Which bump stops for C Street autocross NC2 MX-5?

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Which bump stops for C Street autocross NC2 MX-5?

Postby Shawn.Alexander » Tue Jan 19, 2016 10:07 am

I have a 2009 MX-5 Touring that is being built for C Street autocross duty. This car will hopefully be Nationally competitive. I have moved to this car from a ESP Mustang that was very competitive and don't want to run at the back of the pack if you know what I mean.

Setup:
09 Touring w/sport suspension, LSD
Koni sports/ stock springs
Kosei K1 TS 17x17 +50
RE-71R's 225/45/17
RoadsterSport Race single
Rear Sway Bar (Various being tested, Whiteline 16mm currently)
K&N drop in filter

So my question is what bump stops do I use? What have people found that will work with a rear bar and not a front. I have to worry about the rulebook that states the bump stops can't be contacted any sooner than stock. With the Koni's giving an ever so slight drop in ride height, I have worried about legality.

What I'm thinking is a firmer bump stop up front, due to the stock front bar, and a softer bump stop out back. Thoughts? What about lengths?
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Re: Which bump stops for C Street autocross NC2 MX-5?

Postby Brian » Wed Jan 20, 2016 1:01 pm

Indeed the rule book is telling you stock length on stops. Most are running stock rear sway, upgraded front sway and using stiff rear stops to help rebalance the setup. Since you are using upgraded rear sway, indeed you may need some stiffer front stops to avoid having too much oversteer.
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Re: Which bump stops for C Street autocross NC2 MX-5?

Postby Shawn.Alexander » Wed Jan 20, 2016 6:28 pm

What lengths should I start with?
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Re: Which bump stops for C Street autocross NC2 MX-5?

Postby DavidM938 » Wed Jan 20, 2016 9:13 pm

hi Shawn I don't have an answer regarding bump stops but have a question regarding tire sizes on the required 7" rims for c street. I have a 13 club and am planning on getting the re-71r's this spring. what are the pros and cons of using such a wide tire (225) on the 7" rims vs. either 215s or even the 205s which would have less grip but better sidewall support and steering response? whats the consensus? thanks david in nashville
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Re: Which bump stops for C Street autocross NC2 MX-5?

Postby Brian » Wed Jan 20, 2016 9:58 pm

Wide tires on the narrow rim feels sloppy. Autocrossers in Street Class make it work by doing everything they can under the rules to help the car keep the tires flat to the pavement (stiffer front sway usually, adjustable Koni Sport shocks, stiffer bumpstops, etc). Typically run a lot more pressure than normal in the tire so it doesn't roll over as much. Get all of these details right and running the fat tire squished onto the 7 inch wide wheel can result in a faster result, even if it feels sloppy and less precise (which it does).
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Re: Which bump stops for C Street autocross NC2 MX-5?

Postby Shawn.Alexander » Thu Jan 21, 2016 6:57 am

They don't offer the "feel" of a smaller tire, but they do allow for a precious few more MPH in 2nd gear.

So back to bump stops. What lengths are people starting with????
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Re: Which bump stops for C Street autocross NC2 MX-5?

Postby Brian » Thu Jan 21, 2016 5:25 pm

Yes, taller tire helps that 2nd gear go faster.

You need to measure those stock bumpstops, I simply don't recall at this moment the length of both front and back...not that I would ever do it from memory anyway, as a competitor YOU must make sure your car is legal and you must make the measurements so you are confident you pass a challenge if that comes (which only comes at national level if you blow everybody's doors off). You buy the stiffer stops, trim them as needed, per this rule:


Here's the rule:
13.5.
D. A suspension bump stop is considered to be performing the function
of a spring. Therefore, the compressed length of the shock at the initial
point of contact with the bump stop may not be increased from the
standard part, although the bump stop may be shortened for the purpose
of installing non-standard shocks. Bump stops installed externally
and concentric with the shaft of a shock may be drilled out to fit a larger
diameter shock shaft. Bump stops may be substituted for the purposes
of installing non-standard shocks.

Notice you must avoid contact the bumpstop earlier in the stroke. You will need to measure your OE and Koni Sports to figure lengths and trim the bumpstop length per the rule above. Sorry, no shortcut here, I can't 'give' you the answer, reading the rules and figuring your interpretation of the rules is part of the SCCA game, welcome to the fun part!
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