NC - autocross ride height & suspension setup questions

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Brian
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Post by Brian »

Here we go: 18x9.5 all four corners with 45mm RP-F1 18x9.5 and Kumho 285/30/18, pictures courtesy of rpmchris:


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Brian Goodwin
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rpmchris
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Post by rpmchris »

mr2ace wrote:That's Great Chris Congrats !

A couple of questions your jack point measurements exactly where on the lower frame are you taking these from the seam, or inside if the seam, or ?
Your cross weight is that "without" or "with" driver? I assume "without" but if it is "with" can you share how this was accomplished?

Thanks
Richard
I'm referring to the seam where the factory jack "connects" to the frame rail, and although it will get "used" and not be 100% accurate over time, it's "close enough for gov'ment work." I took before and after measurements just for comparison.

For competition setups, I always include the driver's weight - use the plates from a barbell placed in the driver's seat (or gallons of water at 8.35 lbs per gallon) to approximate your weight. Or, do like I did and pay a professional to do it while you sit in your car reading a magazine! :lol:
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Reitrof
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Post by Reitrof »

I'm new to autocross, for adjustable coilovers, do you set the damping to the max?

I can't believe you get 285's to fit, awesome
2008 PRHT Touring True Red, MazdaSpeed body kit
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Post by rpmchris »

Damping and tire pressure settings are all done at the event, and are fully dependent on weather and surface conditions. My typical setup starts with fronts fully stiff and rears a full turn from stiff. That being said, I've only had one event on this setup, so I'll be playing with suspension settings and tire pressures for a while.
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Post by Brian »

Likewise, I usually start with the fronts stiffer because the front tires heat faster and this avoids that first run spin potential. After the first run the rear tires will be up to temp and I usually add stiffness to the rear to balance the car out.
Brian Goodwin
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Reitrof
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Post by Reitrof »

Thanks Guys. I will start with that.
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vman
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Corner Balancing and heights

Post by vman »

Guys,

As Clydesdale sized driver to corner balance the car, even with motons meant big perch height changes, I assume that Ron being of more normal size probably did not have to adjust as much, at the end of the day squaring it up proved better for us...thanks for the feedback on that one Ron. As to ride height, tires and flares, for those running Hoosier A-6's in a 285 30 18 on the RPF1 18 x9.5 wheel Brian sells you will find them about a 1/4 inch per side wider than the similar size Kumho's, that is why we went with fender flares after rolling and rolling those stock flares. I think Brian is now planning on going with the AWR flare kit as well after several rounds of battling the stock flares.

As far as why Ron would see different turning affects with regard to ride height the only thing I can think of is that we are running Mazdaspeed RX8 bars both front and rear, and I think Ron is not running a rear bar, that could change allot in how to approach overall setup.

As to ride heights I have notes from folks running MX 5 cup cars saying 24.6 inches at fender height in front and 24.6 rear is great for an mx5 cup car is the correct height, they then max out front camber around -2.8 to -3 (they compromise caster to get max neg camber) and rear -2.5 to -2.7. In testing the rear of car will toe out on last 1/3 of stroke, the bump steer we see all the CS cars do, so you want pretty stiff rear springs. Due to change in toe due to rubber bushings for big track cup cars run 0 toe front and rear (Note - I realize for Autox we need more toe out in front). In contrast people I know who work closely with Mazda have gone on record to say the car's suspension performs best at 25.5 inch ride height. All the above testing was done with stock 17 inch tires with R compound tires....so the first thing I found when we tried to go really low is we ripped off the front fender liner with the tire (because our wheel and tire combo is 42 mm wider per side than the stuff the cup cars run we hit the wheel well in a different place...ie. we can't go as low as a cup car with the super wide wheels and tires). Note - when we had the fender rub issue we were running a 245 40 17 24.7 tall street tire on a 9 x 17 wheel RPF1, at the time we were trying to max caster and then camber so that could have added to our clearance issues.

Vasilis
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Re:

Post by Reitrof »

Brian wrote:Likewise, I usually start with the fronts stiffer because the front tires heat faster and this avoids that first run spin potential. After the first run the rear tires will be up to temp and I usually add stiffness to the rear to balance the car out.
Returning to an old thread Brian.

What do you stiffen, bump or rebound?
2008 PRHT Touring True Red, MazdaSpeed body kit
RSII, RX8 Sways, KW V3 Coilovers
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Reitrof
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Re: NC - autocross ride height & suspension setup questions

Post by Reitrof »

Brian or anyone?
2008 PRHT Touring True Red, MazdaSpeed body kit
RSII, RX8 Sways, KW V3 Coilovers
Kenwood DNX7100, Nautilis Horn
Ryan @ GWR
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Re: Re:

Post by Ryan @ GWR »

Reitrof wrote:
Brian wrote:Likewise, I usually start with the fronts stiffer because the front tires heat faster and this avoids that first run spin potential. After the first run the rear tires will be up to temp and I usually add stiffness to the rear to balance the car out.
Returning to an old thread Brian.

What do you stiffen, bump or rebound?
Most adjustable coilovers and replacement shocks are single-adjustable. In the case of a single adjustable shock it's a common simplification (perhaps over-simplication?) to just say "stiffer" or "softer" when speaking of shock adjustments because that's how most people think of the changes they're making when adjusting singles. Singles nearly always adjust rebound only, so you can safely assume when hearing references to adjusting singles that people are referring to rebound and not compression/bump.

With a double (or triple, quad, etc.) adjustable shock you have a lot more freedom to change the attitude of the shocks. This is good and bad - you can tailor a double to meet your own needs on a specific track/autocross better than a single, or you can get the settings wrong much worse than with a single. Hey, that's why there are whole books dedicated to suspension tuning! Nothing beats first-hand experience in the car though, so go out there and play with the adjustments. Change only one thing at a time so you can see what that does. If you're new to this type of shock don't just change things a little bit; set compression to full soft and go for a run. *Take it easy at first because the handling will be funky. Then adjust compression to full stiff and try it again. And again, with compression at about the mid-point. Then do the same with rebound. Big changes will give you a broad idea of how that one variable affects the overall feel of the car. As you get more familiar with the differences between compression and rebound adjustments, you'll find you can make educated guesses to which one to change in order to make a certain desired affect to the car. This learning process comes with the territory of having an adjustable suspension and increases with the more things you can adjust, but if you embrace the process of learning your car it can be an enjoyable journey rather than a tedious chore. Have fun!

Keep in mind that shocks are only one part of the suspension, and not always the thing you want to adjust depending on what you want to change about the handling. Competition Car Suspension Has a great flow-chart that covers most scenarios to help you work through when to adjust what. It's worth picking up this book or one similar if you want to dive deeper into suspension tuning:
CompCarSusp-Understeer.jpg
CompCarSusp-Oversteer.jpg
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