wanna911

So adding canistet pressure is like adding spring rate, but I have heard that running at zero is not good for the shocks. Is there therefore no way to utilize your actual spring rates with no additional added from the canisters?

ProCoach

I defer to Tim Olson and Angelo Zarra on this one, but I KNOW you never want to run external canister pressures at zero. Most shock manufacturers specify a starting or nominal pressure to prevent aeration of the oil and insure adequate fluid control.

https://rennlist.com/forums/racing-a...pressures.html is a good start.

Veloce Raptor

What Peter said. Suspension is an integrated system...

Matt Romanowski

Canister pressure is dependent on the shock brand and design. Companies like Penske and Ohlins run fairly low pressures and I know engineers who have run Penske shock as low as 50 psi. Companies like JRZ, Moton, and MCS (all based off the original JRZ design) typically run more pressure, larger shafts, and work by moving a larger volume of oil. You have to run a minimum amount to avoid cavitation at the piston regardless of shock design and manufacturer.

The spring rate change is from the volume taken from the shaft entering the shock body and displacing fluid. You can get around this with through rod dampers, but those are not cheap or common at the club ranks. Some of the Cup Cars have run them. It's more important to have a good shock engineer that can make your shocks work rather than having a specific brand.

ProCoach

The Penske 8100's on my successful sports racer (and the dozens of similar cars I ran for two decades) specified 200psi canister pressure and did not specify or recommend any change from that range of settings.

More good info from multi-time National Champion Dave Weitzenhof here:

"IMO, adding canister pressure DOES NOT ADD BUMP DAMPING AND/OR DIMINISH REBOUND DAMPING! It DOES shift the raw (gas load not subtracted) damping-force curve upward as explained in the next sentence. The real effect is to add to the load that the damper carries statically (and offsets the damping curve - that is why the pressure load needs to be subtracted from the damping curve, so you can see the actual DAMPING force values), similar to cranking up the spring perches and compressing the spring. The result is that you need to set canister pressures before setting ride height. If you set the pressures after setting ride height, and make a big pressure change, the ride height will also change. In addition, there is a minor increase in spring rate and seal friction when canister pressure is increased.

On checking damper pressures - when I check with the Penske unit (pressure source not attached), the unit's volume makes the damper pressures decrease by about 7 to 8%, based on absolute pressure (gauge pressure + 14.7 psi)."

A great manual covering all ranges of Penske shocks (including tuning methodology) is here: http://www.penskeshocks.co.uk/downlo...TechManual.pdf

Cory M

Joe Stimola told me that 50# of canister pressure was equal to approximately 15# of spring rate on Motons, about the same for JRZ. Once you've arrived at the right spring rates you may be able to use the canister pressure to tune. Considering most race cars are running relatively stiff springs, from a percentage standpoint it doesn't appear that the canister pressure would be a big contributor to the overall spring rate (at least for the Moton numbers Joe quoted). You can't "utilize your actual spring rates with no additional added from the canisters" unless you reduce your spring rates to match the increased rate from the canisters. You definitely don't want to run zero pressure, unless you ride a pogo stick for fun. I ran a session with almost no pressure once by accident and discovered several bumpy sections of track, that had always been smooth before! Not fun!

ProCoach

I heard that "50psi is equal to 8-15 lbs spring rate" anecdotal observation, too. Man, I miss Joe...

Here's another good resource: http://www.shock-shop.com/Thoughts_o...ock_Tuning.php

Matt Romanowski

That is the pretty old recommendations. Penske more than anyone is running lower and lower pressures to reduce stiction and hysteresis. Check out http://www.penskeshocks.com/assets/T...ing%202-04.pdf They used to have another document about running low can pressures and how it helped with shock function, but I can't find it right now.


You can with a through rod damper. That is why they developed them.

Cory M

you already said that in your first post

I'm assuming the OP doesn't have through rod dampers

JoeMag

Hey Guys -- Regarding shock pressure to increase spring rate... It really does not do much of anything. Try it your self... I assume most do some sort of corner balancing. Run your shock perch down a few inches, put a scale on a jack under the rotor, and set shock press at 100psi, then move jack up and record difference in height and change in weight on scale (springs being rates in pounds/inch). Set shock press at 200psi and so same thing and see how much spring rate went up. Note you have to get perch low enough so spring is just "floating" and not touching top perch during this test.

This is what I did and found out the change in spring rate by increasing pressure is very small compared to race spring rates.

forklift

Can't add much here except to say that the worst my car has handled is when I lowered my JRZs to 100 lbs years ago. They like around 200 or so in my car.

ProCoach

Agreed. If the spring rate needs to be changed, change the spring...

wanna911

Very good info, thanks. Doing some suspension tuning this weekend and my mind goes numb thinking about all of the various combinations. I like to have at least some idea of what does what to give feedback to my mechanic.

Matt Romanowski

Buy this e-book from Ross Bentley and you'll have a great, short primer on your shocks and tuning them. It will help you immensely. The best $2.99 you'll spend today.

https://speedsecrets.com/product/shocks-for-drivers/

wanna911

Bought it and read it. Now I need to find out what 2 way JRZ RS Pro's Im running are doing with turns of the ****.

Tim? How much high speed is being adjusted with the low speed per click?