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FC motion ratios

Old 10-31-13, 04:04 PM
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FC motion ratios

I know the front is pretty much 1:1 probably closer to 0.95:1.

Just Curious if anyone knew, or has measured what the rear motion ratio was on the FC?


for anyone that doesn't know, motion ratio is the ratio of the distance the spring moves in relation to the distance the wheel moves. You use it to find out the actual spring rate at the wheels (wheel rate)
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Old 10-31-13, 07:43 PM
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there is a thread about it in the suspension section, its hard to find though, because the search doesn't separate FC and FD very well

i wanna say, .86 is the number, but don't hold me to it
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Old 10-31-13, 09:10 PM
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Old 11-01-13, 01:10 PM
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.88 through a 4 inch sweep around ride height, iirc. Measured it when I was buying shocks last.
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Old 11-01-13, 01:17 PM
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Originally Posted by HotRodMex View Post
.88 through a 4 inch sweep around ride height, iirc. Measured it when I was buying shocks last.
Awesome, thanks!

did you measure the fronts? what size tires was that with? (would that change it much?)

I imagine my 315 width front tires make my struts significantly less than 1...
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Old 11-05-13, 01:38 PM
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That was at the hub. I guess I didn't think about wheel width affecting the measurement significantly enough for my purpose. I didn't measure the fronts for the same reason (adapting dampers from another strut car). I've seen most strut fronts be in the range of .95 though.
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Old 11-06-13, 01:13 PM
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Asked two friends, a Formula SAE student and then an alum, and they both said wheel width didn't come into the calculation of motion ratios.
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Old 11-06-13, 01:34 PM
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Originally Posted by HotRodMex View Post
Asked two friends, a Formula SAE student and then an alum, and they both said wheel width didn't come into the calculation of motion ratios.
I don't know how offset can't be involved if camber is changing as the suspension compresses. But I think that's splitting hairs...

A relatively general .95/.88 figure is fine for most needs.
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Old 11-08-13, 01:16 PM
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I wonder if the tire contact patch evens it out somehow.
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Old 11-10-13, 01:28 PM
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Originally Posted by eage8 View Post
I don't know how offset can't be involved if camber is changing as the suspension compresses. But I think that's splitting hairs...

A relatively general .95/.88 figure is fine for most needs.
Motion ratio in suspension of a vehicle describes the amount of shock travel for a given amount of wheel travel

so its wheel travel/shock travel.

i don't see how camber matters in making the calculation?
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Old 11-10-13, 08:38 PM
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Originally Posted by j9fd3s View Post
Motion ratio in suspension of a vehicle describes the amount of shock travel for a given amount of wheel travel

so its wheel travel/shock travel.

i don't see how camber matters in making the calculation?
going to extremes... if you had a foot long wheel spacer, if there was any camber gain in the suspension travel it would make the tire move up in relation to the bottom of the shock.
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Old 11-11-13, 11:39 AM
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Originally Posted by eage8 View Post
going to extremes... if you had a foot long wheel spacer, if there was any camber gain in the suspension travel it would make the tire move up in relation to the bottom of the shock.
hmm, yeah i guess with your wheels that is worth a look.

actually speaking of i wonder what the um steering axis thing does with your ginormous tires?
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Old 11-12-13, 03:10 PM
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Originally Posted by j9fd3s View Post
hmm, yeah i guess with your wheels that is worth a look.

actually speaking of i wonder what the um steering axis thing does with your ginormous tires?
scrub radius? it's not terrible. the new 18x12s fit with a -10 offset... which is 50mm different from stock aka, 2". My increased kingpin angle from the camber plates fixes some of that... so it's probably not that bad. I haven't actually measured it though.
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