. He commented that damping should be arrived at to achieve "critical" setting - in other words just enough damping to stop oscillation of the suspension...

 

So sensibly enough I wound off all my damping to "0 clicks" and went for a drive. Oscillation. Obviously. There was something else, though. The car was MUCH SOFTER. Not just "under damped" but very, very soft.

 

I gave it incremental increase in "clicks of damping" and tried it again each time, and each time the oscillation became less and the stiffness became more.

 

Beyond about 8 clicks, the car began to bounce around and was *very stiff* so I settled on 6 clicks and was pleased with the results - a really supple and feelsome road car that absorbed the bumps well and rode nicely.

 

Off to the track, then.

 

Oh dear. Oh deary me...

 

I have some pictures, but trust me, it rolled *a lot* - and dipped too much, and pitched too much - in fact the car still felt well balanced but was clearly nowhere near stiff enough for track speeds.

 

I was having some problems with a disintegrating A frame bush later that day so never got chance to adjust anything, but on returning home I am left with a quandry:

 

If increased damping stiffens a car up, how do you set your spring rate? I initially started by thinking you set the spring rate to stop the car grounding out, and then wind in some damping to stop the oscillation - but then the whole car gets stiffer, rather than just being "damped".

 

So my guess is that I need slightly stiffer springs and slightly less damping, but it seems that you can't separate the two functions (spring suspension and damping) - that they are inexorably linked.

 

I just can't figure out the link in my mind to be able to visualise what is happening.

[Sheds Moderator]

Hi Julian,

Part of the problem is that you are dealing with transient states here. In a static state then dampers don't figure in apparent roll or spring rate. However in the real world some heavier dampers will make the springs *appear* to be more effective as you have discovered.

 

In your shoes I would be inclined to contact someone like Freestyle or Nitron with your corner weights and they will give you some decent start settiongs to play around with. Alternatively you can always take the empirical approach of establishing what is too soft or hard then selecting a figure midway between the two for your optimum.

[Julian Thompson]

I think I understand.

 

So, for example, here I'm turning up my dampers more than I would really like because I'm undersprung. I can see now how *people* 😬 could easily try and correct this condition by increasing the damping and end up subjecting themselves to a royal pounding unnecessarily....

 

....maybe this also explains why, sometimes, on here, one chap with a certain type of 7 might say "I run 300lb springs on the front" and someone else with an identical car might be running 200lb springs and saying the other chap is over-sprung?

 

 

[Richard Price]

Julian,

The car is not running any anti roll bars since I have enough to appraise without this complexity added in.

but trust me, it rolled *a lot*

 

The quickest track sevens all use ARB's, with springs of a similar rate to yours.

With a car as light as yours is, I'm sure that the springs are quite stiff enough.

 

 

 

Edited by - Richard Price on 20 Aug 2007 18:03:28

[Julian Thompson]

I see how you've arrived at that conclusion Richard but I'm not prepared to run an ARB until I can definately see I need one - many people just copy everyone else so it is not always fact that there is good reason because everyone does it.

 

The arb won't help the nose diving under braking, or the fact that the car grounds out over bumps, so I am fairly sure it needs stiffer springs first before we can assess the roll.

 

Also, I'm not sure that the fact that a bike engine car has MORE percentage of its weight over the nose has been considered. If you think about it, the BEC has BOTH the engine and gearbox up front, wheras a Car Engine Car has only the engine, with the gearbox much further back over the C/G of the car.

 

If the car didn't hit the floor over bumps and stayed flat under braking I'd be inclined to agree about the ARB but right now I think 300lb springs rather than 200lb springs might be a plan....

[metal mickey]

Julian -

'Also, I'm not sure that the fact that a bike engine car has MORE percentage of its weight over the nose has been considered. If you think about it, the BEC has BOTH the engine and gearbox up front, wheras a Car Engine Car has only the engine, with the gearbox much further back over the C/G of the car.'

 

The zetec I removed was 92kg, the busa is approx 80kg. I don't know the percentage weight split front to rear but i'm pretty sure its heavier at the rear. FWIW I removed the front arb at Oulton on the club day and have never run a rear arb and the turn in seems sharper and the understeer has definately reduced. I have similar spring rates to yourself but dedion chassis.

[Julian Thompson]

Interesting Mike - was thinking that because the gearbox is essentially over the centre of gravity in effect a BEC will be more nose heavy than a CEC

 

Also interesting about your ARB.

 

When you say same rates as mine do you mean 200/150 or 300?

 

 

[Simon.Rogers1]

Hi Julian

 

The main principal as far as I can understand from the reading and "set up" I have done is that you should try and run with the softest spring rates possible. To achieve this you must normally increase the damping.

 

As Richard stated above your initial spring rates seemed not so far wide of the mark.

 

In general the quick guys at the sprints with "k's" are running 225 front and 150/140 rear with Nitrons. Myself I run 225 and 135 but will be changing this winter to 210 on the front and adjusting the valving of the dampers to increase the same at the front.

 

Given the lighter car I would suggest 200 and 135 - f/r

 

However without knowing the valving of your particular dampers this is all guess work. You may well be under damped and you are therefore using the increased spring rate (you increased the fronts) to cope with the soft dampers. ie you are using the spring as a damper.

 

So really you need to have the dampers calibrated against another known and recognised damper that "seems" to work.

 

 

[Paul McKenzie]

Julian,

 

IMHO Richard is right -you need to put arb's on , then sort out the springs, if need be. I know from experience that Caterhams with even a weedy front ARB and standard (read soft) '96 set-up, roll like a hippo when cornering on track 😳.

That's where you need to start..not spring rates

 

Paul

[Richard Price]

Also, I'm not sure that the fact that a bike engine car has MORE percentage of its weight over the nose has been considered. If you think about it, the BEC has BOTH the engine and gearbox up front, wheras a Car Engine Car has only the engine, with the gearbox much further back over the C/G of the car.

 

Julian,

The Corner weights of your car suggest otherwise! (or they did when I checked it for Nick). The Figures may not be absolutely accurate, but they are repeatable.

Your car was 204Kg front and 203Kg rear. Where my k car, ready to sprint, weights 242Kg front and 251Kg rear. For comparison, your old SLR weighed in at 245Kg front and 240Kg rear (on 15" 10 spokes wearing list 1A normal road tyres).

 

[Julian Thompson]

Hmmmn - those figures will do for me Richard - I'm sure they are close enough. (Some changes since so it will now be actually tail heavy by a small margin like battery in the back etc..) So BEC is slightly less nose heavy but not very much - not as much as people would have you think.

 

I guess, then, that the thing here is that when you take the arb off you lower the spring rate in roll anyway...

 

...but despite this, and despite my car obviously running similar spring rates to *the norm* if I went out with the damping set to a "nice" setting and drove the car even "enthusiastically" I would get about 2 miles up the road before having to go looking for pieces of sump pan...

 

...this was the same on the SLR, too, which I actually DID manage to remove the sump pan off in very short order 😳

 

So there is something fundamentally wrong with both cars....

 

which the ARB just will not fix....

[Gambo]

What static ride height are you running. (sump clearance.) ?

Run it too low, and It'll not matter a jot what spring rate you have, you'll smack the sump.

FYI my vx ran 300lbs springs at the front and had a 48/52 front /rear weight balance with me in it. Total weight with me 647kgs.

Difficult to compare now as I have put pushrod on the front. New front springs are 250 but the front of the car is stiffer but does not use ARB.

 

RED 2.0 HPC 230BHP here

[Julian Thompson]

Hi Gambo

 

It's a wishbones level ride height at the front as generally suggested, IIRC.

[Julian Thompson]

 

What about making a new bit to move the tops of the damper towers outwards and upwards a bit. Would this reduce the "falling rate" with deflection issue?

[DSL]

My Busa weight split with me on board at 90-kg with 7ltrs of fuel was 44.61% front & 55.39% rear, R400 split was 46% front & 54% rear.

 

Whilst the Busa was spot on for the Hillclimbs with 150lb rears & 225 fronts, adj bar on front & none on the rear, when I took it on a track day I thought it was pretty dreadful ☹️.

 

Different springing & damper requirements for different circumstances I guess, if you want to get the best out of the car

[Gambo]

Julian, yes that is what they usually say, but the cr500 175/50 is a very low profile tyre, and lowers your ride height from say a185 60 048 buy about 20 mm.

Indeed I have seen the top mount mod that you mention done. It would decrese the falling rate of the spring. One of the sprinters,Tony Abbitellio had done it. He was going to do it for me , but I went pushrod instead.

 

RED 2.0 HPC 230BHP here

[Julian Thompson]

DSL - thanks - it's interesting that I'm not far wide of the mark about nose weight. Yes, as Richard and your figures point out, it is not clear cut, but if you'd asked me a few weeks ago I would certainly have not thought that a BEC was even close to as much weight at the front as a K car etc.. Truth is, that % split is *very similar*.

 

Gambo - right - I see - well, that sounds like a good plan. Do you know how far out he took the mounting? Looking at it there is a likely spring fouling issue on the wishbones if you go very far.

[Julian Thompson]

Forgot to say - it grounded out on A048's too, before I fitted the CR500's!! (So got to be really careful at the moment then - thanks for heads up on that...!)

[Gambo]

Julian, what He did was to make a rocker that fitted to the top mount, he brought the damper out about 20-30 mm and then tied the inside of the rocker, which was inside the chassis down to the lower wishbone mount with a metal bar IYSWIM.

 

RED 2.0 HPC 230BHP here

[Julian Thompson]

I do SWYM!

 

That sounds a well thought out way to transmit the loads....

 

....I've been reading up a bit, and the sump grounding issue is definately a problem that *people* seem to have (not just me), so surely either the suspension design is wrong (in which case how come it's STILL wrong after all this time 😬) AND/OR we're all running too soft a spring (as I suggested before)

 

At the moment I'm driving around and every time I hit a bump I kind of wince as I wait for the bang...If that's you as well then something IS wrong, because I don't do this when I drive any other car!

[Mark Durrant]

Julian

 

The first thing I would check is the ground clearance. I have my car set up at 70mm at its lower point and with 225lb springs on the front I have not had a problem with grounding out although I do take care on roads with a severe camber.

 

Mark D

Comp Sec

 

[Julian Thompson]

Got at least 70 mm Mark, - must be the springs on mine. Then again, I'm using the car on extreme roads I guess up around Derbyshire area with big elevation changes and massive compressions. I guess that if you lived in Norfolk or somewhere the same extremes would not be present.

 

Compounding that is the fact that I'm striving for a (possibly impossible!!) setup that will do this kind of roller coaster road AND flat circuits....

 

😳

[Gambo]

I think Julian you must take into consideration that the suspension design is as old as the car and that in the early days the ride height were higher due to taller profile tyres.Up until quite recently most of the caterham who ran 13" wheels and tyres ran 185 70 profile. They also ran 14" wheels with 185 60. these are much taller wheels /tyre combos, thus giving higher ride hights.

Now ofcourse tyre sizes are very limited in 13", and I do not know about you but I do not think the caterham suits 15" wheels and low profile tyres very well(sorry).

 

RED 2.0 HPC 230BHP here

[Julian Thompson]

I think this is all true!!

 

Shall almost certainly have a go at the damper mounting mod though, and will report back once it's done with a progress report.

 

Thanks for all your comments and help chaps - really appreciated

[Simon.Rogers1]

Julian

 

I do not think it is the spring rates. As I said before and others have supported they are well within the norm.

 

The unknown is the dampers. They would certainly be my first port of call.