Hi all,

I've been running my 2008 Cooper S in D-stock this year. About 6 weeks ago, I installed a H-Sport rear bar as the rules allow in stock class. The bar was set to the tightest setting.

At subsequent events I have noticed that I am more inclined to spin out. It seems to happen at the slightest of inputs. Yesterday for instance I noticed that shifting into second caused a slight wiggle in the rear. Initially I made the assumption that it was all due to tire pressure but I have yet to find the ideal pressure for my tires (Michelin Sport Pilots (215/45/17) so that the car does not feel very loose in the back during autocross. Tires have about a year and a half worth of driving on them so I don't think they are the culprit. Suspension other than the rear sway is stock.

I, maybe wrongly made the assumption that the middle of the three settings was closest to stock? Perhaps I need to change my driving habits slightly to compensate for the bar. Just curious to hear some opinions and thoughts.

 

That is the 19mm right? The softest setting is just a bit better than stock IIRC. I don't know the percentages but I know all settings are stiffer than stock.

 

Yep, it is the 19mm. The documentation with the bar didn't say which setting was closest to stock so I guessed.

 

I recently put the 25mm hollow bar on my car for auto-x. I couple of things that might help:

As you said, you have it on the stiffest setting. I'm not sure what the size of your stock bar was, but even the softest setting might be stiffer than stock. You might try one of the softer settings if adjusting tire pressure doesn't help.

The convention while running a stock rear bar for auto-x is to increase the rear pressure enough to remove grip from the rear tires in order to reduce understeer. With a stiffer rear swaybar, you have the advantage of not needing to run the rear pressure as high because the stiffer rear swaybar is loading the rear tires more heavily in turns as compared to the stock rear ar. As a result, you can lower the rear tire pressures to a level that will give more grip. As an example, pre 25mm hollow bar on my car I had the rear pressure at ~45 psi. Afterward, I've been able to lower the rear pressure to ~36psi and still have what I consider a well balanced car. That's just an example of my non-sport suspension car. But generally, the lower you can get the rear pressure while still maintaining the balance you want and not have the tires roll over on the sidewall too much the better. Long story short, try lowering the rear pressure to increase rear grip and keep from spinning out.

 

Hey Josh,

I did play around with my pressures quite a bit yesterday but nothing seemed to really help. It's possible that I had them both to low and too high. In any event I never found that happy medium. If the softest setting is in fact still stiffer than stock, I'd be inclined to change it to that setting for this weekend and work from there.

 

Sounds like tire pressure alone isn't going to eliminate the oversteer. The amount of grip your tires have (along with alignment, course conditions, ect) will also dictate what setting on the rear bar you will like best. Someone that has the same rear bar but different tires may like the stiffest setting the best. Putting it to the softest setting and going from there sounds like a good idea to me.

On the tire pressure, I've found a couple psi can make a pretty big difference. If you made big changes each adjustment, perhaps you missed the sweet spot.

 

There sounds like there are issues. We added the solid Hotchkiss bar and run it at the middle hole with 3 degrees negative camber up front and don't spin. We also went with a smidge of toe in out back again.

I'd go with the middle hole on yours and try even pressures front and back. If it's cold out then nothing will fix it but rotating tires between runs to build heat.

 

David,

Out of curiosity, what kind of pressures are you running on your STF car? What wheel/tire combo?

 

From my own experiences, I ask you if you are giving up throttle when you begin to feel the back end come loose. Until I could get it through my head that I needed to give it gas in those situations I was sitting backwards on the course quite often.

I have a 19mm bar set at the strongest setting with 49lbs pressure on Star Specs. I now wish I had a 22mm or a hollow 25mm so I could lower the pressure in the rear tires.

 

I would have to say most likely the answer would be no. I'm relatively new to this and still learning how to properly drive a course. So it is possible that the setting I had it at was just too aggressive for my current driving level. In any event, the day did go much better with it set at the softest setting. I managed to take 9th pax of 140 and win my class. I was running 38 front 36 rear and while there was some play in back it was nothing like it had been the past couple of events.

 

9th PAX out of 140???? I'd say you did really well. So from here, just tweek things a little bit at a time.

9th.... WOW!

 

Thanks, it was a good day. There was just a bit of looseness in back but it was very manageable. I think from here I'll just work with tire pressures....and of course my driving skills.

 

You'd definitely gain some overall grip by doing that. Might not cost that much after selling your current rear bar. TireRrack had the best price on the HSport 25mm hollow.


Glad to hear that helped. Driving style can have a huge influence on how a car behaves. Even within good drivers, a car that seems loose to one person may be perfect for another guy.

 

Busy at work these days...

Pressures range from about 40 hot to 48 hot depending on the surface and the wear I'm getting now. The 48 hot was grippy concrete with about 2.3-2.6 camber. The 40 hot was once I got 2.7-3.0 camber up front. Out back it's about 0.5 camber and 0.20 degrees toe in.

rear bar is the hotchkis solid race bar (25mm solid and only $150) on the mid setting. As for driving...I had to change my driving style from HS to STF. I brake a bit earlier and get on the gas before I enter the corner. That helped a lot.

Also...the speed in which I loft off the throttle is the speed of how quickly the back end rotates...for a smooth slow to no rotation I lift very smoothly and slowly..Quick lifts result in a 20-45 degree pop out of the back end.

As for tires it's on Konig 15x7.5 wheels (rollers or litespeeds) in a 32mm offset I believe. weight on the roller is about 14.5lbs and 13.5 on the litespeed. tires are 225 hankook RS3s. No shaving.

 

Interesting...the hankooks must like higher pressures than the dunlops. Running 205's on 16x6.5's, my car likes about 42psi in the front on concrete surfaces. Any higher and it loses grip in the front. I'd expect with more camber and wider wheels, you'll be able to run lower, but the RS3's must like more pressure.

Next step on my car is to fiddle with the alignment. What's your logic for running toe-in on the rear?