winders

Larry,

This can be taken too far though. Otherwise, no suspension would be superior to having suspension.

I suspect compression-biased damping is more common in Europe than North America simply because the tracks in Europe are generally much smoother than what you find here in North America.

The very idea of suspension in race cars is to keep the tires on the ground so they can do their job. Bumpy tracks require a certain amount of compliance and you need softer suspension on bumpier tracks.

On top of that, even if you have glass smooth tracks, you want to slow the load transfer forces so tires don't get loaded so quickly that they lose traction.

Scott

the90

Scott,

You have hinted at one of the most fundamental compromises that the engineer or designer is faced with: track surface irregularities require softer spring rates, while inputs to the suspension in the form of driver input are reacted more quickly with stiffer springs, to reduce the time required to achieve steady state tire lateral forces and chassis position. Remember, springs aren't the only thing to react suspension loads.....

winders

There are many compromises. Springs, anti-sway bars, tires, dampers, and chassis react to suspension loads.

On race cars, we want to eliminate any mechanical issues that cause driver input to be delayed. This is why we remove as much rubber as possible (all, if the rules allow) from the suspension and steering systems and a big reason why we want the cars to be stiff.

Yet we don't want the suspension to instantly transfer these loads to the tries. This is why race cars typically have roll centers low to the ground or slightly under ground. If the roll centers are too close to the cg's, jacking forces are high and jacking forces are instantly transferred to the tire. The tire works much better if the forces are allowed to build up over time. This is why a little bit of roll is always good and no roll is bad. But, you don't want the transfer of load to take too long either. Too much roll is likely worse than not enough.

Of course, how this all works is dependent on tire construction as well. What might work for radial slicks can be quite different for bias-ply slicks. Higher profile tires can provide more compliance than lower profile tires so you might be able to get away with more roll stiffness if it is needed.

Of course, you then have to factor in driving style and driver confidence.

Very complicated.....

Scott

Larry Herman

But that is what makes it so damned interesting, right?

winders

I think I would be happier overall if suspension setup was not "so damned interesting".

When I was racing bikes, we worried about: rake, trail, swingarm pivot height, front and rear ride height, chain angle, fork and shock preload, free sag, rider sag, tire pressures, tire compounds, high speed compression, low speed compression, high speed rebound, low speed rebound, fork oil height, steering damper settings, and shock bellcrank curves. I am sure I am forgetting a few things.

Thank goodness bikes were simpler to work on!

Scott

Land Jet

Larry, after reading Jan Zuijdijk's book, it seems he is saying that compression settings should be higher than the rebound settings. From your quote above it looks like you are in agreement unless I am reading it completely wrong. Most people I know have their settings the opposite of this. Could you elaborate on this a little more please? I know that spring stiffness plays into the equation but what would be typical settings for Moton 2 ways on a 911 type car?

Are you by any chance coming to Summit Point with Reisentoter next month?

utkinpol

can anybody who knows this subject give a quick overview of most common shocks available on the market - JRZ RS, JRZ RS pro, similar motons, 2-3-4 way ohlins etc - are they all generally of same internal design or considerably different?

all i know is that JRZ RS is dual tube and all pro shocks with canisters are monotube but when it comes to valving etc details - no clue. can anybody explain how do they all really differ?

the90

Ohlins TTX and Dynamic DSSV are spool valve, and capable of linear damping setups without hysteresis...all others mentioned are shimmed (I believe).

Larry Herman

Larry, I do agree about having compression damping higher than rebound damping. The caviate is that compression damping numbers may not exactly equate to the same in rebound; i.e. a setting of 6 on compression will not have the same damping force as a rebound setting of 6, especially if cannister pressure is all over the board.

Most people like the feel of more rebound, especially with softer springs. It makes the car feel less floaty, more "locked down". It does not result in more grip, usually just the opposite and can make the car slower.

I was not planning on coming to that event, but will do some RTR stuff later in the spring.

the90

Shimmed dampers will tend to cavitate and produce large amounts of hysteresis when asked to produce significant compression damping. So asking your JRZ or Moton units to do so might produce less than satisfactory results.
Most of the pro sportscar teams are using DSSV dampers, which may also be found on most of the F1 field. Not cheap at about $1800 per unit for an off the shelf piece, but they do work...

I posted an earlier suggestion that represents a good compromise when using a shimmed unit: digressive valving for compression, progressive for rebound.

Larry Herman

Every time I read your posts it seems like you are quoting from an engineering manual. Would you mind explaining these items:

1) Shimmed dampers will tend to cavitate and produce large amounts of hysteresis when asked to produce significant compression damping.

I thought that hysteresis is what a shock did, retard or delay a change in motion. Also, why will they cavitate?

2) Digressive valving.

Do you mean the opposite of Progressive as in Regressive?

utkinpol

i also got puzzled but found good article about this:

http://autopitstop.net/car-improveme...ssive-dampers/
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Valving Characteristics
A shock absorber uses different valves and springs to adjust the dampening force of that particular shock. There are three types of valving characteristics that dictate how the shock will perform. They are progressive, linear, and digressive. Progressive shocks start out soft and get stiffer quickly as the shaft speed increases. Linear shocks uniformly increase stiffness as the piston speed increases. It is similar to the progressive valving but does not make dramatic changes at higher speeds. Digressive is just the opposite of progressive. It starts out stiff, then it tapers off.
----

where I am getting lost is what known makers make what shocks and to what degree all that (i mean valving type) can be controlled or adjusted or altered. so far I see only Ohlins are there? even as purely theoretical topic it is quite an interesting subject.

winders

Larry,

This is from my motorcycle damper days, but it applies:

There are three basic types of valving: linear, progressive, and digressive.

Linear valving increases stiffness at a linear rate as the shaft speed increases.

Progressive valving increases stiffness at a rate higher than the shaft speed increases.

Digressive valving increases stiffness at a rate lower than the shaft speed increases.

Regressive? If that term is used, it probably is the same as digressive.

Scott

Larry Herman

Got it. Thanks Scott & Utkinpol.

Matt Romanowski

Ohlins and Penske shocks will do all three forms of valving. If you need stuff rebuilt or want a real custom shock, talk to Angelo at Anze http://www.anzesuspension.com.