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Motion ratio calculation anomaly

Suspension Tuning & Optimization

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Hey guys. Had a really weird situation happen when I was calculating my cars motion ratio. I measured in 1/2 inch increments (stateside, apologies), and everything was all fine and well. When I started calculating the motion ratio from my wheel travel vs the spring travel, I came across a situation where the numbers just jumped really weird. I’ll attach a pic, but basically, the motion ratio went: 1.246, 1.538, 1.054, 3.378 and 1.196. Is it normal for the suspension to have that big of a swing in such a small increase in travel? Thanks for any info!

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What sort of suspension system do you have?

I have seen a similar measurement happen with the bell crank on a push-rod activated system going over centre but this then ended up with the damper length measurement increasing on the other side.

I cannot visualise any other suspension layout that would easily allow for this to happen.

Did you remove the springs and depressurize the dampers? if not, then you may have actually rolled the chassis as you raised the suspension.

That does seem like an unusual situation that would be difficult for most suspension geometry to achieve. David's suggestion is worth a thought though - I'd suspect at this stage that it might be an anomaly with your measurement strategy so I'd be inclined to go back and double check your raw measurements.

Yeah thats what I though, that I messed I messed up the measurements somehow, but I retook the measurements 3 times and aside from very small variations, came up with the same thing every time. I have the spring off the coilover, the suspension is upper and lower control arm. The Eclipse front suspension is weird in that it had the compression arm is anyone is familiar with the DSM platform. I’ll check it again when I get home from work. I’ll also attach some pics of the front suspension so you have a clearer picture of whats going on in the front. In some of the pics, the spring is still on the coilover, but I assure you it is not on there now. And how do I relieve pressure in the shock? I have a very basic coilover with a single adjustment that controls both damping and rebound. Hopefully at some point I can get on a better coilover.

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Your dampers are likely sealed, so no way to easily depressurize them. I work mostly with pure racing dampers that often need to have their gas pressure checked regularly, so we can also depressurize to reduce any "gas spring" affect.

Also, the cars I work with (small formula and sports racers), usually only weigh a few hundred pounds, so holding the chassis down with any suspension spring/force is tricky. For you -- just keep an eye on that jack stand near the suspension corner you are working with to make sure the chassis hasn't lifted off it.

I'm presuming that you are measuring the wheel travel from the centre of the hub or a similar location in relation to a datum point, are you also checking the angle of the hub? I'm wondering if you are getting camber gain behaviour at that point that is skewing the measurements.

What happens if you do that same test but also measure the upper and lower ball joint positions as well, this may show that there is a point in the suspension movement where the arms move in a non linear manner in relation to each other which is causing the hubs vertical travel to be minimal, with the movement being taken up as a angular change.

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