Advisable GC springrates for a canyon carver...

[JeffRR]

Thanks, I just enjoy the topic of vehicle dynamics and am lucky enough to work in the field.

I'm no expert, the guys that have been doing it for years are the experts and the ones that I really respect.

I've been in the field for about 10 yrs, dam that makes me feel old.

[DSP74]

The only time I've seen people get into trouble with to much rebound is when it was combined with to high spring rates, and low speed rebound, and it was causing the car to be skatey on turn in. And in which case it's obviously wrong....

A couple other instances on big money Z06 Penske shocks was so much rebound that the shock never recovered before the next bump and the car actually packed down because of it. And of course once all out of travel you start doing all kinds of bad things.

[Fair!]

The only time I've seen people get into trouble with to much rebound is when it was combined with to high spring rates, and low speed rebound, and it was causing the car to be skatey on turn in. And in which case it's obviously wrong....

A couple other instances on big money Z06 Penske shocks was so much rebound that the shock never recovered before the next bump and the car actually packed down because of it. And of course once all out of travel you start doing all kinds of bad things.

Agreed... setting a multi-adjustable shock to super high rebound rates is normally avoided but it can happen in some cases. I've seen the situation where the car went over a series of bumps the a very high rebound setting (not on some of our shocks, but another) caused the shock to steadily ratchet down onto the bumpstops... funny to watch, not funny when you're driving it. :D

As for the poster asking about vehicle dynamics, a good place to discuss the performance end of vehicle dynamics (if you have thick skin) is here. Lots of good discussions from people inside the automotive industry, from professional racers, and other folks with a lot of "book learnin" and seat time...

[Slyder]

Just came across this post and I see a lot of good information, especially from Jeff, DSP and Terry.

I wanted to share a little of my experiences with the suspension, especially concerning the spring rates and the shocks. I agree that what you seem to be having is a condition of over-lowering (travel) and the softness of your springs. I always tell people (especially when they're looking at coilovers) that the lower you want to go, you will need a stiffer spring to compensate for lost travel.

However...you also need to consider the sort of surfaces that you will be driving on. If most of your roads are fairly smooth, moving to a stiffer spring may not be so bad. In your case (judging from the photo),my opinion is that you're a little low for the rates you are running. As a result, you're most likely riding the bump stops and giving the shocks hell. The 'zip-tie' that Terry mentioned is a good, inexpensive way to find out if this is actually the case.

One of the things you can do to help some of your lost travel in the front is to switch to a shorter body shock with appropriate valving. For your purposes, a VW G60 Corrado insert would help and it can handle spring rates up to 450#/in. In the rear, the e30 M3 shock is also a good alternative as it is 1" shorter than the regular e30 shock when fully compressed and more aggressive valving. Also, for DD cars and aggressive driving, as well as some track use, 6" 450/in springs in the front combined with 5" 750#/in springs in the rear offer the best of both worlds without being harsh. Stiff, but not harsh.

[DSP74]

[quote=Slyder;1026048]
One of the things you can do to help some of your lost travel in the front is to switch to a shorter body shock with appropriate valving. For your purposes, a VW G60 Corrado insert would help and it can handle spring rates up to 450#/in. In the rear, the e30 M3 shock is also a good alternative as it is 1" shorter than the regular e30 shock when fully compressed and more aggressive valving.

I completely agree and this is the issue we are dealing with here. The strut housings need to be shorter. I don't beleive that as a street driven vehicle that those rates are too soft. BUT they are to soft that low. And IMO the car isn't to low either. Properly matching wheel travel to ride height is key to not bottoming with soft springs...In my old ESP car you were REALLY close to being to low. As rediculas as it sounds 1/8" of ride height to low had a noticeable effect.

Either go stiffer or do things right and shorten the strut housing.

All IMO but I think I would pass on shorter than stock rear shocks unless bottoming the rear shocks becomes an issue. Using stock length shocks on a lowered car affords more droop travel which is a good thing on the rear of these cars..

[JeffRR]

[QUOTE=DSP74;1019922]The only time I've seen people get into trouble with to much rebound is when it was combined with to high spring rates, and low speed rebound, and it was causing the car to be skatey on turn in. And in which case it's obviously wrong....




I think in an auto-x you might be able to get away with a little more of a non-balanced valving. For the most part auto-x surfaces do not have large elevation changes which cause large primary ride type body motion. Patched asphalt / large sharp edge pot holes also might not be very common either. Basically the inputs are pretty low in amplitude which give you some latitude in your tuning.

Street & track use with large amplitude inputs, such as elevation changes, square edge potholes/ patched asphalt, ¼ ~ ½ wheelbase inputs. Over damped in rebound will cause tug, like you said where the car packs down and will not return to the static ride height. The wheel drops into a dip or hole and instead of the wheel dropping quickly and the contacting the ground so the sprung mass will compress the spring/shock, the entire corner drops into the hole since the wheel dropped too slowly to support the sprung mass.

I’ve had experience during development of watching the axle of cars/truck actually hanging up for a split second after an input. We also use a large smooth input to excite the primary ride mode (large amplitude/low frequency), with a car that is over damped in rebound you’ll be able to feel the tug as an abrupt shock after the car initially compress but you can also see it from the outside.

I whish I had some video, its pretty cool in a dorky kinda way.

I have heard that very high rebound damping has been used in NASCAR to keep the car lower hence reducing the airflow under the car for qualifying. The car hits a few bumps on the way out to the track and packs itself down and by the time its back in the pits to be measured its back up to its legal min. I don’t know how common that is just a story that was passed on to me.

Speaking of balancing comp/reb damping, there is a technique that I think is somewhat common in pro series. I sat through a lecture on using it but I haven’t had a chance to try the technique myself.

Basically you measure the shock shaft speed at each corner and analyze the shaft speed data in a histogram format. Ideally you would work towards having an equal number of accounts in each speed bin on the compression and rebound side. The valving can be adjusted accordingly.

Here’s a screen dump from a pdf I found to give yall an idea of the shaft speed histogram.

[el_gringo]

Thanks for assuming I don't know what to drive either. Oh my gosh a guy that wants to modify his car for performance? Not just looks? That must be completly foreign to you guys...

take a deep breath. count to ten. relax. Your gonna find yourself gettin pineapples shoved up your ass from these OG's

[Kilomph]

Great thread. I'm going to re-read all of that again.