Different diameter wheels front and rear

[PeteKz]

I searched this forum a couple ways for anyone who has done this, but didn't get good results. Surely someone has used, say 15" wheels on the front and 16" or 17" on the rear? It would provide a way to get lower profile tires on the front of the car--either to match the profiles on the rear, or to lower the front of the car 1/2"-1"--without changing the suspension and steering geometry. It might even look pretty cool too.

[TwoBluPorsches]

Check out a 992 Carrera with 19 front and 20 rear or 20 front and 21 rear.

Even the AWD versions have staggered diameter wheels


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[PeteKz]

On new cars yes, I mean for older air-cooled cars.

[Qtrfoil]

I may be missing something, but lower profile tires on a smaller wheel of the same width will result in a smaller outer diameter.

I'd also wonder about two other things, just guesses. One, I'd be nervous increasing lateral grip up front on a rear weight biased car. The wheels are wider (on later cars) to try and add more grip at the rear than up front, this would be reversing that. The cars understeer until they don't, and I can imagine the "don't" coming on even more suddenly. Second, with lower profile tires the front end is going to be less damped over irregularities in the surface, especially with no change to the front shocks. For cars that already get light up front, I'm not sure I want the front end being even stiffer and "bouncing" more.

[Bill Verburg]

Originally Posted by PeteKz

I searched this forum a couple ways for anyone who has done this, but didn't get good results. Surely someone has used, say 15" wheels on the front and 16" or 17" on the rear? It would provide a way to get lower profile tires on the front of the car--either to match the profiles on the rear, or to lower the front of the car 1/2"-1"--without changing the suspension and steering geometry. It might even look pretty cool too.

1) You want the tire OD f/r to be fairly close to each other, it's way easier to set the car up that way
2) you want stagger in tread width and stiffness depending on power transmitted through the tires. one measure of stiffness is sidewall height which is why the rear usually has a lower section ratio and greater tread width. The more power the more stagger, but this also needs to be coordinated w/ the rest of the suspension and weight specs.

here is a summary of tire and wheel sizes that fit a 911 SC/Carrera chassis



say you wanted a 245/45 x16 on a 9 in back, @24.7" OD then you are looking for a 15 w/ ~24.7" OD for the front, there really aren't any, 215/60 on a 7 front is the closest, this gives you a really tall hard to fit tire w/ sidewalls that are very squishy compared to wants in back, hardly good fo setup or handling

You have better luck w/ say a 245/40 x17 on a 9 in back @~24.6" w/ a 225/50 on 7 front, the width stagger is ok but the sidewall stagger is still on the squishy side in front

lastly you don't want to over tire the car, a 275/35 on a 9.5 is a great rear w/ a 245/40 front on 8 but for 99% of the 911s out there not needed and adds some relatively minor fitment issues, this is why I broke it down by hp levels.

the sweeet spot in the table for most 911s is 17, 2215/45 on 7 or 8 and 245/40 on 9 or 9.5, unless the car is really underpowered and need the assistance of one of the 15" selections

[PeteKz]

Bill, others, thanks for the info.

[H-viken]

Take a look at the 935. Think it had 16” up front and 19” in the rear

[IROC]

I did this for years. I ran 8x16s in front with 245/45s and 9x17s in the rear with 275/40s. I did it to maximize the amount of tire I could fit under the car. It worked. Zero drivability issues and the grip (I was running r-compound tires) was astounding on my 2350 lbs car. For me, the difference in OD was meaningless and transparent. In fact, a lot of people did this exact setup back in the '90s (I didn't figure it out on my own).

I have pics...

I have since modified my front wheels (they are 3-piece) to 8x17s only because of tire selection.

[Qtrfoil]

Different-sized wheels is not unusual but your fronts are higher profile, with section height on both ends of 110 mm. The OP wrote about LOWER profile up front. I'm not nervous about different-sized wheels, I am a little concerned about increasing lateral grip up front with a stiffer sidewall without doing the same at the rear. I think that would all be fine until it very suddenly wasn't.

[Bill Verburg]

Originally Posted by IROC

I did this for years. I ran 8x16s in front with 245/45s and 9x17s in the rear with 275/40s. I did it to maximize the amount of tire I could fit under the car. It worked. Zero drivability issues and the grip (I was running r-compound tires) was astounding on my 2350 lbs car. For me, the difference in OD was meaningless and transparent. In fact, a lot of people did this exact setup back in the '90s (I didn't figure it out on my own).

I have pics...

I have since modified my front wheels (they are 3-piece) to 8x17s only because of tire selection.

first issue ~1" difference in f/r tire OD, that front tire must need a bunch of extra height in front if on Fuchs 8ET23.3

second issue compared to a properly matched tire set like 245/40 & 275/35
you are losing ~148b-ft, most people will find that quite noticeable but you can get used to most anything

3rd issue the front tire really want an 8.5 but if using 8 you want ET 27,(not available in Fuchs)

[Bill Verburg]

Originally Posted by H-viken

Take a look at the 935. Think it had 16” up front and 19” in the rear

also had ~1000hp, they were the ultimate point and scoot car

[IROC]

Originally Posted by Bill Verburg

first issue ~1" difference in f/r tire OD, that front tire must need a bunch of extra height in front if on Fuchs 8ET23.3

second issue compared to a properly matched tire set like 245/40 & 275/35
you are losing ~148b-ft, most people will find that quite noticeable but you can get used to most anything

I don't know what the units 'b-ft' are, but this setup worked great. It was common on POC (SoCal) cars back in the day and many of us were running it in the southeast. My fronts actually measure out to about 8.2". No issues.

Lap times told the whole story - this was by far the fastest setup I ever ran.

[Bill Verburg]

Originally Posted by IROC

I don't know what the units 'b-ft' are, but this setup worked great. It was common on POC (SoCal) cars back in the day and many of us were running it in the southeast. My fronts actually measure out to about 8.2". No issues.

Lap times told the whole story - this was by far the fastest setup I ever ran.

10000s of lemmings follow the leader

If you are happy,great, my point is that there are much better ways to go, why not choose one of the better paths and then why not optimize it.

be serious, typo, lb-ft


don't know what you are measuring but a 245/45x16 on a Fuchs ET23.3 x16, will have a front space of ~99mm where 89mmm is a more normal max. These are facts not made up BS, to use these the front end is jacked up more than is desirable. which is likely what you did given the 1" difference in tire heights. Alittle front down is desirable a lot not so much. Se all those entries in red, they will be problematic wrt fit.

[RarlyL8]

Here is what a 16 front 17 rear combo looks like.
Obviously this is not a track car so perfect balance is not needed. It drives great, brakes predictably and handles great on the street. The staggard sizes were done strictly for cosmetic reasons. I wanted the car to sit level (not crouching) with the tires filling the fender arches the same front and fear.

[r lane]

What is the physics applied to the balance of front and rear tire sizes. Obviously they are loaded differently, so how does one design a setup. I have 245/45/16 on the back of narrow 78 911 and 205/45/16 on the front. 1 1/2''-2'' different in diameter. To my butt, the car feels good and stable. I have built a set of wheels that will allow me to put 225/50/16 on the front. I will then have front and rear tires with similar diameters. May need to drop the front as I am now higher on that end. Also am concerned about having enough room now with that bigger tire and what the extra front grip will do to handling balance. Any good info is appreciated, Bob

[Bill Verburg]

Originally Posted by r lane

What is the physics applied to the balance of front and rear tire sizes. Obviously they are loaded differently, so how does one design a setup. I have 245/45/16 on the back of narrow 78 911 and 205/45/16 on the front. 1 1/2''-2'' different in diameter. To my butt, the car feels good and stable. I have built a set of wheels that will allow me to put 225/50/16 on the front. I will then have front and rear tires with similar diameters. May need to drop the front as I am now higher on that end. Also am concerned about having enough room now with that bigger tire and what the extra front grip will do to handling balance. Any good info is appreciated, Bob

tire choice distills to slip angles

understeer is when the front tire slip angle is greater than that at the back

over is opposite of that

neutral is when the f/r slip angles are similar

in a 911 where the weight is biased to the rear you want more grip in the rear, to get it you want wider, lower profile tires on wider wheels.

spring rates and shock damping also need to be considered but for a typical 911 w/ typical suspension setup
205/45 & 245/45 will have less grip(greater slip angle in front) then is desired, lots of hp can and will exacerbate that but again most older 911s don't really have lots of hp(400ish) so generally the car understeers more

Factory base setup is 205/55 & 225/50 205/45 greatly reduces the slip angle in front, 205/50 less so. 245/45 doesn't change rear slip angle much.

using 8s instead of 7s also reduces slip angle no matter the tire

typically for track use a 911 responds really well to 8 w/ 225 front and 9 or 9.5 rear w/ 245 or 255 depending on hp, 225/45 is shorter than 225/50 and has a lower slip angle, 245/40 on a 9 balances the 225/45 front nicely. Again assuming typical suspension setup and weight distribution, ie for tack use 23mm front t's and 30 or 31 rear t's which themselves reduce understeer

tire od differences mostly affect torque and gearing costs and setup issues not handling per se

so in general you want balance that is somewhat less understeering than stock which comes from 8 & 9 225/45 & 245/40 , for less over (or more hp 255/40 on 9.5)

you can get these combos in 15 or 17 but not 16 and not w/ Fuchs

225/45 & 245/ 45 x15 on 8 & 9 gives the best acceleration at ht the cost of top speed and more shifting 225/45 & 245/40 on 8 & 9 x17 isn't far behind and 9.5 255/40 for a higher hp car.

[r lane]

Thanks for all the info. Looking forward to seeing the change the wider fronts create. If significant, may take a weekend or two to adjust. Will report back. Bob