Shocks & Motion Ratios

[Chris Boon]

Hey all,
To those teams running the Penske 7800's or any shocks that arent adjustable for that matter, I was just wondering what you guys did to get past the fact that their are no external adjustable nobs on them. Im guessing a lot of testing is in order, or adjustable sway bars?

Also, I have heard through the grape vine that design judges dont like motion ratios that deviate too far from 1. My thinking is that with large deviations from 1 you would get too much variation in your spring and shock forces as the rocker rotates because your motion ratios actually change slightly as they rotate. This is because the moment arm changes slightly as the rocker rotates. My explanation is a worded pretty crap but anyway, what do you guys think?

Cheers,
Chris

[Marc Jaxa-Rozen]

You can get linear geometry (or your choice of progression) mostly independently of motion ratio, I'd look at wheel and damper travel first - with typical FSAE travel values, something close to 1 is usually a good choice.

Doug Milliken did, however, mention that a 1:1 ratio would make it easier for the target weekend autocrosser to select wheel rates.

Marc Jaxa-Rozen
École Polytechnique de Montréal
http://www.fsae.polymtl.ca

[GSpeedR]

The reason (I assume) that judges don't like MRs far away from 1 is because if you don't travel the shock much then seal friction will become a singificant component of the total damper force. If you travel too much, your shocks are unnecessarily big, bad for weight and packaging.

[jack]

penske makes shocks?? thats weird, i thought they just made dampers.

waht are shocks anyway? should i get some?

[Chris Boon]

Well I consider a damper and shock to be the same thing but if you consider a shock to be the damper + spring then yes they only supply the damper. The Penske 7800's I think are called 1/4 midgets in the States. I was talking to one of their reps and he said they could supply us with springs that would suit their dampers if required.

[jkaz]

There are several reasons why you want motion ratios of 1:1, NOT less than 1:1.

As far as springs are concerned, the lower the motion ratio of shock/spring to wheel travel (shock/spring travel less than wheel travel), the higher the spring rate required for a given wheel rate. In other words, for a wheel rate of 18 N/mm with a motion ratio of 0.75 you would need a spring rate of 32 N/mm! The higher rate spring is heavier, puts more load into components, and causes more friction in the shock because of side load from the spring.

As far as shocks are concerned, shocks want displacement and velocity to work properly. The lower the motion ratio, the smaller the shock displacement and the lower the velocity. A shock is a velocity sensitive device; the higher the velocity the more force it produces. Back to our 0.75 motion ratio example. So, with a 0.75 motion ratio 25 mm of wheel travel will result in 18.75 mm of shock travel. 50 mm/sec of wheel travel will result in 37.5 mm/sec at the shock. So with your 0.75 motion ratio your shock now has to produce the same force at 37.5 mm/sec as it would at 50 mm/sec with a 1:1 motion ratio. This increase in shock force results in high pressures in the shock, often requiring higher gas pressure. It also may mean higher seal friction.

So, consider carefully. A small weight penalty for a higher motion ratio (larger rocker arm and longer shock) must be compared to heavier, higher rate springs and shock displacement/velocities that make the shock work less effectively.

If you have any questions, you can write me at jkaz@kaztechnologies.com

[Sormaz]

Originally posted by DohertyWins!:
The reason (I assume) that judges don't like MRs far away from 1 is because if you don't travel the shock much then seal friction will become a singificant component of the total damper force. If you travel too much, your shocks are unnecessarily big, bad for weight and packaging.

You have mentioned a negative of running too small a motion ratio as well as something bad about running too high a motion ratio, however these arguments say nothing specific about the number to shoot for.

If you had a super sensitive aero car, would you still be shooting for 1:1?