Confirmation of Motion Ratio

Hello, I'm making my way through the Suspension course and enjoying it lots. I have an old British sports car from the '70's, with a few track days completed I'm looking to make some handling improvements.

I'm having trouble being confident with the rear motion ration that I have calculated.... well.... calculated is a bit of an overstatement, as it's [1]!!! (Maybe a little less, if you take into consideration the 9 degree angle of the coil over. The arrangement is a simple double wishbone independent set up, but the design has the shock attaching to the hub carrier, so the shock has the same displacement as the wheel, so a 1:1 ratio.

I'm rather constrained by the OEM suspension geometry. Max droop (rebound) is limited by the UJ's binding on the drive shafts, max compression is limited by tyre/wheel arch contact and also limited by the very little ground clearance (exhaust under chassis).

I measured the original rusty springs that where on the car before I rebuilt it, they were 140 lbs/inch front and 130 rear. With the un-sprung weights I measured when assembling the car, the rear frequency was 1.666 Hz and the front was 1.060 Hz (because of the motion ratio at the front).

A common change by folks rebuilding these cars is to use the adjustable spring seat and single adjustable rebound/compression coil over units as pictured.

I needed smaller springs than the originals to fit these shocks, so purchased 130 in the correct size.

Damn, it was so soft!! I had to screw in so much preload just to maintain ride height, that the spring would coil bind over just a medium bump. So then I thought, I should fit a longer spring, so there would be more travel before coil binding happens. This was technically correct, however the spring (only 2" longer than the previous one) would 'banana' and catch on the top of the shock body. Not acceptable at all!

In an effort to fix this issue, I decided at the time (before finding your course!!) to increase the spring rate a bit and see what happened. I ended up with 155 lbs/inch springs, 10.5" long. So not too short to coil bind, and not too long to banana. The car felt good (still feels good!). If anything, it felt adequately soft enough….. By my "bouncing my hands on the back of the wheel arches" test and doing the same with friends cars, it was soft. At this point I began tuning the dampers (still learning on this one), starting from full soft and working my way harder doing different drives, different road conditions.

Now…. Doing your suspension course, following the maths (and verifying it with another online calculator) I get a ride frequency of 1.819 Hz - this seems a lot, and quite stiff.

Considering the suspension frequencies you showed in the course, I ought to be aiming for a lower number at least nearer 1.5 Hz - for a fast road car (occasional track use).

Could I indeed have 1.819 Hz even though I think it is soft?

Any suggestions on how I could reduce it, considering my past experience?

Because of the limited suspension travel, I end up hitting the bump stops quite often on bumpy English country roads! I currently have fairly solid rubber/poly bump stops that are quite savage, but give me the most 'normal' travel possible. I have also managed to get it so that the bump stops come into play at exactly the time the exhaust scrapes!!

I've tried playing around with the damper settings, but they don’t seem to have much effect on these 'big' bump situations. Considering the frequency is so high, I am not inclined to increase the spring rate…. Or should I be not so worried and increase the spring rate to reduce the amount of hard bump stop situations?

I was considering swapping to foam bump stops, but the last time I had the shocks out I struggled to remove the top eyelet as it had been locktited on!! And was nervous applying heat to the shock.

Thanks a lot for the great course so far… looking forward to learning more and applying some modern thinking and knowledge to the old car!!

I suspect a couple of errors.

The first, assuming because it's attached to the hub carrier it's doing to have the same 1:1 ratio.

In practice, the uneven wishbone lengths will mean the carrier rotates as it moves through its travel, the greater the distance from the wheel from the damper mount, the greater the affect will be. You can plot this on paper, if you have the correct measurements for the pivots and lengths, or you can use software which allows you to articulate it. In the distant past I've even made up card representations of suspensions to better visualise their movement.

The second is the damper angle needs to be when loaded as I would expect, from the picture, it will substantially change through the suspension travel.