13 or 15" wheels for autocross

[oilteq]

I have always liked the idea of 13” wheels instead of 15” wheels.  They put us an inch closer to the ground, lower our gearing, are lighter weight, and the front tires can be wider because they are lower than the fenders.  The down side is selection.  I am further limited in that I do not have a trailer or a sag wagon/companion, so I have to drive to an autocross.

My question is, would I be better off with 13” wheels and a set of Falkens or Toyo Proxies, or is it better to go with 15” wheels and figure out something with fenders to fit wider tires? 

[ProfGriff]

I pondered on this for a while after I bought my 7 last year. 13's have a definite advantage for autocross, but you are limited for tire choice unless you go to Hoosiers. If you get the right compound, Hoosiers will definitely out perform Toyo's or Falkens. Me, I decided to go with 205/50s on 15x7's because of the greater tire choice, and it's also what I'm running on the street. Also, I'm cheap, and my Maxxis RC-1s were waaay cheaper than Hoosiers. I can also use the RC-1s for track days and not totally use them up.

[BirkinBernie]

Before I bought a trailer, I got a deal on a set of Kodiak wheels and Hoosier slicks.  Compared to 15x7s and street tires, the Kodiaks and slicks cut 60 lbs off the car - and rotating mass at that.  In order to use the 13's I built a rack to carry them on the Birkin.  It worked great.  Got some really strange looks from people on the highway though....

 

 

 

 

[Jesse D]

I autocross my Seven a lot and switched to 13s and slicks at the same time- first set of slicks were Hoosiers but now I'm on my second set of Avons (from Roger Krauss). I like the 13s and I like the Avons better than Hoosiers- my feeling is the Hoosiers are made for Miata's and Avon understands how light a Seven is. The Miata drivers that ride/drive my Seven all love it, other than the pedal box and seat width:-)

 

In Maine I'm automatically a modified car, being a kit car, so no class penalty for slicks. I'd switch to 13s rather than my 14s for the street too if I could find a set of 13s- I think our cars were designed for more sidewall.

[oilteq]

Thank you for the comments.  It sounds like 13s are the way to go.  I know it sounds wrong, but I do not like really sticky tires on the street.  They pick up every little stone and grain of sand and hurl it back, scratching the car and rear fender guards, and getting some of it in the car.  So I will keep my dried out 15" tires and look into some 13 inch wheels and tires.  It is not the direction I want to take, but I love the roof rack, Bernie.

[RedbeardRacing]

You guys mind sharing what rear brakes you had that fit inside the 13" wheel? Front brakes too for that matter. 

 

I have outlaw 2000 front brakes and will only clear "real wheel" 12.2" ID, all the cheap steel wheels (Bassett, diamond, aero) ID is too small to clear the caliper. They are all about 11.25" ID 

 

On the rear I have a 2011 VW caliper from what I can find, and am looking at an LD-19 formula Ford caliper and matching rotor. About 450 per side. 

 

Any guidance is great. 

[BirkinBernie]

HI Redbeard,

My Birkin has the factory supplied VW front calipers, and Honda rears.  The only clearance issue I have with the 13" Kodiaks is the rear bleeder fittings.  But I ground a little off the top of the bleeders and solved the problem.

[RedbeardRacing]

Would you mind sending me a part number for the Honda calipers and rotor? Those Kodiaks look like really nice wheels. 

[Davemk1]

I use 13" x 10" Real Racing Wheels on my Westfield for autocross. I run Avon Avon slicks and love the combo. They are light and strong and I have many hundred runs on them without issue.

 

It's worth checking with Real Racing wheels. I think they are much less expensive than Kodiaks and he will make them with any width, bolt pattern and backspace that you want.

 

dave

[BirkinBernie]

On 3/10/2023 at 12:17 PM, RedbeardRacing said:

Would you mind sending me a part number for the Honda calipers and rotor? Those Kodiaks look like really nice wheels. 

I found a parts cross-reference put together by one of the members of the old "Birkinowners" Yahoo group.  It lists the rear calipers as "Honda Ballade (Prelude) 130/150 1986".  It lists the rear rotors as "Birkin manufactured" but ISTR one of the fellows on the list providing an ID for a rotor he bought at a local parts store.  I'll search the archives of the list and see what I can find.

[RedbeardRacing]

You're the man, thank you for the info. I think early 90s Miata rear brakes should work as well from what I'm seeing. 

[RedbeardRacing]

That Westfield looks awesome! I have spoke with real racing wheels, and their ID should clear my outlaw 2000 front brakes by 1/16". Awful close. Force racing wheels UK should also fit, they are more than the real wheels and shipping from the uk, but have design options at least. 

[oilteq]

If you are risk averse, you could check the return policy and then order one wheel to verify fitment before ordering the other three.  This would probably work better with Real than a UK based vendor.  With only 1/16" clearance, make sure the wheel weights are offset from the brakes when balancing.

[BirkinBernie]

I spent some time digging through the email archives.  I cannot find anything conclusive about either the front or rear rotors from that era. At this point, I believe the best course of action is to talk with Tom Carlin the Birkin dealer in Colorado.  I suspect Tom has addressed this issue and can provide reliable info.

[papak]

I researched the rear rotor with Tom a few years ago. I have a later Birkin (2015 SS3XS) but the problem is probably the same. Birkin uses essentially the same hub on all four corners (the component pieces assembled a bit differently of course, front to rear). On my model year, they used a 2005 ROW VW Golf GTI rear caliper in order to provide a manual parking brake mechanism. In order to make the caliper work with the hub dimensions, they had a machine shop backcut the rotor down to the stated minimum rotor thickness (about 10.5mm as I recall). I had been experiencing a unique wear pattern on the rear rotors from new where the outer 2/3s of the pad made perfect contact but the inner ⅓ made hardly any. I eventually came to the conclusion that the inner and outer rotor surfaces are not perfectly parallel. With the piston being on the inside of the caliper, it exerts a bit more pressure on the inner side of the rotor and the caliper flexes a bit to accommodate this, hence the unusual wear pattern. After replacing the rotors once, I found a "workaround" by using softer rear pads (Porterfield RS-4 AF340). I see more brake dust from the reds but the wear pattern is more uniform now that the rear pads are fully bedded. I never experienced a real problem with braking capability, given the weight of the car. It was always a question of "if I assembled it correctly, why is it doing that?".

 

Tom Carlin has always been enormously helpful and incredibly patient with all of my many questions over the years. 

 

 

[RedbeardRacing]

Thanks for the write up! I did get a hold of Tom and the birkins came with two different factory rear brake setups. The later was a very common VW caliper and rotor, the older was a 1900 Honda crx hf caliper, reversed side to side and a 90s for escort rear rotor with the center machined larger to fit. 

 

That setup if way cheaper than the ld-19 caliper and formula ford rotor I was looking at.