Good Book For rally suspension?
Posted by aj_johnson
A.J. Johnson Godlike Moderator Location: Pendleton OR Join Date: 01/07/2011 Age: Midlife Crisis Posts: 1,381 Rally Car: 88 Audi 80 |
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A.J. Johnson Godlike Moderator Location: Pendleton OR Join Date: 01/07/2011 Age: Midlife Crisis Posts: 1,381 Rally Car: 88 Audi 80 |
John Vanlandingham Mod Moderator Location: Ford Asylum, Sleezattle, WA Join Date: 12/20/2005 Age: Fossilized Posts: 14,152 Rally Car: Saab 96 V4 |
A.J. Johnson Godlike Moderator Location: Pendleton OR Join Date: 01/07/2011 Age: Midlife Crisis Posts: 1,381 Rally Car: 88 Audi 80 |
John Vanlandingham Mod Moderator Location: Ford Asylum, Sleezattle, WA Join Date: 12/20/2005 Age: Fossilized Posts: 14,152 Rally Car: Saab 96 V4 |
Right here under cars and construction. Search "manuals scan". We think of youse guys in advance cause we just that sorta guys. We're not like the others.......... |
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Good grief! wait till its all waaaaaaaay bad and idling like an old school fuel dragster ba ba bop ba babop!!! VVRooooooom! ba ba bop! You gotta get some dark glasses---which you wear 24/7, and a flat brim hat. 
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starion887 Ultra Moderator Join Date: 09/06/2006 Posts: 798 |
AJ,
Do you feel you have a decent understanding of why different spring rates are chosen? It sounds like you do, if you understand why stiffer rear springs fight understeer. If so, then you are long way towards understanding what you need to know. A lot of the rates we use have been empirically found by lots of rallying experinence. So don't be afraid to go with what is known to work for rally. And the great thing about spring rates and free lengths with coilover suspensions is that you can start with a set of springs and then change them easily and pretty cheaply if you need to adjust them. Most of the cost is in the coilover structure and shock, not in the springs. So don't be overly concerned with the springs. Adjusting that in is more matter of time than money. The next thing to understand is why a damping rate is chosen, and what happens when it gets off. This can be found in engineering books describing either mechanical or electrical damped systems (late sophomore or early junior mech engineering or EE books). If you find some descriptions of damped systems and saw graphs of overdamped, underdamped, and critically damped motion or electrical response, then you would instantly understand the principal. Good spring and compression shock rate result in a slightly under critically damped response upon a landing form a jump, for example. Refer to this grpah from this Wikipedia page: http://en.wikipedia.org/wiki/Harmonic_oscillator , which I have attached below. The red line, is the crtically damped response, for the factor mu=1. Once that is understood, then it is again back to empirical knowledge. What actual shock rate works best to get to this response is a matter of experience and field work. Additionally, differences in rebound and compresion damping rates, and why a ceratin rate works for given car front and rear (with weigth differences), is also moslty empirical in nature. At best, IMO, you can learn the underlying princles of damping as above, and then use that to make decisions on how to adjust if, for exmaple, you want to change wheel hop of small surface rocks. I would spend time trying to prise info from others on how they initially set and adjust their suspension and why, and the resulting changes. (This is kinda hard, admittedly, in the N.A. rally world, as we are so scattered.) A few points that might be worth considering and trying to understand: 1) Fixed damping rate shocks will have limits in how soft you can go on spring rates. A fixed rate shock suspension will become underdamped (an oscillatory boing after each bump) with too soft a spring (look at those diagrams for mu=2, and mu=6 for a broken shock valve). Going hard will not be ideal, but at least you will not 'boinging around'. 2) As for shock rates for rally relative to street, I would expect rally to be softer for ths simple fact that spring rates are lower. They go hand-in-hand to keep mu close to 1. 3) As for the differences in compression and rebound rates, just think about the differences in the way the suspension has to respond upon landing (compression rate controls the action) versus going light (going over a rise, where the rebound rate is the controlling factor). Upon landing, the shock has to control the suspended weight of the whole car as it tries to compress the suspension. On rebound, only the wheel and axle weights are working to extend the suspnsion length; a softer rebound rate is needed to allow this smaller weight to extend the suspension quickly to try to keep the tire in contact with the ground. 4) Once you accept that the shock rates have to be different between compression and rebound, then you realize that the spring rate can only be perfectly matched (mu =1) to one shock rate or the other to give a critically damped response. In reality, a compromise is needed between the 2. If you have adjustable damping rates, then know that getting the compression and rebound shocks rates further and further apart makes the spring selection, and thus the damping action, more and more of a suboptimum compromise. 5) One thing not yet mentioned: Different suspension rates (the time rate of the basic natrual oscillation of the suspension and mass) differs between front and rear is normal in passengar cars for good ride. I don't know if it is better to make the equal front and rear for rally use; that would be interesting to know. This is not normally discussed in suspension talk; I can't see where there is much you can do about the suspension rate as it mostly depends on the car's mass. So again, take the knowledge already there and go with it. So, other than experimentation, getting info from others on what works is the best way to get quickly to a good suspension setting and set-up. Understanding the principals of under-, over-, and critically damped system response will help understand what will happen when you start to adjust things. And....don't forget about suspension travel! Regards, Mark B. |
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