Cal Club Autocross Forums

I'm doing the math to figure out ideal shock damping rates, and its becoming a bit confusing. My ideal rate puts me at about 19lb in/sec at one corner, but that seems to be a bit at odds over what I've heard fast people run.
My current shock rates are already way above that, so I'm having a hard time grasping what I'm doing wrong and what works.

What class?
Stock shocks are often quite wacky to make up for too soft springs. FWD's go thought he roof with rear rate to get car to turn..

Most of what I've been looking at is with ST class cars. Not all of them are McPherson so not directly comparable, but the compression and rebound numbers are way higher that what should be required for critical damping. With the rates Bilstein chose for me, it seems to follow the 65% critical damping formula, but from what I've seen from some of the faster cars that doesn't necessarily translate to autox.

I guess I'm just not trusting my math, because the numbers its spitting out don't really reflect what everyone else is doing.

Because AX'ing ...

It was funny when Rad told Ferrari that he needed stiffer springs in his Fiat for AX'ing. "Michael Schumacher decided which springs to use!"

I would pay money to see Ferrari doing AutoX testing.

We have two very talented suspension guys in our region, I would talk to them before you try to calculate shock valving on your own. Guy Ankeny mainly works on Penske's but can also service some Bilsteins. Bret Norgard worked for Bilstein Motorsports for years but now has his own business, you can reach both of them through this forum.

I have no intention for using any valving numbers I come up with, suspension tuning is very much a mystery to me.

This question sort of started because of a conversation I had with a shock engineer I work with. He plugged my car into Matlab and came up with some very basic numbers that didn't quite follow what I've seen other people run.

Bobby Beyer wrote:I have no intention for using any valving numbers I come up with, suspension tuning is very much a mystery to me.

This question sort of started because of a conversation I had with a shock engineer I work with. He plugged my car into Matlab and came up with some very basic numbers that didn't quite follow what I've seen other people run.

Ok, that makes sense.

If you're not already a subscriber, there's a lot of good info in http://www.racecar-engineering.com/" onclick="window.open(this.href);return false;

And this is an interesting read from one of the founders of JRZ http://www.amazon.com/VEHICLE-DYNAMICS- ... 1477247378" onclick="window.open(this.href);return false;

Throw the math book out the window.

As Jason hints at in his post, shocks are often used to make up for other limitations in your car's suspension. Having had some direct experience with this, I could quote you numbers that you wouldn't believe. Spring and shock rates you car needs for racing are far beyond what any road car engineer would recommend: remember that in auto-x we don't really have things like potholes and bumps to contend with.

Talk to an expert.

Talk to Shaik(?) at FatCat Motorsports. He's an expert on Bilsteins and his website has spreadsheet data, etc. for where you should be regarding particular rates, set-up, etc.

Steve Lepper wrote:Throw the math book out the window.

As J̶a̶s̶o̶n̶ ̶ Eric hints at in his post, shocks are often *wrongly* used to make up for other limitations in your car's suspension.

FTFY

Well Marshal, you can change all kinds of parts in your class so you don't have that problem...

My stock class guys with those shocks? They have championships.

Steve Lepper wrote:Well Marshal, you can change all kinds of parts in your class so you don't have that problem...

My stock class guys with those shocks? They have championships.

I thought we were talking ST. Same suspension allowances as SP.

I agree that when you've got nothing else you gotta do what you gotta do.

Marshall Grice wrote:

Steve Lepper wrote:Throw the math book out the window.

As J̶a̶s̶o̶n̶ ̶ Eric hints at in his post, shocks are often *wrongly* used to make up for other limitations in your car's suspension.

FTFY

^yeah that ;). We are talking about ST* which gives you more than enough allowance to correct the poor stock suspension. As stated before, speak to Guy, Bret, or Angelo at ANZE for autox shock experienced builders.

Bobby Beyer wrote: I guess I'm just not trusting my math, because the numbers its spitting out don't really reflect what everyone else is doing.

the math tends to workout fairly well, though not always. You might need to consider that with struts the usual 2:1 rebound/compression split might shift closer to 1:1. The optimum should still stay around an average low speed damping ratio near .7.

that said 19 lb/in/s seems pretty damn low. I'd look for a math mistake.

When the guy was showing me the numbers he was looking at optimal ride and handling on the sine wave as to not go too deep into over-damped territory. The engineer works primarily with off-road vehicles and MR shocks so on road stuff isn't his expertise. I'm guessing what we estimated for tire spring rate is probably way off, didn't have any info on hand so we made an assumption at 18% deflection. With the math model I used which is based off white-noise suspension movement, the numbers did line up fairly well with how Bilstein had my shocks currently setup.


Spoke with Brett but he isn't quite ready to start working on sealed Bilstein shocks yet, and Guy wasn't sure if he would be able to work on my setup.

I assembled my first set of Bilsteins and am currently designing and building a shock dyno, so this is fresh on my mind.

What do you mean by "19 lb in/sec"? Do you mean 19 lbs per in/sec? Is that rebound or compression damping? Is it purely linear or is there some digression to a lower damping rate above some velocity? What are your corner weights? What is your unsprung weight? What is your spring rate? What is the installation ratio of the spring and the shock? What is the spring rate of the tire?

It's not possible to say if "19 lb in/sec" makes sense or not without answering all of those questions. As far as being in the right order of magnitude goes, 19 lbs at 1 in/sec is in the ballpark for low-speed (0 to 3 in/sec) compression damping on the suspension for a non-aero car with a 1.33 motion ratio (you may see it written as the inverse, 0.75), 900-lb corner weight, 110 lb unsprung weight, and 900 lb/in springs.

If you want to make sense of it all yourself, here's a great resource to start with: http://www.kaztechnologies.com/fileadmi ... ch_Tip.pdf" onclick="window.open(this.href);return false;

If you know the basics (ride frequencies, motion ratio, wheel rate, critical damping, etc), you can skip straight to page 13.

Dennis Grant's "Autocross to Win" site is also a fantastic resource, and easier to read than the Kaz Tech Tips. Here are the relevant pages:
Shocks: http://farnorthracing.com/autocross_secrets6.html" onclick="window.open(this.href);return false;
Buying shocks:http://farnorthracing.com/autocross_secrets18.html
Shock forces: http://farnorthracing.com/autocross_secrets19.html" onclick="window.open(this.href);return false;
Shock dyno plots: http://farnorthracing.com/autocross_secrets20.html" onclick="window.open(this.href);return false;
Shock tuning payoffs: http://farnorthracing.com/autocross_secrets23.html" onclick="window.open(this.href);return false;

Dennis Grant (and others) advocate ~65% critical damping from 0-3 in/sec with digression to 15% above 3 in/sec. My only gripe (so far) with that "rule" is that it doesn't take into account the installation ratio's affect on the velocity ratio... a strut moving at 3 in/sec represents very different chassis movement than a shock in a 1.6 motion ratio SLA suspenison.

Spoke with Brett but he isn't quite ready to start working on sealed Bilstein shocks yet, and Guy wasn't sure if he would be able to work on my setup.

What shocks do you have? (part number)

Steve I'm not exactly sure what shocks I have since they are H&R Bilsteins, I know Bilstein has worked on them in the past but beyond that I'm not sure. They did stamp a number into the rear shocks but I didn't mark it down, I do know the shocks are sealed so there very difficult to work on without the correct equipment.

As for the 19lb per inch sec, well that's a bit more difficult to explain, as this answer comes from a shock engineer. The model requires you input tire spring rate, suspension frequency, corner weight and motion ratio. From there the model simulates all the possible forces the shock could see ie bumps, road embankments, everything inbetween, from there it generates a median wheel force and from that median wheel force it generates a sine wave in which you can select where you want to critically damp. At 65% critical damping it recommended 19lb per inch force with a linear curve, I don't remember what the rebound curve but it was a little less than double the compression curve. After we ran the math model he told me he would typically put a load cell on the shock to see exactly what forces the shock is actually seeing.

^My head hurts now.

Steven Snyder wrote:
Dennis Grant's "Autocross to Win" site is also a fantastic resource, and easier to read than the Kaz Tech Tips. Here are the relevant pages:

I remember Dennis....

This is a good place to learn and ask questions
http://www.eng-tips.com/threadminder.cfm?pid=800&page=1" onclick="window.open(this.href);return false;

Bobby Beyer wrote:Steve I'm not exactly sure what shocks I have since they are H&R Bilsteins, I know Bilstein has worked on them in the past but beyond that I'm not sure. They did stamp a number into the rear shocks but I didn't mark it down, I do know the shocks are sealed so there very difficult to work on without the correct equipment.

It's possible to convert the sealed shocks to take-aparts. The procedure is very straightforward. You just have to breach the nitrogen compartment, weld on a steel plate, drill and tap it, and install a Schrader valve. Then they are just like the take-apart shocks.

As for the 19lb per inch sec, well that's a bit more difficult to explain, as this answer comes from a shock engineer. The model requires you input tire spring rate, suspension frequency, corner weight and motion ratio. From there the model simulates all the possible forces the shock could see ie bumps, road embankments, everything inbetween, from there it generates a median wheel force and from that median wheel force it generates a sine wave in which you can select where you want to critically damp. At 65% critical damping it recommended 19lb per inch force with a linear curve, I don't remember what the rebound curve but it was a little less than double the compression curve. After we ran the math model he told me he would typically put a load cell on the shock to see exactly what forces the shock is actually seeing.

That all sounds correct.

So going back to your original post, where you said "that seems to be a bit at odds over what I've heard fast people run": some people are fast not because of their setup, but in spite of it.

I would start with the values calculated by that shock engineer (but maybe make the damping rate digressive beyond some point, like 3 to 5 in/sec depending on the motion ratio).

"Weird" valvings that people run are sometimes due to a lack of knowledge or understanding of damping... but other times it's because their class restricts spring or damper choice and they are using weird damping rates to make up for those arbitrary limitations. Sometimes you can get away with that when the suspension rarely operates in the range that causes problems for such a setup, like on one specific autoX course.

I just tracked my new spring and damper setup at Buttonwillow over the weekend, and all I can say is WOW. I aimed for 1.8 Hz front and 2.0 Hz rear (10% higher in the rear for level ride), and valved my front dampers to around 0.65 critical with high speed digression (I need to finish my shock dyno before I can confirm the exact numbers). I set my Koni rears to full stiff which is as close as I could get to correct until I'm done building my Bilstein struts. Even without proper rear damping, the car feels amazing. Going over even the biggest bumps does not upset the car in any way. Even tall curbing wasn't a problem. My lap times dropped by 4 seconds, despite me taking out some front camber, burning up my brakes, and losing my radiator fan.

Toby Larsson wrote: I remember Dennis....

This is a good place to learn and ask questions
http://www.eng-tips.com/threadminder.cfm?pid=800&page=1" onclick="window.open(this.href);return false;

Sounds like there's a story there. Andy Hollis seemed to think his Bilstein compression mod was worth doing.