Choisir les bon amortiseur(anglais)

[vince3004 0]

Information tirer de Club4ag.com
Liens:http://club4ag.com/faq%20and%20tech_pages/Dampers.html

Selecting shocks with you in mind

About shocks... Yes, Bilstein are excellent shocks but I only say that in terms of quality and design. The question here is, does it offer the characteristic that you are looking for? All shocks are valved differently over many models and in different markets. It's hard to say since I have no experience with using all possible ones that fit the AE86. Saying that, we should look at which type of valving is correct for you with any brand you buy. Japanese market dampers usually have much stiffer compression rates than rebound (generally speaking). European shocks have more even damping generally between the two figures. It's also important to look at what rate of spring the shocks are designed to work. Adjustable struts and shocks have a set valving range and this may not always be within the parameter needed. So where do we stand? It's impossible to buy everything that is available right? Here's some advice.

First measure your spring’s rate to see if you have the correct spring rate to start with. You can roughly check by using the highest grip tire you will be using regularly. In a wide-open space, circle around like a skidpad test...be sure you don’t induce pitching. The steering input and throttle should be constant. The point of this is to figure out you maximum stroke and body roll in conjunction at constant velocity maximum lateral force. Now to make things easier, tie a cable tie around the shock piston. (Make sure the road is smooth as possible too) This way, you can see afterwards how much of the suspension stroke you used during maximum G's. You should generally be at 80% of maximum stroke... You leave the last 20% for undulations in road and for transition driver input, which put more force. You have to remember; the instant the shocks bottom, the contact patch will deform to accommodate tire deformation, resulting in sudden loss of grip... If it's less than 80% you may increase tire grip or reduce spring rate, or choose a shorter spring. If it's more, you will need to raise the car (longer spring, reduce shock length, or reduce tire grip) Anyway, I hope you get the idea. Now, don't consider understeer or over steer at this point, that is done later after you match your tire with spring rate.

Now when you are satisfied with spring rate, then you move onto choosing dampers (shocks). Its job is to not make the suspension stiffer or softer as many believe. That's the spring's job. The shocks are solely for the purpose of controlling the roll speed.... and this is how we adjust handling of the car. This is how we adjust what the car should be doing against driver input. That simply said is perhaps the most difficult part of suspension along with alignment. But here's a guideline. If your spring was matched to a specific shock, it's advisable to stay close to those valving rates. Using that as starting point, we go on to real suspension tuning. To increase the contact feel at the rear of the car, we can make the rear shocks do more work in balance with the front during cornering. If the loses grip in the rear suddenly, you can do one of the following. You can reduce the dive speed of the front suspension by valving the shocks stiffer on the compression side. Keep in mind that you are not changing the static balance of the suspension by doing so…or simply, you have only reduced the speed of the nose dive and not the amount of nose dive, the latter is done by springs. The car should begin to display some initial understeer followed by the same balance you had before as the suspension reaches its maximum travel. This should make the car feel more stable and predictable. Now similar effect can be achieved by raising the rebound rate in the rear, but most of the initial roll is created upon front dive during braking so you need to come up with a good balance of the two. By adjusting the rear rebound (expansion) stiffer, you will reduce lift (speed) in rear and thereby contribute also to reducing roll (dive) speed. The same amount of constant velocity balance is maintained while you have just improved the predictability of the car. This idea holds true to do other tuning. The key is balancing your driver input against what you can feel the car doing, making things predictable. Too soft of dampers will control speed of the spring’s actions less and too stiff will make the spring not work fast enough. Who wants to be tilted at the end of the corner when you need to be accelerating straight right? By not changing the spring rate, you have maintained what the car will do in the skidpad test earlier while making things more controllable. If you like the “amount of oversteer” but want to make it controllable, it’s the job of the shocks. If you did not like the amount of oversteer on the skidpad, then it’s not a matter of controlling your roll speed but amount of roll itself (spring, and sway bars) or your weight balance (ride height between front/rear). Unless your springs are very mismatched, the shocks generally can compensate and do a very good job of making the desired handling. (For fun and learning…but more detailed and expensive research is needed for actually raising the speed of the car’s cornering limit, which I cannot cover with the scope of today’s writing.)



Anyway, applying the knowledge of common enthusiasts, we can now try to fix your complaint and take some mystery out of your initial confusion.



“I'll try and define it, but it is just that, a tendency to oversteer too
easily. It has an unnerving sensation that the rear wants to 'lift',
and swap around to the front. … With minimal input the car takes a turn, the front takes it,
but the rear seems to want to pitch up and out.”

And…

The best I can describe the handling sensation that I'm looking for is
mild understeer to neutral going into a turn and a touch of 'tucked'
oversteer coming out with throttle.



Reducing roll speed with shocks can eliminate the suddenness that the car falls into oversteer. By working on the rebound side of the rear shocks, you can reduce the lift at the rear and stiffening the compression side at front should keep the rear planted to do its job. All this until you reach the desired cornering attitude predictably.

Next, we work on the throttle induced oversteer… We want to decrease the level of traction so that the car will lose grip with addition of excessive power. To grasp this look at what happens when we step on the gas. If the rear grip doesn’t change while cornering and we add additional force of the engine, we bother the contact patch at the rear. Right? Thus the rear breaks loose. If rearward squat is created quickly upon accelerating, the car is shifting its weight quickly to the rear. This puts more grip at the rear and less in front producing a tendency to push forward upon throttle. So we want to reduce nose lift and tail squat…WAIT! Isn’t this reverse of what we did before? Yes, but now we look at it in conjunction. If your car was oversteering in the skidpad… Then from maximum cornering load, the rear has already a tendency to tail slide…in which case you may not have to do much at all. If it’s the other way and was understeering like a pig, then we need to think. To adjust this by shocks, you will have to go backwards of what we did and that’s not a full solution. So try adding the sway bar to the rear now… We have now added spring rate to the rear suspension only against sideways load. The stiffer rear spring (in terms of lateral motions) should give earlier signs of oversteer, but we can still do more adjustment to the shocks to smooth the curves. The same holds true for reducing the sway bar in front. This is dependent on how much roll you had in the first place. So you can go back to the first paragraph and skidpad test to figure what’s best. Just remember, springs will change both fore-aft roll and lateral roll while sway bars only alter your lateral roll.