STR PRHT Coilover spring rates

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STR PRHT Coilover spring rates

Postby Castro » Tue Jul 14, 2015 6:14 am

Anybody has any experience with spring rates on a PRHT for STR class AX? Ready to pull the trigger on Ohlins coilovers.

9kg front and 6kg rear seems like a popular choice for soft top cars. Would this work for a PRHT?

Currently this is my setup...

Progress/Koni's
FM sway bars front full stiff/ rear middle position.
17x9 RPF1 on 245's Rival S.
Alignment... Camber: -2.5 front/ -2.0 rear Toe: 0 front / 1/16 toe-in rear.
OEM LSD

I'm averaging 3 autocross events per month or about 75% AX, 25% street so being competitive is definitely my priority over comfort. I do 2 events in smooth asphalt lots (Turner field and Atlanta Motor Speedway) and 1 on rough asphalt.

I thought abut running the 7/4 springs rates and upgrade later but regretting not going with coilovers from the beginning I want to do it right this time. Any suggestions?
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Re: STR PRHT Coilover spring rates

Postby Brian » Tue Jul 14, 2015 11:47 am

Yes, everything that works for the soft top guys will work for the PRHT, the slight additional weight on the PRHT is enough for the factory to adjust rear spring rates by just a few pounds, which is nothing once we are talking 6kg rear rates.
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Re: STR PRHT Coilover spring rates

Postby Curves » Tue Jul 14, 2015 12:35 pm

I upgraded my Ohlins springs to 9/6kg and like it much better than the stock 7/4kg. Much more responsive with no noticeable loss in comfort on the freeway. With the higher grip levels and speeds of the RE-71R, I would like to try higher spring rates esp. on the high grip El-Toro surface. But that would mean the time and cost to re-valve. I may wait a little to see how the ND turns out in AX.
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Re: STR PRHT Coilover spring rates

Postby Castro » Wed Jul 22, 2015 11:04 am

The coilovers are on the way...

I'm going with the 7/4 springs for now and got a question on sway bars... I'm currently running FM sways and still have the stock sways laying around in the garage.

With Koni/Progress and the bars at full stiff front and the middle setting in the back the car is neutral with maybe a touch of oversteer.

Any recomendation on what setting to set the bars to begin with? I know it will take some trial and error but if anybody has any experience with similar thickness bars it will be muc appreciatted!
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Re: STR PRHT Coilover spring rates

Postby Brian » Wed Jul 22, 2015 11:09 am

I don't know the FM bars in particular, but the 7/4 spread on the Ohlins is enough that I don't expect you will suddenly have too much oversteer. Thus I would leave the sways alone on whatever settings you have... until you get a safe autocross opportunity to test the limits.
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Re: STR PRHT Coilover spring rates

Postby rastap » Wed Jul 22, 2015 1:45 pm

Brian wrote:I don't know the FM bars in particular, but the 7/4 spread on the Ohlins is enough that I don't expect you will suddenly have too much oversteer. Thus I would leave the sways alone on whatever settings you have... until you get a safe autocross opportunity to test the limits.

Would this the general recommendation when going with coilovers? Leave the sways alone until you see how the car responds?
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Re: STR PRHT Coilover spring rates

Postby Brian » Wed Jul 22, 2015 1:52 pm

rastap wrote:
Brian wrote:I don't know the FM bars in particular, but the 7/4 spread on the Ohlins is enough that I don't expect you will suddenly have too much oversteer. Thus I would leave the sways alone on whatever settings you have... until you get a safe autocross opportunity to test the limits.

Would this the general recommendation when going with coilovers? Leave the sways alone until you see how the car responds?


No, it depends on the spring rate spread. For example, if the customer had our TEIN coilovers with rates 6/5 then most full sway sets will give them more oversteer than most folks would want and we would suggest they actually pull off the rear sway entirely until they get a chance to try the new balance under controlled autocross conditions. They might thereafter put back the stock rear sway but that front to rear spring rate spread is so tight that most are not going to want upgraded rear sway with the TEIN coilovers.
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Re: STR PRHT Coilover spring rates

Postby Curves » Wed Jul 22, 2015 10:08 pm

I run RB Front set to middle and Rear Eibach bar disconnected for Ohlins with both the 9/6 and 7/4 setup. 1/4 inch rake. Some local folks connect the rear sway on our grippier El-Toro surface and disconnect it at Qualcomm where there is much less grip. I tried it re-connected at El-Toro 2 weeks ago, alternating between the four 3-run practice sessions. It was too loose for my liking.

Ken Montonishi who has won a few Nationals including 2010 STR in a MX-5 drove my car on Sunday for two runs and he said the balance feels good. Jeff Cawthorne who drove my car when I had the rear bar connected thought it was a little too loose. So I think I am on the right track.

Of course YMMV. It depends on surface, temps, course and driving style. Bring tools so you can adjust...
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Re: STR PRHT Coilover spring rates

Postby Castro » Mon Jul 27, 2015 7:02 am

Update:

Got the coilovers in with the 7/4 springs. Got the car aligned with the following specs

Ride height: 12.5 F, 12.75 R
Camber: -3 Front, -2.5 Rear
Toe: 1/16 out Front, 1/16 in Rear

Took the car to a regional event yesterday. With the FM front bar full stiff and the rear full soft the car was faster but a tad loose. Still set FTD in the morning runs. After the lunch break disconnected the rear and it was pushing a lot but improved by half second. Tweaked the rebound settings to try to dial out the understeer and improved another 3/4 of a second but still pushing a bit. Set FTD with a 1.2 second lead of a SSM turbo NA.

Going to try the OEM rear bar and see if I can get it more neutral with just a touch of oversteer which is my preference.

Here's a pic of the car with the Ohlins in and a link to a video of the fastest run. Any feedback is appreciated!

https://youtu.be/jLIqdIA8d24
Attachments
IMG_6799.JPG
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