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I'm considering upgrading the wheels on my 997.1 C4 and am trying to determine what offsets are considered "in range" to the original factory ones.
Few aftermarket wheels are the same offset as the OEM ones - particularly if moving to a larger 20" wheel and I think I remember reading somewhere that the wrong offsets can damage the AWD differential.
So I guess that's a second question.
Anyhow appreciate any wisdom for someone who has ventured down this road (slight pun intended).
Tire size is the factor that will destroy differentials if not kept with the same/near ratio to stock. Offset is a measurement to show where the rim face sits relative to the hub. For an AWD 7.1, you'll want to maintain a 2.7% difference from front to rear. You can calculate this by hand or using one of the many available tire size calculators online. The easiest path would be to reuse the stock tire sizes (235/35/19 and 305/30/19). You can deviate a bit from that but really try to keep the difference between the two tires no more than 1% (1.7%-3.7%).
https://tiresize.com/comparison/ is a good way to check the deviation between tire sizes. You'll also want to keep your overall tire diameter within 3% of the stock size to keep your speedo from reading too far off. This may seem like a lot of numbers to keep track of at first but you'll get the hang of it as you research more about wheel sizing (diameter, width, offset) and tire sizing.
Edit: Forgot to mention the reason you want to keep the F&R ratio as close to the same is because the front diff is geared to roll 2.7% faster than the rear diff so that they spin at the same speed when going down the road. Changing this ratio too much causes the diff to slip in order to maintain the same rolling speed as the rear which is what causes the differential damage.
The wrong ratii will also throw off the PSM system and you may find the engine cutting power at off times be user it thinks its seeing slippage.
Also, I've never seen a 997 that looks good on 20" wheels. Find a set of forged 19s you like and use the tire sizes mentioned above and don't create other issues for yourself.
Never has a 20" wheel and tire been lighter than the same wheel and tire in a 19" and that's all unsprung weight. It effects acceleration, braking, steering feel, etc.
Wow, thank you so much for this. I had been trying to figure it out and am so thankful for the help and extremely knowledgeable answer. I've got to say, I've lurked for a while but answers like this impress me. I'm always going back and forth about mods, but at least I can do it from an informed position - first do no harm!!
Many thanks, @andino
And @Petza914 I've enjoyed the heck out of your posts! Agreed on your points. Very valid. The Carrera Classics I have aren't bad, I'm just looking for something a little more aggressive and/or a different color. BTW - my car is going to the detailers next week in prep for a photoshoot. I've already picked out the location, you guessed it, an elevated parking garage
Wow, thank you so much for this. I had been trying to figure it out and am so thankful for the help and extremely knowledgeable answer. I've got to say, I've lurked for a while but answers like this impress me. I'm always going back and forth about mods, but at least I can do it from an informed position - first do no harm!!
Many thanks, @andino
And @Petza914 I've enjoyed the heck out of your posts! Agreed on your points. Very valid. The Carrera Classics I have aren't bad, I'm just looking for something a little more aggressive and/or a different color. BTW - my car is going to the detailers next week in prep for a photoshoot. I've already picked out the location, you guessed it, an elevated parking garage
I still have the polished 19" Champions for sale if you want to go lighter and stronger with a unique finish (only set of these I've ever seen that were polished). 11.5" wide rear with to better support the 305 tire width. Rears are NB fitment though, so you'd need some spacers on a AWD WB car.
@ncracing You can go more aggressive in 19s by adjusting offset and width for a better fitment than stock. There's also the option of lowering your car to reduce the wheel gap if that's an option for you as well. The combination of a more aggressive wheel (either through offset or combined width/offset) and lowering will make the car look much aggressive since there won't be as large void in the wheel well area. I'd suggest deciding if you want to lower your car first before choosing a wheel spec because it'll change slightly when you lower the car depending on the amount. If you're after a "perfect" combo, that'd be the most optimal path. Otherwise, you can always add a spacer after you lower your car if your wheels don't quite fit the way you want after lowering.
I went with 19" HRE's on my C4S and they have been perfect (I considered 20's for a brief second before deciding to pass for the same reasons as Petza914 states).
As for offsets and making sure everything was in range, I did a bunch of research and felt comfortable HRE had done their homework to ensure a perfect fit.
I'm considering upgrading the wheels on my 997.1 C4 and am trying to determine what offsets are considered "in range" to the original factory ones.
Few aftermarket wheels are the same offset as the OEM ones - particularly if moving to a larger 20" wheel and I think I remember reading somewhere that the wrong offsets can damage the AWD differential.
So I guess that's a second question.
Anyhow appreciate any wisdom for someone who has ventured down this road (slight pun intended).
Please post a pic of your current setup so we have an idea of what you are working with. You got some great feedback already about the AWD issue really being the circumference of the wheel/tire relationship between front and rear, not the offset.
On wide body cars, and for a flush wheel to wheel well relationship, you will want your offset in the rear to be as close to 48-50mm as you can. For the front a 53-50 mm offset works well too. The lower the offset # the further the wheel gets pushed out.
Think of it this way. If the mounting hub inside the wheel were at center, then the offset would = 0 mm. Let's say with an offset of zero at the hub that this would cause the outer wheel to stick out past the body of the car 50 mm. This would look ridiculous and cause major rubbing issues. Now, as the offset number increases and gets larger, then imagine you are "removing" material from the inside wheel hub and that brings the wheel closer to the suspension components and body of the car or wheel well.
So in the case of narrow body 911s versus wide body... Narrow bodies usually have a factory offset of 63mm on the rears. On a wide body 911 we now have extra space to fill. So we want a "lower" offset number or to add more material back to the hub in order to push the wheel out more. So for wide body cars a lower offset of 50 mm works better. If you decide to use a narrow body offset wheel of 63 mm, then you will need to add a spacer of at least 13 mm, which is like adding material back on to the wheel hub, to equal of 50mm offset for the rears. Is your C4S lowered at all?? If it is, then you may get away with a 15-17 mm spacer for a total of 48-46mm offset when working with narrow body 63mm wheels. Any more than that and I think you will be pushing the wheel too far out. It comes down to preference on stance. Some will recommend offset of 44-46 to push the wheel as close and flush to the body as possible. Then you tire choice will also matter in order to have the best side wall look and roundness.
@Hilltopper Super nice - the duck tail and wheels are iconic. One of the things I most love about the 911 is exactly that.
I probably need to start getting out to some events, maybe I'll see you at one!
@qikqbn That makes perfect sense and I have to give credit to you and other fellas for helping me really understand the concept. My car isn't lowered (I'm resisting the urge to mod and its damn challenging. Most things I've done are cosmetic and easily reversible - lights etc, the gundo hack isn't lol). That having been said, you mentioned spacers. other than instances where you would add them to get to the correct offset, do people add them primarily for looks? Or does it provide a difference in handling?
For frame of reference, my sports car before this one was a '93 MR2. I upgraded the stock 15inch wheels for big 16" ones
Spacers work by spacing out the wheel from the hub. It does change the effective offset of a wheel but at the expense of changing the scrub radius. Not an issue on the rear but on the front, it can cause clearance issues when approaching either end of your steering lock. With a stock width/tire plus being stock height, you're unlikely to run into any issues if you're just adding spacers to your stock wheel/tires. If you're looking for aftermarket wheels, then it would be ideal to get the proper width/offset combo to use without spacers but small adjustments should be fine with a spacer. But you don't want to add a 25mm spacer on the front axle as that'll push your wheel out from the hub a whole inch making your scrub radius way different than stock. This would effect handling and such as well but you may not notice until you're approaching the limit.
03-24-2021, 11:46 PM
AWD / Aftermarket Wheel Questions
