Tire size question
11-11-2002, 10:59 AM
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Tire size question
I've been searching high/low for a decent street tire in the size 285/35/17. Have come up empty thus far. Though there's some mention of that size being the stock rear size on NSX's.
Anyone have insight on 285/35/17's? It specs out perfect with the original stock size (fronts) of 225/50/16, though I'm trying to fit the 285's to the rear of my 88 951S.
Anyone have insight on 285/35/17's? It specs out perfect with the original stock size (fronts) of 225/50/16, though I'm trying to fit the 285's to the rear of my 88 951S.
11-11-2002, 03:22 PM
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just fyi, 285's are ENORMOUS.
good luck doing huge smokey burnouts with those things <img src="graemlins/burnout.gif" border="0" alt="[burnout]" />
but really, i don't know of any tires in that size, and 285/30/18 is only a tiny bit too big and has a great variety avaliable. of course you need 18" rims...
good luck doing huge smokey burnouts with those things <img src="graemlins/burnout.gif" border="0" alt="[burnout]" />
but really, i don't know of any tires in that size, and 285/30/18 is only a tiny bit too big and has a great variety avaliable. of course you need 18" rims...
11-11-2002, 05:22 PM
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Yes, I'm sure. I have 18" hollow spoke rims now but am trying to move down to 17"s. My car tramlines so badly you have to keep two hands on the steering wheel at all times. I'm hoping 17s will help. Besides, my front rims are only 7.5" wide, and I want at least 245 width tires on the front.
On the other hand, I've yet to upgrade to the 968 (or better) camber blocks and associated hardware. Anyone know whether or not this would help my tramlining? If I can solve my tramlining, I will keep my 18" Technology hollow spokes.
And yes, I'm aware of the size of 285's. And since I'm not 17 anymore I don't have the need to do "burnouts." That's just noise and wasted money.
On the other hand, I've yet to upgrade to the 968 (or better) camber blocks and associated hardware. Anyone know whether or not this would help my tramlining? If I can solve my tramlining, I will keep my 18" Technology hollow spokes.
And yes, I'm aware of the size of 285's. And since I'm not 17 anymore I don't have the need to do "burnouts." That's just noise and wasted money.
11-11-2002, 05:24 PM
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Yes, I'm sure. I have 18" hollow spoke rims now but am trying to move down to 17"s. My car tramlines so badly you have to keep two hands on the steering wheel at all times. I'm hoping 17s will help. Besides, my front rims are only 7.5" wide, and I want at least 245 width tires on the front.
On the other hand, I've yet to upgrade to the 968 (or better) camber blocks and associated hardware. Anyone know whether or not this would help my tramlining? If I can solve my tramlining, I will keep my 18" Technology hollow spokes.
And yes, I'm aware of the size of 285's. And since I'm not 17 anymore I don't have the need to do "burnouts." That's just noise and wasted money.
On the other hand, I've yet to upgrade to the 968 (or better) camber blocks and associated hardware. Anyone know whether or not this would help my tramlining? If I can solve my tramlining, I will keep my 18" Technology hollow spokes.
And yes, I'm aware of the size of 285's. And since I'm not 17 anymore I don't have the need to do "burnouts." That's just noise and wasted money.
11-11-2002, 07:29 PM
#6
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"My car tramlines so badly you have to keep two hands on the steering wheel at all times. "
The tramlining is due to the width of the tires, not really much with the diameter of the wheels (each point of tire in contact with the ground for less time). I had really bad tramlining when I went to 265/45-16 Kumhos on 16x10.25' wheels.
The solution was to go with the aligment specs outlined in TSB#9303 on 17" wheels. Basically you want to increase caster to around 3.5 which is about the maximum you can get out of the adjuster. This is actually a happy medium because more caster may be more stable, but the car definitely feels a lot more sluggish and slow to respond to steering inputs.
"just fyi, 285's are ENORMOUS."
Wait 'til you see my car with 315/35-17 tires on 17x12" wheels, heh, heh! <img src="graemlins/jumper.gif" border="0" alt="[jumper]" /> <img src="graemlins/jumper.gif" border="0" alt="[jumper]" /> <img src="graemlins/jumper.gif" border="0" alt="[jumper]" />
Also I want to see a shot of Paul Bloomberg's car from the rear with his 335/30-18" tires on 18x13" Simmons wheels !!!
The tramlining is due to the width of the tires, not really much with the diameter of the wheels (each point of tire in contact with the ground for less time). I had really bad tramlining when I went to 265/45-16 Kumhos on 16x10.25' wheels.
The solution was to go with the aligment specs outlined in TSB#9303 on 17" wheels. Basically you want to increase caster to around 3.5 which is about the maximum you can get out of the adjuster. This is actually a happy medium because more caster may be more stable, but the car definitely feels a lot more sluggish and slow to respond to steering inputs.
"just fyi, 285's are ENORMOUS."
Wait 'til you see my car with 315/35-17 tires on 17x12" wheels, heh, heh! <img src="graemlins/jumper.gif" border="0" alt="[jumper]" /> <img src="graemlins/jumper.gif" border="0" alt="[jumper]" /> <img src="graemlins/jumper.gif" border="0" alt="[jumper]" />
Also I want to see a shot of Paul Bloomberg's car from the rear with his 335/30-18" tires on 18x13" Simmons wheels !!!
11-11-2002, 07:31 PM
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I recently looked into this. I also have 7.5 & 9 X 17. I could have done 245/45-17 and 275/40-17 and maxed out the rim reccommendations. However, I went with 235/45-17 and 255/40-17. This was a little bigger in the front, but it's a better match front to rear and I am pleased. I am also pleased with the kuhmo 712's I got for them too. great in teh rain and the dry.
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11-11-2002, 07:57 PM
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Hey Danno-Thanks for the caster recommendations. Jesse Jackson said it best: "Keep hope alive!" <img src="graemlins/drink.gif" border="0" alt="[cherrsagai]" />
11-11-2002, 08:53 PM
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OK, hold the bus, I have a couple more points of contention.
Danno tells me that tramlining has more to do with the tires width. My 225/50/16's in front do NOT tramline at all, but my 225/40/18/s in front tramline like the bumper cars at Great America. Both tires are 225 wide. I am assuming tramlining is a front tire peculiarity, not rear.
Theories?
And in Porsche Technical Service Bulletin 9303 about the upgrade to 17" tires/rims, it says to use a 17mm spacer on the rear(s). I wonder why? And do you suppose they mean 17mm each side?
Seeking enlightenment.
Danno tells me that tramlining has more to do with the tires width. My 225/50/16's in front do NOT tramline at all, but my 225/40/18/s in front tramline like the bumper cars at Great America. Both tires are 225 wide. I am assuming tramlining is a front tire peculiarity, not rear.
Theories?
And in Porsche Technical Service Bulletin 9303 about the upgrade to 17" tires/rims, it says to use a 17mm spacer on the rear(s). I wonder why? And do you suppose they mean 17mm each side?
Seeking enlightenment.
11-11-2002, 09:27 PM
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Patrick, what is the width and offset of your front wheels? I assume the 16" wheels are ~55mm. A smaller offset will typically cause more tramlining due to more tire/wheel being on the outside of the spindle rotation axis. The geometry of the '86 944 turbo with stock wheels should experience more tramlining than that of a '88 944 turbo with stock wheels. The flip side to the increased tramlining is often increased steering response. If I had my VanValkenburg book and a scanner with me, I'd scan a diagram and post it for you. Maybe Danno has something he can share.
The caster angle affects the degree to which the vehicle weight is used to return the wheels to the straight ahead position. Increasing caster increases this effect, and thus helps counteract tramlining. The caster will actually cause the inside front of the car to raise and the outside front to lower. The degree to which this happens depends on the caster setting. The caster also affects the dynamic camber.
The caster angle affects the degree to which the vehicle weight is used to return the wheels to the straight ahead position. Increasing caster increases this effect, and thus helps counteract tramlining. The caster will actually cause the inside front of the car to raise and the outside front to lower. The degree to which this happens depends on the caster setting. The caster also affects the dynamic camber.
11-11-2002, 09:58 PM
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This is starting to get interesting.
Steve: The offsets on my 18" Technology hollow spokes is 50. Not sure what it is on the 16s, but if it is approx. 55, not much difference eh?
Steve: The offsets on my 18" Technology hollow spokes is 50. Not sure what it is on the 16s, but if it is approx. 55, not much difference eh?
11-11-2002, 09:59 PM
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This is starting to get interesting.
Steve: The offsets on my 18" Technology hollow spokes front rims is 50. Not sure what it is on the 16s, but if it is approx. 55, not much difference eh?
Steve: The offsets on my 18" Technology hollow spokes front rims is 50. Not sure what it is on the 16s, but if it is approx. 55, not much difference eh?
11-11-2002, 10:24 PM
#13
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" My 225/50/16's in front do NOT tramline at all, but my 225/40/18/s in front tramline like the bumper cars at Great America. Both tires are 225 wide."
Hmmm yes, I guess I should go with 'tread width' rather than 'section/sidewall width'. Same section-width tires would have tread-width on the ground that's about 0.5-inches wider in 18". But I don't think that would explain all that's going on.
I think Steve was alluding to scrub-radius? That's the distance between where the steering-axis's rotation meets the ground and the center of the contact patch.
The '86 cars had positive scrub-radius meaning the tire's contact patch's center is outside the point where the steering axis hits the ground.
The later cars had negative scrub-radius with the contact patch inside the steering axis.
I'm not sure how much of a difference this makes in tramlining. When a tire hits a bump, the force pushes the tire back (relative to car's momentum and direction of travel). This force then causes a torque on the steering in proportion to the distance of the scrub-radius between the contact-patch and steering-axis. Then if the scrub-radius is positive, it makes the left tire dart left, a right tire dart right. With a negative scrub-radius, the left tire darts right and the right tire darts left.
If both wheels hit the same bump, then the two forces oppose each other and there's no darting. Ideally, you would want zero scrub-radius, so that the tire won't want to yank the steering no matter what kind of bump it hits. You can do this by increasing offset on '86 cars while decreasing offset on '87+ cars.
Ok, I left out one important point, which is the length and stiffness of the sidewalls. Whenever a tire hits anything on the road, all those forces has to act through the sidewalls before it gets to the rim, which gets to the spindle, which twists and goes to the steering. So... if you have shorter, stiffer sidewalls like with 18" wheels, then yes, those bumps and twisting forces will be transmitted better though the shorter, stiffer sidewalls and the result will be a more nervous and darty front end.
I guess I'm so used to my car jumping around on bumps and road imperfections that I take it for granted. I've always got a firm grip on the steering and am constantly making corrections to keep the car straight. In fact, I tried out 275/40-17" tires in front on my 17x10.5" wheels before and if I happen to so much as sneeze, it would make a complete lane change!
Try setting additional caster up front (max out adjusters) and see if the car's more well-behaved. Also tires with softer sidewalls, like Michelins would tend to tramline less...
Hmmm yes, I guess I should go with 'tread width' rather than 'section/sidewall width'. Same section-width tires would have tread-width on the ground that's about 0.5-inches wider in 18". But I don't think that would explain all that's going on.
I think Steve was alluding to scrub-radius? That's the distance between where the steering-axis's rotation meets the ground and the center of the contact patch.
The '86 cars had positive scrub-radius meaning the tire's contact patch's center is outside the point where the steering axis hits the ground.
The later cars had negative scrub-radius with the contact patch inside the steering axis.
I'm not sure how much of a difference this makes in tramlining. When a tire hits a bump, the force pushes the tire back (relative to car's momentum and direction of travel). This force then causes a torque on the steering in proportion to the distance of the scrub-radius between the contact-patch and steering-axis. Then if the scrub-radius is positive, it makes the left tire dart left, a right tire dart right. With a negative scrub-radius, the left tire darts right and the right tire darts left.
If both wheels hit the same bump, then the two forces oppose each other and there's no darting. Ideally, you would want zero scrub-radius, so that the tire won't want to yank the steering no matter what kind of bump it hits. You can do this by increasing offset on '86 cars while decreasing offset on '87+ cars.
Ok, I left out one important point, which is the length and stiffness of the sidewalls. Whenever a tire hits anything on the road, all those forces has to act through the sidewalls before it gets to the rim, which gets to the spindle, which twists and goes to the steering. So... if you have shorter, stiffer sidewalls like with 18" wheels, then yes, those bumps and twisting forces will be transmitted better though the shorter, stiffer sidewalls and the result will be a more nervous and darty front end.
I guess I'm so used to my car jumping around on bumps and road imperfections that I take it for granted. I've always got a firm grip on the steering and am constantly making corrections to keep the car straight. In fact, I tried out 275/40-17" tires in front on my 17x10.5" wheels before and if I happen to so much as sneeze, it would make a complete lane change!
Try setting additional caster up front (max out adjusters) and see if the car's more well-behaved. Also tires with softer sidewalls, like Michelins would tend to tramline less...
11-11-2002, 11:02 PM
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Several good things to ponder. Thanks for your insights, Steve and Danno. I think I need to enlighten my tech when he does the alignment this time.
Why the hell do I keep double posting? It's not Groundhogs day yet!
Why the hell do I keep double posting? It's not Groundhogs day yet!
11-12-2002, 12:43 AM
#15
Danno: What is the largest size 17" rubber that will fit inside the fenderwell of a 944 turbo? I have a set of 17" 968 Turbo Cup style wheels and need new rubber. I just got a car with 17x9 and 17x11 Kinesis K20 wheels (not sure of the offset but they look great on my 914 with spacers and 911 hubs) with new 315/35 & 275/40 rubber on them. I'm tempted to try and put the Kinesis wheels on the 944 - seeing as how it looks like it will be a long wait for a 6 cylinder engine for the 914.