Race Miata

I've mentioned it in another topic in the UTH forum but I think motion ratios are too technical for the UTH and is more suited for the R&C forum. If anybody could point me to any numbers, that would be much appreciated.

In particular, I'm wondering if we have a lower front motion ratio than rear? If that's the case, I can't understand how Honda came up with stiffer rear springs than front.

TIA

twohoos

For the AP1,
0.82 front
0.77 rear

Race Miata

Thanks for the quick reply, John. Are those the numbers that have been flowing around?

I forgot to mention that I've seen those numbers as I mentioned in the UTH thread. I doubt the numbers are correct. It looks to me that the shock absorber lower mount point is no further than 75% of the lower arm (both front and rear). That means the motion ratios have to be lower than 0.75. Plus the front shocks seem to lean more than the rear. That suggests lower front motion ratio than the rear.

Anybody else heard different numbers?

TIA

Kel

What is the suspension motion ratio and why is it important(or whats it do)?

Orthonormal

The motion ratio represents the leverage that the wheel has on the shock/spring due to their relative mounting locations along the suspension arms. You multiply the spring rate by the square of the motion ratio to get the effective stiffness acting on the wheel.

Race Miata

Suspension motion ratio is the mechanical leverage of the suspension geometry to transfer the load from the tire to the spring. The lower the motion ratio, the stiffer the spring needs to support the same weight of the car on that axle. In the higher level of suspension tuning (above swapping off-the-shelf springs/sways, etc.), you start from a set goal of wheel rates and work back thru' motion ratio to get spring rates. With a good setup, wheel rate for each axle should reflect the corresponding weight on that axle except for about 15-20% bias towards the non-driving axle (i.e. 15% towards rear for FWD and 15% towards front for RWD). Then, you choose sways to fine-tune front/rear balance. This was how my miata and BMW suspension were done.

Since the conversion between wheel rates and spring rates involve the SQUARE of motion ratio, those numbers have to be accurate or your numbers will be way off.

For me for the S for the moment, I want to understand why Honda runs so much rear bias on spring rates on the S with 50/50 weight distribution. If motion ratio is higher on the front compare to the rear, that would be understandable. But according to my educated guess, front motion ratio seems to be lower than rear. And that should require even stiffer front springs than rear.

twohoos

Hmm, my source for those is the same as Windscreen's (the old MVMSS). Spitfire's diagram does seem to indicate something lower, but then the MVMSS is pretty explicit about quoting wheel rates directly, which is really what we're after, right?

Could the geometry of the front upper a-arm (mounted further outboard than the lower arm) effectively increase the motion ratio?

Kel

Cool, thanks for the explaination!

Race Miata

Do you happen to remember what the wheel rates from the MVMSS? From my calculations base on 219/291 spring rates and 0.8160/0.7721 motion ratios, I get 146/173 wheel rates, which is not ideal for a 50/50 RWD car anyways.

I thought about the upper arm geometry and also the rear-toe-control link they could only further "decrease" motion ratio. It's not common (if not impossible) for a double-wishbone suspension to have over 0.8 motion ratio. My miata with very similar suspension pick-up points has 0.692 and 0.766 (IIRC).

twohoos

Yes, they were 146/174.

I've never heard this rule of thumb about why any particular spring rates are more or less ideal? Remember there are swaybars, different tire widths, damping effects... Seems like there's a lot of ways to get to neutral handling.

Anyway, have you ever attempted a measurement of the wheel rates? Might be enlightening.