DuncanG

According to the old Koni doctrine damper bump setting is for controlling the unsprung mass and NOT for bump control which should be controlled by the spring rate. By inference the rebound rate is for controlling the sprung mass.

"Bump damping controls the unsprung weight of the vehicle (wheels, axles, etc.). It controls the upward movement of the suspension as when hitting a bump in the track. It should not be used to control the downward movement of the vehicle when it encounters dips. Also, it should not be used to control roll or bottoming."


Is this still current thinking? Sounds like cobblers to me. If thats the case then why is rebound/bump ratio not around 10:1 for a typical case where the sprung/unsprung weight is around 10:1, instead of the more typical rebound/bump of around 2:1? Also if the damper bump setting is nothing to do with bump control then why do the better units have a high-speed bump adjustment?

Maybe I've just got the wrong end of the stick.

Duncan

AvalancheS8

Your average car damper doesn't have proper speed sensitive valving though, so setting the damping heavy enough to work at the slow shaft speeds required for body control will leave you hideously overdamped for the sort of shaft speeds encountered when hitting a square edged bump at speed. I'd guess it's this that leads to the assertion that the bump damping shouldn't be used for this. It's fairly common practise in fields where current damper technology is well ahead of what is generally used on cars (moto-cross, Downhill mountain bikes, snow mobiles etc)to have proper shim stack speed sensitive set up with a blow off for high speed and then you can tune the low speed damping to control the chassis movement and the high speed to deal with the sharp bumps. This can work very well indeed but will be costly (e.g a 3" stroke single rear damper to control 8-10" of wheel travel on a Downhill mountain bike, custom built and valved to your personal requirements costs around £400 - £500)

DuncanG

Thanks for that Avalanche. The problem I'm having is bottoming on dips. My feeling is that there is a lack of hi-speed bump damping, but that setup guide seems to suggest that I should be stiffening springs instead.

I found an excellent technical document on the Penske shocks site that explains in great detail the operation of shim-stacks, high & low speed damping, progressive, linear & digressive characteristics etc. For hard-core geeks only: http://www.penskeshocks.com/Adjustab...h%20Manual.pdf

911

Struth!
Is it not simpler than this?
If the car has a certian suspension travel/weight then there is a 'perfect' spring rate to assure the car does not bottom out in extreme conditions? This may lead to a stiff car, but expected on a Scooby, M3, 911 etc.
The damper's rate needs to be matched to stop that spring excessively rebounding time and again to 'rest'. The adjustment is there to set the rebound damping and is a compromise setting for slow and fast deflection of the suspension members, suspension friction and changes of all this when hot/cold etc?

Sorry if I sound an amature in all this, but I am, and find the subject very interesting as I drive my Sti v3 on road and in Hill climb competition.

911

jgevers

Nowadays the damper is not just used to control the spring/unsprung mass.

By means of pre-tension on the bump shim stack the damper can be used for a lot more. One of the obvious uses is roll control. By increasing the low speed bump damping the suspension frequency can be increased, which can allow the damper to hold the car up during turn-in. As the damper speeds up, the bump damping digresses and allows the suspension frequency to fall again. This will then allow the suspension to move when hitting curbs, bumps etc.

'Bottoming out' can be a result of inadequate bump damping which allows the suspension to speed up to fast. But if spring rates are to low, you might be asking the damper to do to much.

Having a good bump damping curve can make a dramatic effect on lap times. Why do you think a good chassis engineer/damper technician commands one of the best salaries in motorsport?

regards,

Job

AvalancheS8

Bottoming on dips, or even landing from jumps counts as low speed damping. The low and high speed tags refer as I'm sure you are aware, to the speed of movement of the shock, not the car. If you consider the wheel, and therefore the shock movement to absorb a square edged bump, such as a pothole, hit while the car is travelling at 50 mph, it has to move vertically, by several inches almost instantaneously. Compare this with the movement when the car drives through a dip in the road, where the suspension moves comparatively slowly. That's the difference between high and low speed damping.

AvalancheS8

You'd think huh ?! The problem is that the behaviour of a simple damper (i.e a piston with a simple hole) is highly non-linear. What you want is for the damping force produced to rise in a pretty much linear or somewhat regressive manner with the shaft speed of the shock, what you get is that it actually rises exponentially. This means that if your damper is set up to work well at low shaft speeds (body roll, dips in the road etc) then the damping will be at best far, far too hard for high shaft speeds, at worst you will get spiking. Spiking is where the damping rises so much that the shock to all intents and purposes locks out entirely under sharp edged impacts. Conversly if you set your damper to work for these impacts, it will offer almost no noticeable damping at low shaft speeds. Because of this, damper technology has evolved to reduce the damping as the shaft speed increases in order to get the characteristics we want. The performance difference between dampers using this technology and ones without is huge.

DuncanG

911: so you're in agreement with the 'Koni' philosophy - springs handle the bumps and dampers control spring rebound.

Job: you're saying (I think) springs & dampers work together in dissipating bump energy but they have to be balanced, not all one or the other.

Avalanche: yes I'm talking about shock speed. Clearly cornering control is low-speed and square edged bumps are hi-speed, so sloping-edged bumps or the up-ramp of a dip is going to be somewhere in-between and I suppose will shift from the low-speed to hi-speed regeme depending on car speed.

Another question - given the available damper travel of 150mm what would you recommend for bump & rebound? I have it set for 80mm bump/70mm rebound. Clearly I could dissipate more energy with a longer bump, but I don't want it to be skittish on the crests.

Currently my spring rates give a frequency of 110cpm, I am going to try one stage up which will give 120cpm - just to give the 'Koni' method a try although I have my doubts. I think that rate is quite high for road car - what do you think? Also I'm going to try longer bump-stops.

The areas of road where I've had the bottoming problems with this setup are well known to me and didn't cause a problem with my previous setup at higher speed even with much softer springs. So the old setup must have been absorbing more energy in the damper.

jgevers

It sounds like your dampers do not have enough low speed (upto about 4 inch/s) damping and the car falls straight through. If after falling trought its damping (bottoming out), you do not get a jarring that you can feel through your spine, you might not have enough rebound damping to control the spring. The reason I explain it like this is that I would expect the damper in extreme rebound acceleration, to hold on to the wheel slightly which you would feel through your spine.

What dampers are you running and what usage are they getting. If you have Penske, Ohlins, Reiger, Intrax, Moton or JRZ, I will be able to specify shim stacks and/or pistons for you to give you a decent base setting.

I would expect to see at least 600lbs at 2inch/s through a very rapid rise from zero on your front dampers. I would expect flattening/digression of bumpcurve at about 6 or 7 inch/s. (I am just guessing here).

regards,

Job

DuncanG

Job yhPM.
Certainly feels like it falls straight through.

I not sure I understand what your saying about rebound damping though. Are you saying that if the rebound is too soft then the car will bounce from full bump to full rebound and I'll feel a jarr at full rebound (no rebound-stop)? No I don't feel that. I sure feel it in my spine though when it goes coil-bound at full bump! I didn't mention that before since I'm trying to focus on the springs and dampers, but of course I'll be attending to the stops.

Usage - A/B road thrashing. Not much use so far on this new setup as I can't push it the way it is. I don't want to name the maker/supplier until they've had time to address the problem.

Any thoughts on spring frequency and bump/rebound?

Duncan

PS
These were AST coil-overs supplied by Powerstation. Over a period of several weeks they made no effort to help despite me giving them feedback on 3 different spring setups which I tried at my own expense. The damping of the ASTs simply does not suit fast driving on bumpy roads.

jgevers

When I talk about suspension frequency, I mean the frequency of the whole suspension assembly. With springs I use N/mm or KG/cm or lbs/inch.

Damper ratings are difficult to just write down in numbers. I normally use them in graph form with force against velocity. I quoted some numbers in my previous post.

Just a thought though. Last week I had a Suzuki racing bike in my workshop to re-valve the damper & forks. The bike was bottoming out at the front under braking. The actual reason why it was bottoming out was because the previous damper engineer had allowed some dirt to get into the valve stack. This was allowing oil to bypass the valve stack,resulting in greatly reduced low speed bump damping. (Bike won 3 races last weekend after damper rebuild).

So, the valving might be okay, but a mistake could be made during the assembly.

regards,

Job

911

Job, didn't we have a good thread on this topic about 6 months ago?
Great tech discussion, very interesting to the 'outsider'.
911

jgevers

Yes we did. And it's good to see that more and more people start to realise that dampers play a very important role in keeping the 4 black patches on the tarmac.

regards,

Job

911

It would be good if LEDA came on this thread, some times they respond, the only guys to do so.
Maybe this will wake them up!
911

jgevers

I think they might only build progressive dampers? Digressive might be a it modern for them?

regards,

Job