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Calling on the suspension gurus here at Rennlist... Would really appreciate an expert weighing in on the 996TT motion ratios.
I think the wheel-to-spring motion ratio is about 1.0 for both front and rear. The rear is pretty straightforward since both are attached to the hub and the damper angle is pretty close to vertical. The front strut, especially on the non-standard (funky?) AWD design, is less clear, but I think it is between 0.95 and 1.0. It looks like the strut axis passes really close, but aft, of the ball joint longitudinal axis (for lack of a better term). Plus, I found a strut analysis that says most strut motion ratios are between 0.98 and 1.02...
Why do I care? I'm a curious engineer and have a strange need to understand mechanical things. ;-) More specifically, I'm attempting to do some rough estimates on the 996TT suspension related to wheel rates and ride (natural) frequency for the front and rear. If the motion ratios are near 1.0 and the stock Turbo springs are approximately 33N/m front and 60N/m rear (I know they're progressive, but this is a reasonable average), then the stock ride frequency is about 1.7Hz front and 1.8Hz rear, which seems about right, if a little high. That gives a rear-to-front freq ratio of about 1.05 -- close to the 1.1 to 1.2 rule of thumb for passenger vehicles.
I'm running main springs of 570 lb/in front and 860 lb/in rear now (on good coilover dampers). I love the rear for the street and will never consider anything softer, but the front seems bouncy. It could be that my front ride height is too low (maybe I'm hitting the bump stops?) for the stock suspension geometry, but I think it might have to do with my rear-to-front freq ratio, which is about 0.96 -- well below the target range for street vehicles between 1.1 and 1.2. So, I'm thinking of softening the front a little (and maybe stiffening the rear a little) to push the ratio closer to the desired 1.1 to 1.2 range (thereby, reducing the "bouncy" front perception).
This is purely a concern for my usage, which has been nothing but street to date. It is important to note high end dedicated race/track cars offer improved performance with front frequencies higher than the rear. The Cup and RSR cars are perfect examples -- The RSR front is 2100 lb/in and rear is 2,200 lb/in, with front/rear weight splits not too different from our 996TT.
What is interesting... The stock GT3 frequencies are much closer to the desired range, which might explain why folks don't mind driving them on the street for extended periods: https://www.6speedonline.com/forums/...ml#post3627826
Anyway, all this supposition is predicated on the motion ratios being near 1.0. So, somebody please correct me if I'm wrong.
Calling on the suspension gurus here at Rennlist... Would really appreciate an expert weighing in on the 996TT motion ratios.
I think the wheel-to-spring motion ratio is about 1.0 for both front and rear. The rear is pretty straightforward since both are attached to the hub and the damper angle is pretty close to vertical. The front strut, especially on the non-standard (funky?) AWD design, is less clear, but I think it is between 0.95 and 1.0. It looks like the strut axis passes really close, but aft, of the ball joint longitudinal axis (for lack of a better term). Plus, I found a strut analysis that says most strut motion ratios are between 0.98 and 1.02...
Why do I care? I'm a curious engineer and have a strange need to understand mechanical things. ;-) More specifically, I'm attempting to do some rough estimates on the 996TT suspension related to wheel rates and ride (natural) frequency for the front and rear. If the motion ratios are near 1.0 and the stock Turbo springs are approximately 33N/m front and 60N/m rear (I know they're progressive, but this is a reasonable average), then the stock ride frequency is about 1.7Hz front and 1.8Hz rear, which seems about right, if a little high. That gives a rear-to-front freq ratio of about 1.05 -- close to the 1.1 to 1.2 rule of thumb for passenger vehicles.
I'm running main springs of 570 lb/in front and 860 lb/in rear now (on good coilover dampers). I love the rear for the street and will never consider anything softer, but the front seems bouncy. It could be that my front ride height is too low (maybe I'm hitting the bump stops?) for the stock suspension geometry, but I think it might have to do with my rear-to-front freq ratio, which is about 0.96 -- well below the target range for street vehicles between 1.1 and 1.2. So, I'm thinking of softening the front a little (and maybe stiffening the rear a little) to push the ratio closer to the desired 1.1 to 1.2 range (thereby, reducing the "bouncy" front perception).
This is purely a concern for my usage, which has been nothing but street to date. It is important to note high end dedicated race/track cars offer improved performance with front frequencies higher than the rear. The Cup and RSR cars are perfect examples -- The RSR front is 2100 lb/in and rear is 2,200 lb/in, with front/rear weight splits not too different from our 996TT.
What is interesting... The stock GT3 frequencies are much closer to the desired range, which might explain why folks don't mind driving them on the street for extended periods: https://www.6speedonline.com/forums/...ml#post3627826
Anyway, all this supposition is predicated on the motion ratios being near 1.0. So, somebody please correct me if I'm wrong.
John
I see you did not get any replies. (I was searching for 993 MRs for a friend. No reliable data on MRs there either.)
I'll offer a few comments, engineer to engineer:
1. If you want to know the motion ratios of your turbo, better just measure them. If the 996 crowd is like the 964 and 993 people, anyone who knows is not talking. The ones talking don't have a clue. At least some are honest and will ask you why you want to know and why don't you just do what everyone else does? My friend has been unable to find any well-regarded setup shop for 993's that knows the MRs either.
2. I don't know where you got the figure above, but it is obviously incomplete. Information in your own post shows that you know it is, also.
3. I don't think normal people can eyeball a multilink suspension, like in the back of 993 or 996, and have any idea what the motion ratio would be. Maybe a suspension designer with lots of experience could do it. Maybe.
4. I have no idea what they really are, but assuming MRs near 1, even for McPherson struts, would not be historically accurate. If "near 1" includes anything over 0.85, well, ok.
5. When only one end of a car is bouncy it's because there isn't enough damping compared to the spring rate at that end. It has nothing to do with natural frequency differences front to rear. (Not that such difference isn't important.)
11-20-2018, 01:44 AM
996TT Suspension: Motion Ratios?
