34 replies to this topic

[Bench Racer]

For those who care, here are NASAs SM Shock Testing Procedure

Error report: Sorry, we couldn't find that!

 

[marcusmazza]

 

As promised. Here are images of a spec front and rear damper. Overlaid is what a typical race shock for a Spec Miata should look like. Sorry again, I only have CVP data for the spec shock. If anyone wants the data files, let me know. You will need to download Roehrig's software from their website in order to open them (the demo version is free).

 

-Marc

[RazerX]

Hi Marc,

 

Not an expert, or very knowledgeable but curious.  Help me understand the graphs?  Is the red lines the typical SM and green is not legally modified?  is this supposed to show they are illegal because the are very different curves?  Or that they are not different enough  to matter?  

 

thanks

[marcusmazza]

RazerX,

 

The red curve is the legal spec miata shock curve. The green curve is a very different shock, and would most definitely not be considered legal. The point I was trying to make is that the spec shock (red line) is a very poorly tuned shock, and that matching shocks or building to the edge of the tolerance (something like +/- 10%) isn't going to make a noticable difference. The spec shock has very little compression damping, and way too much rebound damping. Playing within the legal tolerance band doesn't change that. If you truly want to make the car handle better, you need to make a pretty big change to the shock valving, and that change should look something like the shock represented by the green line. The green line, which would definitely be a cheater shock, would make a noticable difference. How much of a difference? It depends on the track, the car setup, and the driver.

 

-Marc

[RazerX]

Marc,

 

Thanks, education much appreciated.  

 

So you have a 1.6 and assuming your run SM.  Would just use the Bilsteins as they came from the factory, or would you try to have them match balanced?

 

Just curious.

[marcusmazza]

RazerX,

 

Yes, I run a 1.6 in SM. My car has the Bilsteins as they came from the factory. I have never run a matched set back to back with a set as delivered from the factory, but I highly doubt anyone would be able to tell the difference in a blind test. Just my opinion.

 

-Marc

[Fat Cat]

So obviously I've been away from this thread for a while and it's time to chime in.

 

We use PVP for our dyno testing.

 

In terms of improvements and value, I'd love to have free reign to make the best possible dampers for Spec Miata racers but unless / until that rule is implemented, we have to work with what's there. I also found the stock Bilstein HD (used as Spec Miata shock) damping to be ludicrously rebound-biased. Compression is almost non-existent so the cars are ALWAYS jacking into the bump stops and depending upon which end is getting worked harder (front vs rear) you get changing balance all the time.

 

Our customers have seen lower lap times and improved driveability / consistency with the SM-X valving (the only one we offer, no point in a lower-level service IMO). There's not much room to work with on compression but we do add a little there to resist that tendency for the shocks to immediately bottom out. We also soften the high speed rebound because it improves compliance on berm hits, makes the jacking down "less bad" and contributes to better extraction of grip from the tires. Driveability on wet / dirty tracks is also better due to the reduced jacking down.

 

Begging to differ, you WILL see an improvement in the car's handling with tasteful changes to the damping, even given the rule limitations. Going outside the rules isn't how I roll and I won't work with customers who want to get illegal shocks. I build the best legal shocks I can.

 

From the test results I saw back from SCCA, there seemed to be a slight negative offset in terms of measured force. All our shocks passed, mind you, but doing some data mining I saw the downward trend. I wonder if that might be why there was a problem with the Valafar shocks - dyno to dyno differences.

 

 

 

A few comments reagrding shocks and testing procedures.

 

1. Roehrig shock dynos are very repeatable as long as they are in calibration, and they are of the same type, i.e. crank dyno vs. an EMA (and I'm not just saying this because I used to work for Roehrig). I'd imagine there will be some variability when comparing data across different manufacturers.

 

2. What is the test specification being used? Is it a CVP test or a PVP test. If its a PVP being used, the variability between different dyno manufacturers should be less. It will also make the use of screwdrivers as clevis plins less of an issue. CVP tests are more sensitive to play in the yoke mechanism, slight velocity changes throughout the stroke, loos clevis pins, etc.

 

3. The Bilstein tuning for the spec shock is garbage. Not just because of the variability, the overall tuning sucks. There is no compression damping what-so-ever, and the rebound damping is almost progressive untill you hit the knee and it starts going digressive. I've seen similar tuning before, mostly on the rear shocks of mountain bikes. Re-building them to match or even re-builing them to the edge of spec is like polishing a turd.

 

4. In order to make a noticable change in the handling of the car, or lap times, the valving change needs to be huge. I have some data on the spec shocks that I plan on posting soon (sorry, CVP only) and some data on a 'cheater' shock (CVP and PVP). The 'cheater' shock data is very similar to what the Brits are using. My understanding is their version of spec miata has open shock tuning rules.

 

5. There are a few ways to tell if you're shocks are going bad. As John pointed out, check for leaks, cracks, etc. Also, you can check the gas pressure by pushing on the shock rod. I believe it should be roughly 20lbs of force to compress a miata shock. After that, there isn't much that goes bad on a shock. The valving doesn't really wear. Really, the only other wear item that could lower the shock forces is the piston seal. No easy test for that one, short of putting it on a dyno.

 

 

-Marc

[Fat Cat]

Marc,

 

We are in agreement on the drawbacks of the stock Bilstein HD / Spec Miata damping and rule limitations. The shocks are deficient in bump, over-damped in high-speed rebound and actually deficient in low/mid-speed rebound as well. An OE Showa would behave better in some ways! The other curve you posted is an improvement and far outside the current SCCA specs. In a series where I had free reign, I'd take the design even farther.

 

From your comments about 'polishing a turd', it doesn't sound like you've tested one of our shocks vs. a non-revalved SM suspension so you are speaking theoretically. I've already had feedback from our customers that our methods work and improve lap times. Not an optimal solution, but better. Higher quality shock oil (AMSoil vs. the OE Bilstein fluid) is another benefit to our rebuilds.

 

The OE rod force of the Bilstein HD as measured on our Roehrig 2VS is between 40-45 lb for the front dampers and slightly more on the rear. We keep the pressures in a similar range.

 

Shaikh

 

RazerX,

 

The red curve is the legal spec miata shock curve. The green curve is a very different shock, and would most definitely not be considered legal. The point I was trying to make is that the spec shock (red line) is a very poorly tuned shock, and that matching shocks or building to the edge of the tolerance (something like +/- 10%) isn't going to make a noticable difference. The spec shock has very little compression damping, and way too much rebound damping. Playing within the legal tolerance band doesn't change that. If you truly want to make the car handle better, you need to make a pretty big change to the shock valving, and that change should look something like the shock represented by the green line. The green line, which would definitely be a cheater shock, would make a noticable difference. How much of a difference? It depends on the track, the car setup, and the driver.

 

-Marc

[38bfast]

I have dynoed many SM shocks. Number one issue is consistency. They are all over the map and yes it will affect handling. Second issue is that off the shelf I have seen some that do not meet spec.

[Fat Cat]

Thanks for adding your experience. I think the inconsistency was the reason the spec was created to begin with, yes?

 

Very good reasons to make sure they're rebuilt and dyno-tested for verification / matching. A shock developing slightly more force goes on the heavier corner, etc.

 

I have dynoed many SM shocks. Number one issue is consistency. They are all over the map and yes it will affect handling. Second issue is that off the shelf I have seen some that do not meet spec.

[cpdenis]

I purchased and installed a Fat Cat -MT- bump stop kit on my 99'SM. The bump stops have rotted. I've examined several other SMs with the same setup, some newer, most older and a few are also rotting. The car is garage kept. No chemicals are used to clean the bump stops and car runs 4-6 times a year in Florida. I've tried calling FC's number but I get an answering machine. Has anyone else experienced this? Solutions?

 

Thanks

 

CP

[FTodaro]

Has to be that fla air i have had mine on for 4 years and look good.

 

other venders can provide replacements. Dave advanced auto, drago, others.

[Alberto]

I purchased and installed a Fat Cat -MT- bump stop kit on my 99'SM. The bump stops have rotted. I've examined several other SMs with the same setup, some newer, most older and a few are also rotting. The car is garage kept. No chemicals are used to clean the bump stops and car runs 4-6 times a year in Florida. I've tried calling FC's number but I get an answering machine. Has anyone else experienced this? Solutions?

 

Thanks

 

CP

 

Rotted?  Got pics?

 

Next time you try to call Fat Cat, try to leave a voice mail.  He always returns my calls.  Remember the time zone difference....

[Bench Racer]

FatCat phone responses have changed, in my words because he has sooo many lookers that were absorbing his time. Send an e-mail and leave a voise message about the EXACT subject. He'll respond.

[cpdenis]

Thanks, all. Voicemail was returned. We are working on the issue.
CP