I'm trying to decide on what wheel/tire combo to use on my 20b project car. It has a ported motor and a 102mm GT42R turbo. My initial thought was to use 285s in the front on 18" wheels with 335/30 18s in the rear. John at CCW suggested I use 19" rear wheels with a 305-325 so that the overall diameter is greater than 26". The theory being that the larger diameter wheel will create a better contact patch than a wider wheel of a smaller diameter.

I plan on putting ~10,000 miles on the car a year, so I'm not interested in anything with a really short tread life -ie street legal race rubber.

I'm curious to see what everyone's thoughts are.

 

I just have a set of Fikse FM5 17" wheels with Kumho MX 275/40/17" on rears and 235/45/17 on the fronts. when just rolling into the throttle it spins first, spins when boost comes on in second, just hooks and goes in 3rd, 4th etc...I am running just stock springs with Koni yellow struts set to stock ride height. The car suprisingly goes really well for the power its putting to the wheels. I am also running a rear diff brace, trans brace, engine torque brace and poly motor mounts, trailing and toe links and rear camber set to max positive (almost in the red) to compensate for the car squating under boost to make the best contact patch.

 

Chris,

What does John think about the minimal sidewall's affect on traction when using 19s? The 18" sizing with 335s sounds pretty logical.

Cheers.

 

I think you should stay with 18's.... for sidewall height considerations. I doubt that going to a +26" diameter will help so much with contact patch as it would by just changing your gear ratio, which you could just do by... cahanging your gear ratio.

What are you going to do about fender clearance? 335's aint gonna fit.

As far as brands, for a "true" street tire, you're probably looking a Michelin PS2's... which I believe are considered the best of the best right now.

 

'glad to hear the project is moving along. given the fact that redrx7 (mike) couldn't keep his 20b from driving sideways, and it was underturboed, you are going to have a handful. i suppose, as peter says, that the 335s will be your best bet. you will want real low rear airpressure. 27 max. tire selection w/b an important issue.

your front interests me more than your rear.... john always recommends 18X10 in the front. i think that's a big mistake. it totally screws your scrub radius. hey, i could see big fronts if the FD was 4 wheel drive. i run 18X8.5 front on 245s. (10s of course in the rear)... FWIW i have yet to be beaten on a road course.

scrub is a really bad thing. tires loose grip when the aren't rolling.

just my 2 cents,

howard

 

Howard, I always *thought* along that line, and wondered why everone wanted such a large front tire in a rear wheel drive car (I always questioned those that would warn about one's car understeering as a matter of course if they ran a smaller front tire than rear).

I run 255/275 on the street, and like the ballance in most cases, but went (currently) with 18x10 all around w/ 285/30/18 for the track because the smaller front would push around slower, tighter corners.

The 10's were engineered to be as close to the stock center as possible (they're Brad B's old custom Technomagnesios) with +50 or +52 offset (whatever stock is). I know the actual center of the wheel is further out due to the width/offset equation, but how bad could it really be? Are your 8.5's over +50?

Sorry for the HiJack, maybe we should move this?

 

If the offset is the same, the scrub radius is the same, because the offset is referenced to the wheel (and therefore tire) centerline. The scrub radius is the distance from the point at which the steering axis intersects the ground and the tire centerline. The stock FD offset is 50mm, to use 18x10's you need about a 50mm offset, so the scrub radius will be the same, or nearly the same. Handling can be balanced with either equal wheel and tire widths or with a stagger, it just depends on the relative roll stiffnesses front and rear.

 

Good discussion!

His thought was that the sidewalls on the 19s would deform better with ~305. I don't remember the aspect ratio off hand and don't have the info near me.

I have Pettit flares sitting in a box. I would love to meet my goals, without bastardizing the body work, but it doesn't look like that is going to be possible. I have been reading about the PS2s. Unfortunately they don't appear to be available in 285/30/18.

Hi Howard. Thanks for the general words of encouragement. Having the car back in my possession has re-energized me, which I needed after writing several hefty 20b-related checks in quick succession.

I guess it is time I educated myself on this topic.

No problem, keep it going. Several people have said that you can compensate for a-symmetric tire sizes with suspension setup. Again, my knowledge is rather limited in this area, but doesn't that fix come at an expense elsewhere? I can see how increasing rear bar stiffness could reduce rear grip and increase over steer, but wouldn't it also make the handling on the limit less progressive. Also, by stiffening up the rear bar aren't you effectively dubbing the rear of the car down to the grip level of the front? Wouldn't it be better to increase the lateral grip in the front rather than decrease it in the rear? Is that not possible with the stock front control arm length due to the scrub radius?

 

With as much power as you will have, you will probably want/need to make some compromises in slower speed handling feel (i.e. a bit more understeer in slower corners b/c of wider rear tires) to be able to have better traction.

My experience has been that generally you can set the rear sway bar setting slightly stiffer to compensate for moving to wider rear tires. It doesn't necessarily reduce rear grip per se but it does make the rear "stiffer" (over simplified but I can't word the technical explanation any better) and handling "feel" will be different especially on turn in and slow speed corners. I'm not expert so I'm sure/hope more experienced peeps will chime in.

Conversely, check out what people like Damon do with a much larger front bar and stock rear bar. Concepetually that would make for huge understeer. I'd love to hear Damon's comments on that for my own education.

 

When tuning for balanced handling for unequal tires (in the case of an RX-7 anyway), you're essentially reducing the grip potential of the widest tires to match that of the smaller ones in order to get balanced handling during steady state cornering. If you don't do that then the handling will move towards understeer, but with power it can be overcome. It's not going to be as fast as if you'd gone to larger fronts though, as you're always going to be limited in corner speeds by the end with the least grip, which will be the end with the smaller tires. Neutral doesn't mean that the understeer can be overcome with power, it means that the whole car will slide at the same time.

Generally speaking, a slight understeer balance is fastest/easiest to drive, as you can be more aggressive with power. With lots of power getting on the throttle out of corners becomes hard, as the rear tire's gip will be all used for cornering, so there's no extra grip left for acceleration, that's where a stagger can come in handy. While maintaining the same maximum grip setting in the front and going with wider rears the balance moves more towards understeer, but you'll be able to get on the power earlier in the corner without oversteer.

Remember, the car's maximum corner speed/grip is determined by the lowest grip end of the car, so going with large tires up front as well is a good idea.

 

^ I was always under the impression that overall grip increases when you have bigger tires, even if you only increase the size of 2 tires. You can change the suspension to decide which end of the car will handle more of the cornering load. For example, if the car is fairly neutral, after adding bigger tires to the back ,the car may feel more understeery. But by making changes to the rear roll resistance vs the front, you can shift more of the cornering load to the rear and thus reducing the load on the fronts.

 

I think the (semi) short answer on wether or not to run the same size all around boils down to means. If the wheels you HAVE only support a 255 front tire, but your rears will support a 275, run the 275. There's no point in reducing the rear tire to match the front out of some sort of blanket rule that says you'll "undesteer" as a result.

However, if you have the opportunity to run a front that's as big as your rear (without, compromizing the rear, and without compromizing something else up front like scrub radius), you should do that too. I don't see any reason to limit the tire size on one end of the car to match the other. That'a for the track. I don't see any reason period to run a huge front tire on the street.

The 18x10 +52, 285/30/18 front i'm using really couldn't be any larger, and matches the rear.. but if I could run a 295/30 or 305/30 hoosier in back, i'd do it.

 

Still haven't decided. Unless I run 285s all around or 285/335s Pilot Sports (not PS2s) I will have to mix and match tires. People talk about this like it is worse than crossing the beams in GhostBusters. Is it really that big of a deal with an a-symetric wheel setup? I miss the days when supercars came with 18" wheels. Everything today is 19+, with run flat technology and huge overall diameters (27"+ for the Enzo!).

 

On the street it's probably ok, but when you start pushing it then the handling might get weird, and it could possibly change over the course of the drive due to heat. It's best to avoid mixing and matching if at all possible.

 

i'm also curious about this topic, being in almost the exact same boat as CCarlisi (20B with 102mm GT42R in an fd)

when i bought my 93 R1 rolling chassis, it had 19x10 in the back and 18x8.5 up front with 275/225. beautiful wheels. . .i don't think that i'm gonna replace them for a long time.

rather than dealing with that, i think that i'm gonna put a ford 8.8 in the back with a 4-link and hopefully that will help some. that combined with tein type flex should make for a reasonably fun ( and mangable ) car i think. to be perfectly honest, i am largely uneducated in this area, so any input is greatly appreciated.

no track around here, so almost exclusively a street car.

thanks
ryan