Inboard suspension and shock hysteresis

[Jsivils]

I am looking at making changes to my race car this offseason. I have an idea but would like some imput from others that have more experience with inboard suspension.

Currently i have a traditional upper/lower control arm with a 7" stroke coil over bilstein ASN shock. Race tracks range from smooth with minor bumps to ruts, shock speed in the <2ips, to 12+ on ruts. Typically not both at same race. Also the shocks can take a beating in a crash. Finally, we get about 6" of travel at the wheel.

Many higher funded teams use integra or ohlins shocks that minimize both rod pressure and shock hysteresis. It seems on the slow speed/travel areas these shocks keep the car in the race track not on top like the rest of us are. We do not have the funds for them.

I am looking at attempting to go to a push-rod rocker inboard suspension. The positives seem to be more adjustment, less chance of damaging an expensive shock in a crash, the ability to use motion ratios.

One idea I have had is to increase my motion ratio to say, 1.3:1 to increase the travel at the shock. the idea is to obtain more shock travel for a given wheel displacement to help minimize the differences between the bilstein and ohlins shocks. I can go with a 9" travel shock to add more travel ability.

My questions, are as follows:

1. Would this help my situation of budget shocks, getting more wheel control?
2. My understanding of hysteresis is a lag in the shock, would the addition of the push rod and rocker increase this, more mass and more connections to add resistance?
3. Any other tips for helping my situation, I have race car dynamics book getting ready to read and looking to get dixons shock technology book, any other suggestions or tips to reduce rod pressure and hystresis?

Thank you for tips and ideas.

Jason

 

[WolfHR]

I'll jump in before more knowledgeable and more expirienced guys (that means just about anybody around here) come- it might help with certain things but you will also introduce complications which might work to your disadvantage if not carefuly considered.

For example, I was tinkering with hypothetical pullrod system and while I managed to get fairly decent force characteristic (linear with slightly degressive top) what took me by surprise was motion ratio (wheel velocity to damper i.e. piston speed) which wasn't constant even in the linear range...

 

[Toddr8541]

First what do you see as the disadvantages of the current shocks you are using vs the shock the other teams are using?

Do they have a greater number of adjustments?
Do the shocks have a smaller change between adjustments?

You need to know the differences in the two before you make any changes. Understanding this will help you make the best design choice.

 

[Jsivils]

I am not a shcok expert and have not had a chance to read Mr. Dixon's book on them yet.

The shocks that many other teams run are double adjustable, ours are not, we can rebuild between races but just not the click easy adjustments.

The more expensive shocks tend to excel in track conditions that are "dry-slick" which are smooth track, low traction, low shaft velocity situations. One major difference that as a driver I can feel is rod pressure, which I cannot find much info on the subject. The higher the gas pressure the less traction we seem to get, it gives the car a on top of the track feeling. My intial understanding of rod pressure is that it is just like spring rate, however, dropping the spring rate by the same amount as rod pressure is not a predictable change. Old twin tubes or high quality mono-tubes really excel in this area. The mono-tubes tend to run as low as 30 psi, we cannot seem to get below 60 psi without seeing cavitation. The second item seems to be in the shocks amount of "hystresis" newer Ohlins LMP and Intergra IDA shocks greatly excel at this when compaired to a standard bilstein for example. The Integra IDA supposibly uses a gas bladder to replace the dividing piston, which helps it to react. Also both use a base valve set up.

Our shocks do not have the base valve set up, and tend to do well on medium and rough race track. When shaft speeds are 3-6+ ips. My thoughts were that by moving to a push-rod set up I might be able to increase the shaft speeds at the shock to the range where mine seem to be able to react well. I am running a standard Bilstien ASN shock, typically mono-tube, no base valve, aluminum divider piston.

Additionally, running progressive type suspensions that are extremely soft then become stiff at a quick rate are becoming popular, this set up may offer more tuning options besides buying stock in a rubber bumper/spring company.

The differnces in the shocks are adjustment but primarily in the quality and the way they react. ONe thing I had read about the nik-link suspension is that it creates more movement for the shock/spring which helps to compensate for a poor shock.

The final reason, is moving an expensive part to a more easier protected area.

 

[GregLocock]

If you got to a 1.3 motion ratio you will magnify the effects of shock friction, which will give you low velocity problems.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[gt6racer2]

Hi,
Rod pressure does have a "spring rate" impact, but this is not so significant. What you are probably feeling is the knock-on effects of that higher pressure on the damper friction/response.
At very low amplitudes the damper doesn't work like the nice curves you get on the dyno, and stiction/friction become more critical. Friction/stiction comes significantly from the seal, and the seal design/friction level is driven by the pressure you need to contain. Typical simple monotubes ( designed for base pressure 200PSI typical )seals have high friction - double tube seals ( typically 50PSI ) have much lower friction.
You can't run low presure in your simple monotube because the compression damping is generated on the piston and the subsequent pressure acts to push the separator, causing cavitation. The base valve monotube design reduces this issue as there is a pressure drop across the base valve, reducing the pressure at the separator. Separator movement friction is also a concern - so the bladder type that don't move for small inputs work well.
Rather than the trouble of total new suspension for the "in the track' issue I'd first invest in some better monotube dampers. If that's out of price range then maybe try some full displaced twin tubes for these smooth tracks.

 

[Toddr8541]

Have you thought about placement of the shock in relation to the wheel itself. The closer to the wheel the more accurate the dampening for the motion of the tire and wheel.

 

[nobog]

"The mono-tubes tend to run as low as 30 psi, we cannot seem to get below 60 psi without seeing cavitation"

We keep the pressure constant (high pressure mono-tube, 200 psi, snowmobile application) and change the valving. The pressure is used to keep the fluid from boiling and eliminate cavitation - not to tune for dampening characteristics.

"Have you thought about placement of the shock in relation to the wheel itself. The closer to the wheel the more accurate the dampening for the motion of the tire and wheel."

That depends - look at any dirt bike, the shock is mounted as far from the rear axle as possible - the linkages (or in some cases no linkages) do just fine.

Jim

 

[Toddr8541]

Jim
I agree with the dirt bike analogy but in that case the shocks work well because the closer the shock is to the pivot point the smaller the shock and still get adequate suspension travel. The shocks are also valved for those conditions. However, lack of adjustability can be offset by changing the shock location in relation to the pivot points to gain better dampening characteristics (provided you have sufficient shock travel).

Todd

 

[Jsivils]

I am thinking of making some changes, but after some great input I think most my changes will be in the actual shock itself, stepping up to a better shock.

About shock gas pressure as a tuning tool, along the lines nobog mentioned, it is used as a tuning tool instead of valving for some people. However, what I am talking about is rod pressure and it's effect on traction, the rod pressure seems to remove much of the drivers feel and decrease traction greatly. It is hard to explain, but the car just has a strange or ontop of the racing surface feel.

I thank everybody for their input, I am going to do some more reading and see if I can get a better set of shocks. Lots of great input on this forum and has helped to point me hopefully to some good results.

Thanks guys.