Van R shocks problem

[Math ze engineer]

Hi
We bought 4 new Fox Racing Van R shocks 2 years ago but we only tested them on a car this year since we failed to get the car done for fsae MIS 2008. Well this year the judges found that our suspension was stiff. Let me explain.

When I designed the suspension, I took one of our Van R without his coil and compressed it on a scale. it gave me around 100 lbf of push to START the compression then it just goes to the end of its travel WITHOUT adding more force. As we had 250lbf/in coils I designed the suspension to have the car go down one inch on its own weigh with the pilot in. In reality, it doesn't even go down by 1/16" (well the tires compress a bit). So I brought them on our schools Instron traction machine (used to break metal samples). I made 6 tests with two shocks: without coil, with 250 lbf/in on without preload and 250 lbf/in on with preload and same thing for the second shock but with a 400 lbs/in coil. We found on the result plots that we have a 300N(65lbf approx) offset on each curves.

Now I notice 3 things. First our scale is a piece of crap. Second, I think I did something wrong on my design because the car doesn't go down even if it was designed for a 100lbs preload. Third, why the hell do our shocks need 65lbf to be compressed!?!?! Most people in competition told me that it is not normal but the 4/4 shocks are having this behavior. Can you help me with this?

Thank you

[Math ze engineer]

Hi
We bought 4 new Fox Racing Van R shocks 2 years ago but we only tested them on a car this year since we failed to get the car done for fsae MIS 2008. Well this year the judges found that our suspension was stiff. Let me explain.

When I designed the suspension, I took one of our Van R without his coil and compressed it on a scale. it gave me around 100 lbf of push to START the compression then it just goes to the end of its travel WITHOUT adding more force. As we had 250lbf/in coils I designed the suspension to have the car go down one inch on its own weigh with the pilot in. In reality, it doesn't even go down by 1/16" (well the tires compress a bit). So I brought them on our schools Instron traction machine (used to break metal samples). I made 6 tests with two shocks: without coil, with 250 lbf/in on without preload and 250 lbf/in on with preload and same thing for the second shock but with a 400 lbs/in coil. We found on the result plots that we have a 300N(65lbf approx) offset on each curves.

Now I notice 3 things. First our scale is a piece of crap. Second, I think I did something wrong on my design because the car doesn't go down even if it was designed for a 100lbs preload. Third, why the hell do our shocks need 65lbf to be compressed!?!?! Most people in competition told me that it is not normal but the 4/4 shocks are having this behavior. Can you help me with this?

Thank you

[Drew Price]

Nitrogen charged dampers will always require force to compress the shock, the amount of force is proportional to area (diameter) of the shaft and charging pressure (which is probably pretty high on new Fox's).

I made some similar errors with my first suspension, the springs I specced didn't settle nearly as much as I thought they should have. Take this opportunity to double check the math in your design, and double check that what's on the car is actually what you wanted! I found out after 3 months of hunting for answers (along with looots of other troubleshooting that took precedence) that the motion ratio of my suspension front and rear was about 35% off from what I thought it was.

It turned out that during an initial re-design of the rockers the geometry got inverted. Didn't find the mistake till after comp. Finally answered my question of why the car needed ~2x stiffer springs than I thought it needed to handle decently.

I found some errors in my suspension design spreadsheet too.

Moral is, try to isolate the things that could be causing the problems and work from there.

Like double check that some of your problem with getting the car to ride height isn't that there's a lot of stiction in your suspension somewhere, especially in the shocks and their pivots with really small motion ratios, make sure you rock the can back and forth so the tires aren't binding ride height changes in scrub, but 60-100 lbf to compress the damper seems pretty high, you might see about how low you can take the nitrogen pressure before you start having frothing issues.

Best,
Drew

[Math ze engineer]

I just thought I could fix this problem by preloading the rocker. If I push the rocker with a screw to preload the shock by 65lbf so it would start working on its linear curve as we get the car on wheels. This screw would act as an endstop on full rebound and cancel the 65lbf offset on spring rates. It would also avoid stretching the shock on full rebound. What do you think about it?

[Zac]

Does this shock have a "pro-pedal" feature? If so, that's probably your issue.

[Math ze engineer]

it doesn't have this feature I think its for air shocks