295-sized rear tires OK for C4S or TT?
03-08-2003, 03:42 PM
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295-sized rear tires OK for C4S or TT?
Hello,
I've been researching whether the 295 sized rear tires are OK to be used in our 993-based AWD cars. So far the searches haven't really indicated any solid information one way or another.
My understanding is that Porsche selected the 225/285 size tires so that there will always be some speed differential between the tires and thusly some torque transferred to the front tires via the AWD system. Will using the 295 size tires, while keeping the fronts at 225, transfer more torque to the front or less torque? My calculations below indicate less torque is transferred if speed differential is used as the means to judge torque transfer.
My calculations indicate that the 225/40/18 fronts have a circumference of 78.81 inches and the 285/30/18 rears have a circumference of 77.7 inches. This means that the fronts roll less for each complete rear revolution; about 1.1 inch less.
With the 295/30/18 tires at the rear, the circumference is now 78.44 inches. This is almost on par with the circumference of the front tire. This means that with each roll of the rear tire the front tire rolls ~.37 inch less than the rear. The fronts still roll slower than the rears but the difference is less than before.
Any comments on the above calculations or the conclusions?
Thanks,
*****
I've been researching whether the 295 sized rear tires are OK to be used in our 993-based AWD cars. So far the searches haven't really indicated any solid information one way or another.
My understanding is that Porsche selected the 225/285 size tires so that there will always be some speed differential between the tires and thusly some torque transferred to the front tires via the AWD system. Will using the 295 size tires, while keeping the fronts at 225, transfer more torque to the front or less torque? My calculations below indicate less torque is transferred if speed differential is used as the means to judge torque transfer.
My calculations indicate that the 225/40/18 fronts have a circumference of 78.81 inches and the 285/30/18 rears have a circumference of 77.7 inches. This means that the fronts roll less for each complete rear revolution; about 1.1 inch less.
With the 295/30/18 tires at the rear, the circumference is now 78.44 inches. This is almost on par with the circumference of the front tire. This means that with each roll of the rear tire the front tire rolls ~.37 inch less than the rear. The fronts still roll slower than the rears but the difference is less than before.
Any comments on the above calculations or the conclusions?
Thanks,
*****
03-08-2003, 10:15 PM
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I suggest you put 245s or something larger on the front if you do you go with 295s on the rears. Check the diameter of your tires and maintain the same ratio as the 225/285 combo. You definitely will put more understeer with larger rears but without the balance put unnecessary stress on the 4WD system.
03-08-2003, 10:32 PM
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Out here in the desert, the low riders all use ultra wide tires with extra offset so the tires extend at least 6" out past the wheel arch.
Looks cool, and remember to wear your cowboy hat!
Looks like that's where you're going. Mud flaps wouldn't hurt.
Looks cool, and remember to wear your cowboy hat!
Looks like that's where you're going. Mud flaps wouldn't hurt.
03-09-2003, 01:24 AM
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I just purchased a set of 996 Turbo rims for my 96 C4S. Wheelenhancement.com says the 295's fit fine on a wide body 993.
My stock "turbo look" rims were shot (too many dings) when I purchased the car and I couldn't see paying the price to replace the solid spoke rims. I wanted hollow spoke and like the idea of a little more tread! The tires arrive on Monday and I'll let you know how it goes.
R
My stock "turbo look" rims were shot (too many dings) when I purchased the car and I couldn't see paying the price to replace the solid spoke rims. I wanted hollow spoke and like the idea of a little more tread! The tires arrive on Monday and I'll let you know how it goes.
R
03-09-2003, 01:58 AM
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Thanks for the replies.
Actually I'm not worried about the fit of the tires so much as the affect it has on the AWD system.
I hadn't thought too much about the understeer that such a setup would have. I'm hoping that the RS rear adjustable sway bars can fix the understeer.
Will 245's work on the 8" rims w/o being bulged out?
R, when you get things setup let us know if it feels different. With less torque up front and more grip in the rear understeer should be more pronounced.
*****
Actually I'm not worried about the fit of the tires so much as the affect it has on the AWD system.
I hadn't thought too much about the understeer that such a setup would have. I'm hoping that the RS rear adjustable sway bars can fix the understeer.
Will 245's work on the 8" rims w/o being bulged out?
R, when you get things setup let us know if it feels different. With less torque up front and more grip in the rear understeer should be more pronounced.
*****
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03-09-2003, 02:19 AM
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*****,
I use 245/35-18 front with the stock 285/30-18 rear on my AWD 993TT. Works fine on both the street and track without any noted ill effects. The overall diameter of the tire is 24.64" front and 24.92" rear. Can't say I've been able to appreciate any difference in the way the torque is split between the front and rear.
My 245's are on Fikse 8.5" rims, but should be fine on 8" rims as well. See <a href="http://www.bobwoodmantires.com/pzeroc.htm" target="_blank">Pirelli P-Zero C page</a>.
The AWD 993's have a tendency to understeer on stock tire sizes, and increasing the rear tire size to 295 may exacerbate that. I would agree with the others who suggested that you should strongly consider a 245 front if increasing the rear to 295. My car handles very neutrally with the 245/285 combination.
I do think a 295 would be OK if this is really the way you want to go. If you look at the actual tire widths of the new R-compound Michelin Sport Cups tires for our Porsches, the specified 285 tire actually has the width of some manufacturers' 295 tire.
It's better, IMO, not to have to solely rely on your rear bar to dial out your understeer.
I use 245/35-18 front with the stock 285/30-18 rear on my AWD 993TT. Works fine on both the street and track without any noted ill effects. The overall diameter of the tire is 24.64" front and 24.92" rear. Can't say I've been able to appreciate any difference in the way the torque is split between the front and rear.
My 245's are on Fikse 8.5" rims, but should be fine on 8" rims as well. See <a href="http://www.bobwoodmantires.com/pzeroc.htm" target="_blank">Pirelli P-Zero C page</a>.
The AWD 993's have a tendency to understeer on stock tire sizes, and increasing the rear tire size to 295 may exacerbate that. I would agree with the others who suggested that you should strongly consider a 245 front if increasing the rear to 295. My car handles very neutrally with the 245/285 combination.
I do think a 295 would be OK if this is really the way you want to go. If you look at the actual tire widths of the new R-compound Michelin Sport Cups tires for our Porsches, the specified 285 tire actually has the width of some manufacturers' 295 tire.
It's better, IMO, not to have to solely rely on your rear bar to dial out your understeer.
03-10-2003, 12:22 AM
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Anir,
Thanks for your tire recommendations and the suspension advice, nothing better than voice of experience. I checked on the availability of the 245/35/18 size and it seems spotty (i.e. not everybody has this size), but as long as there's one or two choices that's enough.
I have been looking at the Michelin Sport Cups but they don't have the 245 size, maybe their 225 is the equivalent to the 235 size.
I'm also planning on installing the PSS-9 kit I just purchased during Gert's sale in Feb. M030 front bar and RSR rear bar as well. I'm hoping between all these components and tire sizing choices I'll be able to get a neutral handling car.
*****
Thanks for your tire recommendations and the suspension advice, nothing better than voice of experience. I checked on the availability of the 245/35/18 size and it seems spotty (i.e. not everybody has this size), but as long as there's one or two choices that's enough.
I have been looking at the Michelin Sport Cups but they don't have the 245 size, maybe their 225 is the equivalent to the 235 size.
I'm also planning on installing the PSS-9 kit I just purchased during Gert's sale in Feb. M030 front bar and RSR rear bar as well. I'm hoping between all these components and tire sizing choices I'll be able to get a neutral handling car.
*****
03-10-2003, 01:37 AM
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You are correct in that 245/35-18 is a rare tire size. In fact, I've only been able to find it in an R-compound Pirelli P-Zero C. This is now a discontinued tire, and its replacement (the P-Zero "Corsa") is not currently available in 245/35-18.
Therefore, I bought two sets of C's recently. I hope some other tire manufacturer will eventually make this helpful size.
Folks seem really happy with the Michelin Sport Cup, but the Cup 285's are super wide (compared to most 285's), which makes we worry about reintroducing the understeer issue.
You are correct in that 245/35-18 is a rare tire size. In fact, I've only been able to find it in an R-compound Pirelli P-Zero C. This is now a discontinued tire, and its replacement (the P-Zero "Corsa") is not currently available in 245/35-18.
Therefore, I bought two sets of C's recently. I hope some other tire manufacturer will eventually make this helpful size.
Folks seem really happy with the Michelin Sport Cup, but the Cup 285's are super wide (compared to most 285's), which makes we worry about reintroducing the understeer issue.
03-11-2003, 12:32 AM
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*****,
I didn't even think about the effects on the AWD until I read your post and replies. Tires are mounted and I'll be the first guy in line early tomorrow to get them put on my car (they were closed by the time I got home today)
I went with the 02 Turbo specs of 295 rear / 225 fronts (continental contisports). The PZero handles well but is gone so fast I wanted something with better noise/ride comfort and longevity.
I don't drive my car on a track and actually don't push it that hard in the Colorado mountains. It is so hard to get in trouble with a 4S you really have to do something dumb! But maybe this tire combo will show how important tire sizes can be..
I'm new to P-car ownership and a sports car of this level in general and curious what understeer is? What does the car do exactly and how can I look for it with my stock 225/285 P-zero's before I drop off the car to have the wheels installed..?
I should be able to reply tomorrow with all the gory details.
Thanks,
R
I didn't even think about the effects on the AWD until I read your post and replies. Tires are mounted and I'll be the first guy in line early tomorrow to get them put on my car (they were closed by the time I got home today)
I went with the 02 Turbo specs of 295 rear / 225 fronts (continental contisports). The PZero handles well but is gone so fast I wanted something with better noise/ride comfort and longevity.
I don't drive my car on a track and actually don't push it that hard in the Colorado mountains. It is so hard to get in trouble with a 4S you really have to do something dumb! But maybe this tire combo will show how important tire sizes can be..
I'm new to P-car ownership and a sports car of this level in general and curious what understeer is? What does the car do exactly and how can I look for it with my stock 225/285 P-zero's before I drop off the car to have the wheels installed..?
I should be able to reply tomorrow with all the gory details.
Thanks,
R
03-11-2003, 03:28 AM
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R,
Understeer is the affect where the front of your car pushes more (plowing) and you have to apply more steering input to get it to turn the direction you want. Oversteer is just the opposite. The car turns in more than desired and usually results in the rear coming around to the front if left uncorrected (i.e. spin the car).
These effects are rarely noticeable under normal driving conditions. You have to be going fast in a turn to observe these effects. Since most of my other vehicles are front wheel drivers, I'm used to understeer, which is what Porsche set the C4S up for. Now the 951 ... that's a different story. Tail keeps wanting to come out under boost ... a little bit scary sometimes.
Where did you purchase the 02 TT wheels/tires? I've seen a few eBay auctions by Ernies wheels ... etc.
Give us a post when you've got the new wheel/tires on. I'm curious as to the effect.
*****
Understeer is the affect where the front of your car pushes more (plowing) and you have to apply more steering input to get it to turn the direction you want. Oversteer is just the opposite. The car turns in more than desired and usually results in the rear coming around to the front if left uncorrected (i.e. spin the car).
These effects are rarely noticeable under normal driving conditions. You have to be going fast in a turn to observe these effects. Since most of my other vehicles are front wheel drivers, I'm used to understeer, which is what Porsche set the C4S up for. Now the 951 ... that's a different story. Tail keeps wanting to come out under boost ... a little bit scary sometimes.
Where did you purchase the 02 TT wheels/tires? I've seen a few eBay auctions by Ernies wheels ... etc.
Give us a post when you've got the new wheel/tires on. I'm curious as to the effect.
*****
03-11-2003, 04:31 PM
#14
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by WillyC4S:
<strong>Understeer is the affect where the front of your car pushes more (plowing) and you have to apply more steering input to get it to turn the direction you want.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">If you are truly understeering, then any additional amount of steering won't do anything. You've already exceeded the grip of the front tires ( that's why you are understeering.. )
If you are new to sports cars in general and just want to know more, you should get a copy of Going Faster by the Skip Barber school. Best $20 driving investment that I"ve made.. Second to rennlist, of course..
The book explains all that you'd ever want to know, and then some. It's a great starting place.
<strong>Understeer is the affect where the front of your car pushes more (plowing) and you have to apply more steering input to get it to turn the direction you want.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">If you are truly understeering, then any additional amount of steering won't do anything. You've already exceeded the grip of the front tires ( that's why you are understeering.. )
If you are new to sports cars in general and just want to know more, you should get a copy of Going Faster by the Skip Barber school. Best $20 driving investment that I"ve made.. Second to rennlist, of course..
The book explains all that you'd ever want to know, and then some. It's a great starting place.
03-11-2003, 04:33 PM
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</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by jwilberdog:
<strong>Had 295's on my c4s with no problems (other than eating them up every 4-5k miles)</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">That's because you have a really heavy right foot Jason!!
<strong>Had 295's on my c4s with no problems (other than eating them up every 4-5k miles)</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">That's because you have a really heavy right foot Jason!!