GeeDee

iTrader: (2)

I’m after some expert advice from suspension experts on here.

I have posted on a couple of threads https://www.scoobynet.com/scoobynet-...ng-issues.html & https://www.scoobynet.com/suspension...ml#post6985433 re my new AST Stage 3 setup on my Spec D not being quite man enough to allow one wheel to hit a bump (eg those single square shaped speed bumps, potholes, etc) without it bouncing into the bump stop. I have not experienced the same thing when both front wheels hit a linear speed bump.

What I am trying to understand is the difference between increasing the spring rate which will give it enough oomph to be able to raise that corner of the car in the event of such a bump and increasing the shock absorber rate to resist the bump in the first place. I was originally set up with the shock settings on 6/4 F/R. I have now upped that to 7/5 but have not found a suitable bump on which to test it yet.

Ultimately, I am after a car with a good ride quality but still maintaining handling. I appreciate there is a compromise there but I was very happy with the balance of my old Classic WRX MY99.

Can anyone offer a suitable explanation or do I need to dig very deep to locate my old suspension tuning book from years (and years) ago?

GeeDee

iTrader: (2)

OK, well maybe I’ll start it off.

My understanding is that it is primarily the spring that absorbs the bumps in the first instance and the shock absorber then absorbs the energy in the spring as a result of it being compressed.

A secondary function of the shock absorber is reducing the amount of energy the spring initially absorbs in the bounce condition but if overworked could lead to overheating, reduced performance and, ultimately, excessive wear. Also, if the shock is turned up towards maximum then there will be little to take up wear thereby reducing its life even further.

911

I am no expert, but i have messed about with this subject seemingly endlessly.

The spring absorbs the initial shock of a road surface change in that particular corner of the car, and is influenced by the others too to some degree.
The damper stops the spring occillating till it naturally settles.

The relationship is a tricky one as do you need a linear damping or one with a non linear characteristic.

I used to engineer springs and dampers in high precision weighing machins including weigh bridges, and controlling a 20 tonne capacity bridge with a muddy full quarry truck on it is not easy.

The damping range and characteristic is governed by the design of the valves to pass the gas or oil during first and subsequent deflection of the spring.

That series of relationships must be very complex and 'infinitly' adjustable.

To your car and a 'bounce' over the bump allowing the system to bottom out to the bump stop sound to be something is wrong with the valving or in the case of the AST's I think the washer stack spacing.

My Sti V3 is a bit heavier than stock (roll cage etc) and was perfect on 50/40 spring rates.

The car has had a few offs on the track and has thumped a few holes too ar slow and high speeds, and has never bottomed out, front or rear.

Om 60/50 springs the same.
This was on one way adjustable dampers.

The same though applies to the 3 way I have where there is considerable scope in the damping to say the least.
The one ways can be tuned to give ride comfort and performance with the above springs, but the 60/50's are hard for the roads we all endure.

Graham.

DuncanG

Thats rather old and simplistic dogma and obviously comes from a smooth track racing perspective. If you were to follow that advice then why have any bump damping at all? Can you imagine a rally car with no bump damping - it would whack on to its bumpstops at every landing.

A suspension setup for bumpy conditions will have significant bump damping and so the damper will absorb a considerable amount of energy before the spring has to deal with the rest. Bear in mind that all the energy absorbed by the suspension gets dissipated as heat in the damper - the spring just stores energy on the bump stroke then releases it back to the damper on the rebound stroke.

An ideal suspension would deal with small and large bumps differently - small bumps would be absorbed with little resistance (but large deflection) while larger bumps would be resisted in order to control the limited suspension travel. In the real world suspension is not that smart an so there is a compromise. Generally smooth surface / track oriented suspension will have low (high-speed) bump compared to rebound rates (expressed as a rebound/bump ratio of around 3-4) while that for a bumpy application will have rebound/bump of around 1.5 (although it could be higher if it also uses hydraulic bump-stops). I've found a rebound/bump ratio of around 2 works well as a bumpy road compromise.

Another effect of rebound/bump is jacking. A ratio greater than 2 will tend to lead to jack-down where on repeated bumps the suspension ratchets lower and lower making the bump handling worse, while a ratio of less than 2 will tend towards jack-up.

'High-Speed' wrt dampers refers to the damper-piston speed which can roughly be considered in 2 speed regimes: low-speed (0-50mm/s or so) controls body motion (roll, pitch etc) while high-speed (roughly 50mm/s upwards) is exercised by bump handling. Modern dampers are speed sensitive and have a non-linear force/velocity characteristic.

Dampers and springs both provided resistance to bumps, but in a different way. Spring resistance is proportional to the compression on the suspension - so the spring has little effect until there is already considerable compression. Damper resistance increases with the speed of suspension movement - so the damper starts working immediately the compression starts and before the spring has reacted at all. The spring and damper must work together to control the acceleration and speed of the compression. Insufficient damping will allow the suspension to accelerate too rapidly at the start of a compression leaving the spring a much harder job to catch up with in the latter stages. However too much damping will lead to a very dead and harsh response to the compression.

The relation of damping to spring rate is called damping ratio and is a proportion of 'critical-damping'. Critical damping (for high-speed) is not the optimum - it would feel very overdamped, harsh and reduce grip. The sums for critical damping can be found in any book on dynamics. According to Millikan damping ratio will be in the range of 0.25 for comfort up to 0.6 or so for maximum performance (tyre dependent). Modern dampers will generally allow increased damping ratio for the low-speed regime, ie body-control or 'handling'.

What do adjusters do? -
It depends. On the KYB-AGX, according to KYB, the adjuster affects both bump and rebound for high and low-speeds roughly proportionally. Thats probably what a naive (most of us) user would expect an adjuster to do and its useful for adapting to different spring rates or for but less good for switching from road to track settings. However on a high pressure monotube (eg AST) the adjuster is probably a simple needle-type bleed valve and that will affect mostly rebound and low-speed proportionally more than high-speed with virtually no effect on high-speed bump. So thats good for tuning body control for track but no much good for adapting to different spring rates. That type of damper requires a revalve to alter its high-speed bump response. High end dampers could be 3-way adjustable and that allows low-speed bump, high-speed bump and rebound to be tuned more or less independently. A good vendor will supply a force-velocity chart showing how the dampers characteristics will vary with each adjustment.

Thats all for now.

911

It's been a long time since you posted on here!?

Great read as ever.
Hope there is more

Graham.