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Progressive rate springs - FB with FC subframe

Old 03-01-18, 10:15 AM
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Progressive rate springs - FB with FC subframe

I am finishing up my FB endurance racer build which included swapping in front FC subframe/suspension.

I have the subframe bolted to the bottom of the frame rails and per discussions around the internet this creates a roll center issue due to the control arm angling up at normal ride height.

My understanding is that this means as the suspension compresses, the roll center lowers, thus increasing the leverage on the springs as it rolls. I understand one solution is to put a big front sway bar on to mitigate roll, but my concern is then that might create excessive understeer.

My question is this then: What about progressive rate springs? The thought being if the spring rate increased with compression, this could mitigate the increasing forces created by the control arm angle.

Has anyone tried this? Truthfully I feel I may be overthinking things but thought I would throw the theory out there.

Thanks in advance for any input.
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Old 03-05-18, 07:36 AM
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I don't think progressives are a good idea for a track car. Problem is that at turn-in you're on the softer rate so you have a high roll rate, then when you hit the higher rate the suspension not only has to deal with lateral g's, but it also has to slow and stop the roll from a faster roll rate.

IMO the roll center being lower when the car is lowered is overstated as being a "problem". Think of it this way, you are reducing roll moment because the car is lower, and at the same time gaining mechanical grip at the front due to the front suspension being somewhat less "anti-roll" in its geometry.

I'd just try it and see how it goes first before trying to fix a problem that might not exist... Or if you do want to compensate, do it with slightly stiffer linear-rate front springs than you would otherwise consider.
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Old 03-06-18, 09:40 AM
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Thanks ZDan. Yeah, I knew I was overthinking it. I'm just going to put a set of decent rate springs on and see where I am.

I might buy a heavy front bar and also bring a stock one to the track and maybe swap them out if it's unbalanced.
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Old 03-06-18, 02:06 PM
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I think it's best to just fix the roll center properly and not chase down an appearance as priority. Progressive Rate Springs + Excellent Damping with near zero high speed rebound and a higher compression to rebound ratio at low speed is going to be amazing. ZDan isn't wrong, it's just down to the damping curve you build in for your car. I'd rather look at Tarmac Stage WRC cars that have progressive spring rates or dual rate suspension w/ excellent damping instead of a car that spends its life on a relatively smooth racetrack as my benchmark.
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