# Grassroots Motorsports Article 10/2010- skinnier tires better?



## nlink720 (Apr 16, 2008)

I compete in the STR solo class in my BMW Z4. Being a new class this year, most people seemed to agree that the widest wheel and tire combination possible would yield the most grip, and the best results. Accordingly, I went with a 255/35-18 setup and 8.5" wheels on all four corners. Keep in mind that my car originally came with 225/40-18 front and 255/35-18 rears on my "sport" version, and the "non-sport" originally comes with 225/40-18 all around.

The Grassroots article was somewhat persuasive in that it uses supposedly accurate data to support its claim that sometimes wider is not always better. They settled in on a 245 tread width as better than a 255. The only vague part was that the 245 was 3 pounds lighter than the 255, so there is most likely some contribution to the result as a result of the reduction in weight too -- what percentage is the part I wish they would have tested to find out. 

Nonetheless, most people seem to agree that wider front wheels cause vague steering feel -- something that I strongly agree with since my former 225 front tires were razor sharp in feel compared to the numb 255 front tire feel. I figure also that my car, being much heavier than the Miata, would probably benefit by spreading the weight more via a wider tire width, but I have to say that I greatly miss the steering feedback I had on the narrower front tires.

So my question is: what if I went from 255/35-18 (current setup on all four) to 245/40-18 on all four? I want to stay non-staggered for handling balance and front-rear rotatability. I much prefer the 255 square balance over my former 225 front and 255 rear stagger. 


There seem to always be a million opinions on the "wider is better vs. xx battle" but any ideas about the GRM article would be appreciated; especially if founded in personal experience.

Thanks


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## controller (Aug 15, 2010)

I just got a z4 3.0si coupe and was planning on getting rid of the stagger myself. I was debating on 245 vs. 255. My concern was that the 255 wouldn't fit. Which tires did you go with? The Dunlops are .5" wider in 255 trim. I was debating if that extra half inch was worth it, considering price, weight and overall diameter. I will say back when I raced in STX with a WRX, a friend of mine would beat me in 235 dunlops, when I had the 245s. Granted he had a lot more skill than me, but still had a skinner and shorter tire. I am thinking about a 17" wheel because its a little lighter as well.

What are you wheel offsets? Did you have to use a spacer?

Also do you have a LSD? which?


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## nlink720 (Apr 16, 2008)

I currently have 18x8.5 wheels with 255/34-18 no problems. I wanted a 9" but it would likely protrude 7mm when the camber is not at max. I am going to 245/40-17 and will let you kno after sunday


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## jadewombat (Jul 27, 2010)

My wife's Mercedes came with factory 225/17 rims, daily driver. I autocrossed it a few times with the street tire, then went with some 16 rims and some slightly stickier 225 Kuhmo tires. I like the way it spins up quicker off the line with the smaller rims, haven't noticed any difference in the steering feel though.


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## RCC55125 (Mar 12, 2010)

Don't forget one reason for the staggered set up from the factory is to promote understeer on the street, primarily as a safety issue. You generally want to promote oversteer for Auto-X. One easy way to do this is with smaller rear tire section. If 225's give good bite on the front then they should be fine for the rear. You will then have a better chance of inducing power on oversteer. I think you will find the best set up for Auto-X is 225's all the way around.
I recently did a few track sessions with 225/40-18 front and 255/35-18 rears which is the standard set up on my ZHP, same as your Z4. Tires were old Potenza RE040 street tires. Handling was fine in fast sweepers and slow corners as well with no hint of oversteer. I then changed to 225/45-17 Yokohama Neova AD07's on all four corners, a much sticker compound that the Potenza's. The handling was excellent with this set up with no hint of oversteer coming out of second gear corners even with traction control off. This it the set up I would auto-X with a 330i.


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## nlink720 (Apr 16, 2008)

RCC55125 said:


> Don't forget one reason for the staggered set up from the factory is to promote understeer on the street, primarily as a safety issue. You generally want to promote oversteer for Auto-X. One easy way to do this is with smaller rear tire section. If 225's give good bite on the front then they should be fine for the rear. You will then have a better chance of inducing power on oversteer. I think you will find the best set up for Auto-X is 225's all the way around.
> I recently did a few track sessions with 225/40-18 front and 255/35-18 rears which is the standard set up on my ZHP, same as your Z4. Tires were old Potenza RE040 street tires. Handling was fine in fast sweepers and slow corners as well with no hint of oversteer. I then changed to 225/45-17 Yokohama Neova AD07's on all four corners, a much sticker compound that the Potenza's. The handling was excellent with this set up with no hint of oversteer coming out of second gear corners even with traction control off. This it the set up I would auto-X with a 330i.


I strongly disagree, with all respect. 255s ont the car, square, gave much more grip than the stock setup of 225 F and 255 R. But there will be a limit. So far, 245 square seems to be an excellent compromise between feedback and grip. I just switched to 245-40/17 and it is outstanding. 225 square would leave huge amounts of grip on the table, while I feel 255 may be too much. The car felt disconnected, and vague compared to reducing to 245.

I wonder how 255R and 245 front would fare, but the sqareness is nice for rotatability.


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## RCC55125 (Mar 12, 2010)

I'm not sure what is correct for your Z4 but if 225's on the front were not washing out under hard cornering you may not have to go much bigger. You mention that 245's allow you to rotate the car and that's what you want for Auto-X. Another way to judge the tire size is how much do you break away at launch. If the 245's hook up within two feet they may be too much tire. You want about 15% tire slip for close to 10 feet for maximum tractive effort. If the 245's are getting slip off the line and you can make the car rotate going to 255's may take that away. If the 245's don't slip that much at launch maybe 235's would be a better compromise. Regardless of what size you pick a larger rear section width only promotes understeer and you don't want to do that for Auto-X.

Here's an interesting link that will fuel the fire, http://www.performancesimulations.com/fact-or-fiction-tires-1.htm.
It starts off:
"Only air pressure and weight (tire load) dictate contact patch size. Tire width has nothing to do with it. If you double the width of a tire you cut the length of the contact patch in half. You get the same contact patch **area** either way. This is described by the following equation reasonably accurately (maybe within a 5-10% error range?):
tire_pressure = weight / contact_patch_area
Therefore: contact_patch_area = weight / tire_pressure."
Enjoy.


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## nlink720 (Apr 16, 2008)

We are digressing, since the original concer was what would be fastest: 255, or 245. The GRM article suggested that 245 was 0.7 faster, and that wider is not always better. So far, I believe them, but have not reached conclusive results with respect to the Z4.

You are righ that the force is the same regardless of width. but many feel thr reason wid is better is because cornering imposes side loads that w wider setup is better able to resist.


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## RCC55125 (Mar 12, 2010)

I don't subscribe to GRM but read the article posted on line. Their test car was a 325i which is heavier than your car with a little less HP. The wheel size and lag times shown are:

Wheel size: 16x7.5 inches 
Tire size: 205/55R16 
Mean lap time: 43.353 sec. 

Wheel size: 17x8 inches 
Tire size: 225/50R17 
Mean lap time: 43.155 sec. 

Wheel size: 18x8 inches 
Tire size: 225/50R18 
Mean lap time: 43.339 sec. 

It looks like the 225/45-17 was fastest. If you have a set of 17" wheels it might be worth a try to go smaller than 245. I still think your best set up will be 225/45-17 on all four corners.


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## nlink720 (Apr 16, 2008)

That's why we were having the disconnect -- we are referring to different articles. I read what you are referring to -- I am referring to this month's article done on the STR Miata.


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## controller (Aug 15, 2010)

RCC55125 said:


> I don't subscribe to GRM but read the article posted on line. Their test car was a 325i which is heavier than your car with a little less HP. The wheel size and lag times shown are:
> 
> Wheel size: 16x7.5 inches
> Tire size: 205/55R16
> ...


Your logic is way off. The 17" was faster than the 18" because the overall diameter was shorter which gave an advantage to gearing. I am sure the weight of the rim/tire combo was less as well, which helped. But 17x8's can handle 245's and I am sure they would be faster on almost any application outside of a very lightweight car that doesn't need the tire, but not the BMW.


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## RCC55125 (Mar 12, 2010)

Why don't you call the guys at Tire Rack and get their advise, they ran the tests referred to in the GRM article.
I don't see how my logic can be that far off when the figures I referred to are for a BMW and have the 17" tire come out faster (43.155) than a 16" (43.353) or an 18" (43.339). These figures should be closer to your car than for a Miata.
I think 245's would be too much for the front even if OK for the rear. That may mean 235's on all 4 corners. However, you said the stock 225's felt razor sharp on the fronts and that's what you want. You don't want a staggered set up but maybe 225 F, 235 R would produce the best results. The reason I keep coming back to the 225 R is for Auto-X you want the back end to be loose. It certainly would not be the best set up for the street, I'm only thinking about the autocross course. If you plan to drive your set up daily then the 245 square may be the best compromise.
Again, call the guys at Tire Rack and tell them what you have and what you want to do. They have a lot of BMW's in their fleet and I'm sure one of them drives a Z4. You can also Email [email protected] he will be glad to help.


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## Uptowngurl (Oct 5, 2010)

Such as a good ideas and interesting link that will fuel the fire.


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## TeamM3 (Dec 24, 2002)

As always, it just depends. First, your car is not a Miata in more ways than you can imagine Second, generalities are only that, regardless of which direction they're applied in. In that case the tire size availability and selection skewed the results IMO. 

In this class you have full control over handling balance and transition feel through shocks/valving, springs, swaybars, bushings etc. There's no reason to choose an undersized tire for this purpose like may be warranted in the stock class. IMO you would be better served to choose a 2006 - 2008 z4 coupe 6-spd for this class. It should be a real contender in STR if properly equipped, setup, and driven. I seriously considered pursuing it myself, but in the end cannot currently justify two competition vehicles and don't want to part with the current one. Given the power output potential of this choice I personally would choose to locate used SSR Type-C Comp (early 1-pc version) wheels and 255 Hankook RS-3 tires. You really can't go wrong with this combination for national level competition.

255s fit and even wider isn't an issue.


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