Custom designed shock length - a question!

[cstreit]

So I'm going to be sending the Bilstein RSR's out to have them shortened, both the piston rod and the housing itself.

Why? Because the RSR shocks once turned upside down are essentially left with no travel at all. SO...

The question of the hour is:

How much?

I am going to be using 8" springs and need to figure out exactly how much compression travel I need. This will allow me to calculate the amount of reduction in the shock body length (since I am at zero travel now). This will then translate into the piston rod length, based on the overall resting length of the shock, in order to center the piston in the body itself...

How much shock travel (on a lowered racecar) does one need on compression? What is typical? 3 inches? 4 inches?

[Kevin Stewart]

two inches is enough but it always seems i get 4+

[jluetjen]

I would think that the question is bounded by how far your suspension can travel from it's static ride height before it hits something else (like a bump stop, tire to fender, etc). You don't want the shock to restrict movement prior to you running out of travel. If it does it will essentially become the bumpstop. This would not be a good thing since a bump stop has some designed in "give" while your shock will just go solid.

Why not just measure the center of the shock's travel now, and then the length that the shock should be when at rest in the car. The difference is what should be removed.

Be aware, that just my SWAG.

[dickster]

John is right. You need to have enough travel to go from full extension to full compression of the suspension - plus a bit for safety.

I think the easiest way to measure would be to measure the full movement of the suspension between the points that the shock will be mounted) wihtout the shock attached.

You do not want to bottom out the shocks. Of course the proper way to do it is to try it on the car. suck it and see.

[Wil Ferch]

I think factory spec for full shock travel is on the order of 7" ....

--Wil Ferch

[cstreit]

Originally posted by jluetjen
I would think that the question is bounded by how far your suspension can travel from it's static ride height before it hits something else (like a bump stop, tire to fender, etc). You don't want the shock to restrict movement prior to you running out of travel. If it does it will essentially become the bumpstop. This would not be a good thing since a bump stop has some designed in "give" while your shock will just go solid.

Why not just measure the center of the shock's travel now, and then the length that the shock should be when at rest in the car. The difference is what should be removed.

Be aware, that just my SWAG.

Thanks John, THat's sort of what I plan on doing. I though to block the car up at standard ride height with the wheels off and struts disconnected... Then measure the required travel by moving the suspension up with a jack... This will tell me how much I have maximum.

I have a feeling that my total available movement will be much more than the 8" springs I'm going to use will allow. I bet you they compress less than 3 total inches. Beyond the spring travel, I then need to leave 2" additional for the bump-stop rubber, and then probably another 1" of clearance "just in case" to prevent hitting the shock piston to the bottom.

I'm assuming that one needs less over-center rebound travel than compression?

[dickster]

dont know, what about when you go over a crest?

it all depends on the springs, weight of the car etc.

[jluetjen]

Chris;
I think that the bump-rubber (fully crushed) will define your maximum compression -- you shouldn't need any additional travel in that direction. In regards to droop, it get's more complex -- how low do you want to go? In general suspension droop is good -- it keeps the wheels on the ground. But in some specific cases, you can have your shocks set to act as droop limiters -- yes many places will do this if you ask.

[MotoSook]

Keep in mind that there will be very little body roll, and this is a track car. Unlike a street car, the "from rest to fully extended travel" won't need to be much. Rebound after compression will be the "from rest to fully extended travel" plus the compression distance should he hit a bump. From rest to fully extended travel is critical should he hit a pothole on the track.

If he goes airborne after a hill it won't matter how much "from rest to fully extended travel" he's got. (enough air and any car will lose all suspension travel) It's a compromise he has to make for a track car. Cresting a hill should result in some compression, but not much. So figure on adding a little of that to the "from rest to fully extended travel."

So is 2 inched "from rest to fully extended travel" adequate, and is 3 inches from "rest to full compression" adequate, for a track car?

With an extra inch of margin to prevent "bottoming" out in either direction, he would need 3 inch from rest to fully extend, and 4 inches from rest to full compression. The bump stop on the piston shaft can be trimmed down to about 1 inch to prevent metal to metal contact on compression beyond 3 inches (note: the bump stop on the shaft will compress some, but not to the point where the piston hits the bottom of the shock housing..a bad situation).

Alas, all of this is just wheels turning in the space between the ears, until he fiddles around with moving the wheel on his car. We'll also want to find out how much travel he's got now, and where a 7-inch travel piston will sit once it's all bolted together. That is, well want the piston in that 2-3 inches from bottom positon... geez..I'm having a hard time picturing all this in my poor head.

[jluetjen]

Souk;
Actually competition cars generally have more travel in droop then in compression. Cresting a hill is one situation where droop counts -- but not necessarily the most important to a race car. You also need to consider the weight transition when braking and turning. When you brake, the car will pitch forward (which may or may not be a bad thing). If you have limited droop in the rear, it will limit the amount of pitch that the car can travel. If you trailbrake into a right hand turn, limiting droop of the right rear wheel will tend to reduce the weight transfered to the Left front wheel and reduce understeer -- until the wheel comes off the ground at which point it's braking contribution will be 0 and the car will most likely suddenly change to understeer.

Now the bump stop should not be considered as something which is there to prevent metal to metal contact. It actually is a rising rate spring to cushion the transition to a solid suspension at the limit of travel. Clever use of bump stops can often provide a "rising rate" suspension in classes where multiple springs are not allowed. Carroll Smith often wrote that tuning the bump stops was one of key aspects of setting up a successful race car.

While cutting down the bump stop will provide a little more travel, it will also make the transition of the car from suspended to bottomed more drastic and as a result remove a degree of "forgiveness". If you cut down the bump stops you will often find that the car will have snap oversteer/understeer whenever the suspension bottoms which is often a recipe for disaster.

I don't know if this made your head feel any better.

[MotoSook]

John,

My head feels worst...

You just added dive to the equation, something I overlooked! Yes, we'll want rear tire contact for sure. How much dive will the car exhibit? Argh!

The thing with the bump stop is that I just don't think one should rely on it. At the point where the bump stop comes into play, the shock will have compressed 3 inches in the above hypo-shock above. If one were really limited in suspension travel, them we'd be looking at a bump stop design as you mentioned. In that case, I better go have a few beers to cool the grey matter. I'm not discounting the benefit of the bump stop, just that it may be beyond Chris' ability ..no really, how would one find a tailor made bump stop? So in the above, the bump stop would most like be useful for that last bit of travel. Remember he has to trim the bumpstop to give the shock more free travel.

My biggest concern for Chris' car is that the adapters that have been supplied for his shock and spring mounting will determine his travel distance, which (my greatest fear) will be smaller than the hypothetical shock above.

But! Knowing Chris, he's probably rolling his engine out of the way as I type to make room for a little soul searching while staring at the suspension from a Yoga position in the engine bay.

[jluetjen]

Tailoring a bump stop is easy, you take out a pocket knife and you...

1) ...Do NOT whittle down the height unless you want to make the bump-stop stiffer
2) ...Can extend the slope of the cone shaped area which will make it softer. By changing the shape, you change it's charactoristics. Thinner cross section = softer ==> more travel. Depending on where you thin it down you can even change the shape of the progression. Stacking up bump stops will extend the progression
3) ...Call up most shock shops (Such as Truechoice in your neighborhood or Essex) and order some different shaped bump stops from them.

[MotoSook]

John, you are taking the fun out of this!

[jluetjen]

'Sorry Souk,
Here, let me argue the conventional wisdom...

Chris don't worry about the shock length. Everyone knows that stiffer springs are better. Mount your shock as it is so that the suspension binds up solid -- that's better then the stiffest T-bars and sway bars! Spend the money that you saved on custom 30 inch wide wheels and you'll really be hauling!!!

[MotoSook]

I'm speechless!

[banjomike]

with Tbars the max droop could be calculated using Hooke's law (unsprungweight=springrate*displacement). You know the weight (or could measure it), and the spring rate, solve for the displacement. If that displacement is greater the the physical limit of the suspension travel then the physical limit is your max droop.

In the case of coilovers, I reveal my ignorance. I'd guess that those new springs are pretty dang tight. Does the wheel ever 'hang' from the springs or can the wheel hang below the max uncompressed length of the spring with the spring loosing contact?

If the wheel does hang from the spring then Hooke's law will give you your number. If not then some other physical restraint will be the determining factor.

[jluetjen]

Mike; in your first paragraph you're talking about compression -- not droop -- aren't you??? Droop is when the wheel travels below the static ride height

As far as coil springs are concerned, the same law applies up until you reach the point of having the coils bind. If the coils are not evenly spaced, then it might be progressive and your into a whole 'nother analysis.

[MotoSook]

I got my speech back (John I hope you're not getting tired of this gadfly act)

The shocks that Chris has have compression and rebound valving that is unlike any street car shocks I have seen. They are very resistant! How fast will they allow "droop" on the track? Droop rate? Is that a technical term?

Droop with a torsion suspension...is there? The T-bar will exert a force on the suspension at all time, expect at that one spot where there is zero load on the T-bar.

[jluetjen]

How fast will they allow "droop" on the track?

The answer to that is on the shock dyno graph.

Droop with a torsion suspension...is there? The T-bar will exert a force on the suspension at all time, expect at that one spot where there is zero load on the T-bar.

Lots!!! Jack up the back of your car and you'll see how high you have to jack it until the rear wheels finally come off of the ground. As far as the load, check it with the car jacked up and the tire clear of the road. Try to lift the wheel in it's travel. Go ahead -- try to lift it!

[MotoSook]

Yes, but it's not really droop, is it?