Welcome! Log In Create A New Profile Recent Messages

Advanced

New Wheels

Posted by daniel 
January 17, 2009 05:33PM
Hello everyone. I am back safely from Venezuela and sometime in the next week (once I have all of my photos hosted online) I will make a nice post about my trip with my favorite pictures. Until then, I have some other news. As some of you may remember, I have an e36 5-lug setup on my e30. After doing a lot of wheel searching, I got a bit disappointed because I really wanted 16 inch wheels, not 17's, and ALL e36 wheels that are 8 inches or wider are 17's. The issue here is that the e36 setup requires an offset of at least ET35 in order to avoid rubbing issues with the fenders, so I thought many other BMW wheels were out of the question. My car isn't super low, though, and after finding out that the center bore of the e36 is the same as every 5x120 BMW except for the SUVs and the e39, I did some more searching and decided to take a shot at some 16x8 ET23 Style 5's. The offset is very low, but as I mentioned before, my car is at a reasonable height (compared to some e30s). I don't really have money to be buying new rimzzz, but I got these Style 5's for $250 (ended up costing $130 to dismount/mount/balance the tires with new stems...ouch). I think I can sell the rare MOMOs for $380 and break even.

Here are some pictures (taken this morning) of the MOMO Mirage wheels I had before. 16x7.5 ET35:


you can see that ET35 is even already a bit low:

not this morning:


and the new wheels (which came from an e38 740i, by the way):







I would LOVE to kill that fender gap in the front, but the fact of the matter is I still scrape that front lip every once in a while, and I'd rather have a car that can go over speed bumps than one that gets stuck on them. I think the rear looks pretty good right now and it surprisingly isn't rubbing, but I might even raise it a bit to make the fender gaps more even. Maybe at the E30 picnic in June I will slam the front or something. Tires are 225/40ZR16 Toyo T1R. Slightly undersized, but I had them on another set of wheels that I bought a year ago, so I am using them before going to a 245/whateverZR16.

Thanks for looking!

PS- i LOVE the way that rear tire is tucked in right inside the fender. mmm mmm mmmm!


January 17, 2009 07:37PM
Sweet!
January 17, 2009 07:51PM
Nice stance and good looking wheels but you gotta do something with the paint man!
January 18, 2009 12:14AM
Quote
Archeo-peteriX
Nice stance and good looking wheels but you gotta do something with the paint man!

yes the paint is terrible. the car was keyed by someone and actually says FUCK ME in the RR quarter panel. Photographs really bring out the terrible paint. it doesnt look as bad in person. It is worst on the hood and the roof (the spoiler is bad too). I could easily/cheaply paint the hood black and the spoiler black, but the roof is another issue. Right now mechanicals are the priority though, so no money towards a paint job. There is also a big dent in the passenger door, which would probably be taken care of easiest with a new lachsilber door. With all that said, the car has been way more reliable and cheaper than any other e30 i have had, so i am quite pleased with my risky purchase over the summer.

thanks for the complements.


January 18, 2009 08:17AM
Nice. A definite improvement over the old wheels!

Wheel selection is why, when/if I get around to my own S52 conversion, I intend to keep the 4-lug E30 setup. I love my Ronal LS wheels, and have no desire for any wheel larger than 15". Plus, I already have the steelies with Blizzaks for ice racing.

I'm glad you are enjoying your new car, even if it does say "F-You" on it. thumbs down

Cab
1990 325i(s)
2004 325XiT
January 18, 2009 11:40AM
Why did you go for Z-rated tyres? You don't need those and could mean a real difference in the price.
The MOMO's weren't bad for aftermarket wheels but these look SO much better.
January 18, 2009 12:01PM
Quote
Cab Treadway
Nice. A definite improvement over the old wheels!

Wheel selection is why, when/if I get around to my own S52 conversion, I intend to keep the 4-lug E30 setup. I love my Ronal LS wheels, and have no desire for any wheel larger than 15". Plus, I already have the steelies with Blizzaks for ice racing.

I'm glad you are enjoying your new car, even if it does say "F-You" on it. thumbs down

Well, having bought the car with the 5-lug already done, I didn't really have much choice. It's a sacrifice I am willing to make though since it also means I have an e36 M3 brake setup in the front. One downside to that is the smallest wheel I can put on the car is a 16". I wouldn't mind having 15x7.5 or 16x7.5 wheels, but the 225 tread width is somewhat of a limiter here. Even in warm weather its extremely easy to spin the tires with all the power the car has. One reason I got the 16x8's is so I could put 245's on it.


January 18, 2009 12:07PM
Quote
Michiel 318iS
Why did you go for Z-rated tyres? You don't need those and could mean a real difference in the price.
The MOMO's weren't bad for aftermarket wheels but these look SO much better.

I have Z-rated tires (technically W, but it falls into the Z category) because over a year ago I bought 4 wheels with those four tires. To save myself some money, I put the tires on my current wheels and sold the wheels without tires. Also, I do not mind the extra grip with an S52 under the hood. So technically I didn't buy the tires (at least not anytime recently). Hopefully these last a while, but the next set of tires I want to buy are 245/40ZR16.

I agree, the MOMOs aren't totally bad, but they were out of place and also not in very good condition.


January 18, 2009 12:10PM
ha! i just did a search for 245/40-16 tires and they don't appear to exist. Oh well, I have quite some time before I need new tires anyway.


January 18, 2009 02:57PM
Quote
daniel
Well, having bought the car with the 5-lug already done, I didn't really have much choice.

Agreed, I wasn't trying to say the 4-lug was necessarily preferred, just saying that when I ever find the money to do this conversion, I'm happy with what I have as far as wheels go. My wheels are 15x7.5, so I may be able to put enough width on there, if I run into the same problem you have. Or maybe at that point I'll decide that the E30 wheel setup is holding it back, and then I'll do the wheel conversion. Who knows. It's all a dream at this point anyway.

I've never felt any problem with the E30 brakes, and while yes, there's a lot more power from the S52, the car doesn't really weigh all that much more, so I would be willing to give the stock brakes a chance before deciding they need to be upgraded.

Cab
1990 325i(s)
2004 325XiT
January 18, 2009 04:18PM
You should go for 245/35ZR16 if you want to keep about the same circumference (or total tyre height).

If you go for 245/40ZR16, you'll have about stock circumference (compared to 205/55R15), but it might rub in your wheelarches.
January 18, 2009 04:57PM
Quote
Cab Treadway
Quote
daniel
....

I've never felt any problem with the E30 brakes, and while yes, there's a lot more power from the S52, the car doesn't really weigh all that much more, so I would be willing to give the stock brakes a chance before deciding they need to be upgraded.

I've never understood the contrary logic used to justify bigger brakes on a car just because it has more hp. If the weight of the care is basically unchanged, then the amount of braking power needed to stop the car from any speed will be the same irrespective of what the power is.

The only time bigger brakes might be an advantage is n track racing where grippy tires, hard braking and frequent braking are needed. For street use or the occasional auto-x, I doubt there is any advantage to be had.
January 18, 2009 05:00PM
I wonder if we need to revisit that old tire 'patch area' thing again?

Or is this just about the macho look factor of the tires?
January 18, 2009 06:29PM
Quote
Archeo-peteriX
I've never understood the contrary logic used to justify bigger brakes on a car just because it has more hp. If the weight of the care is basically unchanged, then the amount of braking power needed to stop the car from any speed will be the same irrespective of what the power is.
Yes, but presumably with more power the speeds attained will be higher.

Quote

The only time bigger brakes might be an advantage is n track racing where grippy tires, hard braking and frequent braking are needed. For street use or the occasional auto-x, I doubt there is any advantage to be had.
I agree. Autocross driving, and especially street driving, is not hard on brakes. Track driving is, but even then it depends on the track, mainly in the number of hard braking zones and the amount of cooling-down stretches between the braking zones. It also depends on the car. My Mazda Miata will run laps perfectly happily all day long on any track, with its small stock brakes (but race pads). Other cars, like the Nissan 350Z for example, have stock brakes that are simply too small to stand up to track use. Any 350Z that spends much time on the track will need bigger brakes.

I should know about how E30 brakes fare, but I don't.

__________
Dave
'91 325iX
January 18, 2009 08:41PM
LOOKS SWEEET!
rkj
January 18, 2009 11:13PM
Sweet indeed smiling bouncing smiley nice choice Daniel thumbs up
January 19, 2009 12:47AM
Quote
Archeo-peteriX
Quote
Cab Treadway
Quote
daniel
....

I've never felt any problem with the E30 brakes, and while yes, there's a lot more power from the S52, the car doesn't really weigh all that much more, so I would be willing to give the stock brakes a chance before deciding they need to be upgraded.

I've never understood the contrary logic used to justify bigger brakes on a car just because it has more hp. If the weight of the care is basically unchanged, then the amount of braking power needed to stop the car from any speed will be the same irrespective of what the power is.

The only time bigger brakes might be an advantage is n track racing where grippy tires, hard braking and frequent braking are needed. For street use or the occasional auto-x, I doubt there is any advantage to be had.

big brakes look cool, too.


January 19, 2009 12:47AM
Quote
Archeo-peteriX
I wonder if we need to revisit that old tire 'patch area' thing again?

Or is this just about the macho look factor of the tires?

yes fatter tires look cooler. why do they make tires of different width if the width doesnt make a difference?


January 19, 2009 12:49AM
Quote
JUMPNYC
LOOKS SWEEET!

Quote
rkj
Sweet indeed smiling bouncing smiley nice choice Daniel thumbs up

thanks guys smileys with beer


January 19, 2009 07:11AM
Quote
Archeo-peteriX
I've never understood the contrary logic used to justify bigger brakes on a car just because it has more hp. If the weight of the care is basically unchanged, then the amount of braking power needed to stop the car from any speed will be the same irrespective of what the power is.

The only time bigger brakes might be an advantage is n track racing where grippy tires, hard braking and frequent braking are needed. For street use or the occasional auto-x, I doubt there is any advantage to be had.

I completely agree. "They" always say that with more power you need more brakes, but I feel the same as you, especially for the type of driving that I would be doing.

For a lot of track driving, you definitely do need to consider your brakes, because with more power, you'll be going faster at most points on the track, so the required braking force will be higher, which cooks your brakes faster and leads to fading if you don't have the right setup. But for the driving I like to do, which is street and autoX, I don't have a need for big brakes. Daniel is right, though, they do look cool.

Anyway, as I figure it, when I finish school and start working on the E30 project, there are several big brake kits available, but I will not be doing one of those with the project. I figure it's not needed, plus it's the kind of project that can be done relatively easily later on down the road, there isn't a significant advantage to doing that when the car is completely apart like it will be doing an engine swap.

Cab
1990 325i(s)
2004 325XiT
January 19, 2009 07:17AM
Quote
daniel
Quote
Archeo-peteriX
I wonder if we need to revisit that old tire 'patch area' thing again?

Or is this just about the macho look factor of the tires?

yes fatter tires look cooler. why do they make tires of different width if the width doesnt make a difference?

Well, assuming the same weight of the car and the same tire pressures, it does change the shape of your contact patch, from long and narrow to shorter and wider. The wider patch gives you different performance in terms of lateral grip, but if you want "more" contact patch, you need to lower the tire pressures. I'm not sure if it's harder to spin tires with a wider patch; I don't understand why it would be, since the same area of road is contacted by the tires.

When you are under hard acceleration and have significant weight transfer to the rear, it may be easier for a wider tire to deform enough to account for the increased weight. A narrower tire would have to "compress" itself farther out of round to increase the patch than a wider tire would. That's just a guess, though.

Cab
1990 325i(s)
2004 325XiT
January 19, 2009 10:00AM
Quote
daniel
Quote
Archeo-peteriX
Quote
Cab Treadway
Quote
daniel
....

I've never felt any problem with the E30 brakes, and while yes, there's a lot more power from the S52, the car doesn't really weigh all that much more, so I would be willing to give the stock brakes a chance before deciding they need to be upgraded.

I've never understood the contrary logic used to justify bigger brakes on a car just because it has more hp. If the weight of the care is basically unchanged, then the amount of braking power needed to stop the car from any speed will be the same irrespective of what the power is.

The only time bigger brakes might be an advantage is n track racing where grippy tires, hard braking and frequent braking are needed. For street use or the occasional auto-x, I doubt there is any advantage to be had.

big brakes look cool, too.

Cool and $1.50 worth of flashy decals will make the car look like it has 20 extra hp to the uninitiated grinning smiley
January 19, 2009 10:07AM
Quote
Archeo-peteriX
Quote
daniel
Quote
Archeo-peteriX
Quote
Cab Treadway
Quote
daniel
....

I've never felt any problem with the E30 brakes, and while yes, there's a lot more power from the S52, the car doesn't really weigh all that much more, so I would be willing to give the stock brakes a chance before deciding they need to be upgraded.

I've never understood the contrary logic used to justify bigger brakes on a car just because it has more hp. If the weight of the care is basically unchanged, then the amount of braking power needed to stop the car from any speed will be the same irrespective of what the power is.

The only time bigger brakes might be an advantage is n track racing where grippy tires, hard braking and frequent braking are needed. For street use or the occasional auto-x, I doubt there is any advantage to be had.

big brakes look cool, too.

Cool and $1.50 worth of flashy decals will make the car look like it has 20 extra hp to the uninitiated grinning smiley

You guys never remember the important stuff...

Don't forget the yellow differential! That makes all the difference!!!!

Cab
1990 325i(s)
2004 325XiT

January 19, 2009 10:18AM
I can buy into the 'cool' factor; after all it's part of what attracts us to the E30 smileys with beer
January 19, 2009 10:20AM
Quote
Cab Treadway
Quote
Archeo-peteriX
Quote
daniel
Quote
Archeo-peteriX
Quote
Cab Treadway
Quote
daniel
....

I've never felt any problem with the E30 brakes, and while yes, there's a lot more power from the S52, the car doesn't really weigh all that much more, so I would be willing to give the stock brakes a chance before deciding they need to be upgraded.

I've never understood the contrary logic used to justify bigger brakes on a car just because it has more hp. If the weight of the care is basically unchanged, then the amount of braking power needed to stop the car from any speed will be the same irrespective of what the power is.

The only time bigger brakes might be an advantage is n track racing where grippy tires, hard braking and frequent braking are needed. For street use or the occasional auto-x, I doubt there is any advantage to be had.

big brakes look cool, too.

Cool and $1.50 worth of flashy decals will make the car look like it has 20 extra hp to the uninitiated grinning smiley

You guys never remember the important stuff...

Don't forget the yellow differential! That makes all the difference!!!!

I still think you didn't give the plaid one a good enough try out smiling bouncing smiley
January 19, 2009 10:22AM
Quote
daniel
Quote
Archeo-peteriX
I wonder if we need to revisit that old tire 'patch area' thing again?

Or is this just about the macho look factor of the tires?

yes fatter tires look cooler. why do they make tires of different width if the width doesnt make a difference?

So people will think they're better. Bigger is always better winking smiley
January 19, 2009 10:34AM
Quote
Cab Treadway
Quote
daniel
Quote
Archeo-peteriX
I wonder if we need to revisit that old tire 'patch area' thing again?

Or is this just about the macho look factor of the tires?

yes fatter tires look cooler. why do they make tires of different width if the width doesnt make a difference?

Well, assuming the same weight of the car and the same tire pressures, it does change the shape of your contact patch, from long and narrow to shorter and wider. The wider patch gives you different performance in terms of lateral grip, but if you want "more" contact patch, you need to lower the tire pressures. I'm not sure if it's harder to spin tires with a wider patch; I don't understand why it would be, since the same area of road is contacted by the tires.

When you are under hard acceleration and have significant weight transfer to the rear, it may be easier for a wider tire to deform enough to account for the increased weight. A narrower tire would have to "compress" itself farther out of round to increase the patch than a wider tire would. That's just a guess, though.

I think some of the misconception about wide tires and grip comes from seeing those huge tires on dragsters. Most people don't realize that those tires are so wise because of what happens when the power is applied. On the dragster, the tires are almost immediately spun up to maximum speed and they become donut shaped with the contact patch getting reduced to a fraction of what it was when the tire was resting.

On a street car, the tires are designed to maintain the same patch size and shape as much as possible so that tire life and handling are optimized. I doubt there is much difference, if any between the wide and narrow tires under acceleration as far as patch size is concerned.

Differences in tire construction would have a greater affect; the number and composition of belts used, side wall stiffness and perhaps tread pattern.

Either way, the coolness factor is there and a wide surefooted stance has the psychological affect of making the care seem faster or better handling.
January 19, 2009 11:09AM
Quote
Archeo-peteriX
I think some of the misconception about wide tires and grip comes from seeing those huge tires on dragsters.
Maybe, but huge tires win the trophies in autocross, too. My track/autocross car is a Mazda Miata. Until a few years ago, the hot tire to have in CSP was either a 225 Hoosier or 215 Kumho V710. Both of those are really fat on a Miata. Then, a couple years ago, Hoosier came out with a monster 275/35-15 in their A6 autocross tire. People tried them on their Miatas, and guess what? They were faster. Now it's nearly impossible to win at the national level unless you're on 275s. IOW, we haven't yet reached the limit of fatness -- fatter is always faster.

On the track, it's a different story. At high speeds, tires that are too fat can slow you down if you don't have the power to drive them. At stock power levels, the fastest tires for my Miata are 205s. Cars with forced induction are faster on 225s. 500 HP Vettes need 315s to put the power down, or else they just spin their wheels at every turn.

Sorry I don't have comparable data for track E30s, but I'm sure the phenomenon still holds.

__________
Dave
'91 325iX
January 19, 2009 12:05PM
Quote
Archeo-peteriX
On a street car, the tires are designed to maintain the same patch size and shape as much as possible so that tire life and handling are optimized. I doubt there is much difference, if any between the wide and narrow tires under acceleration as far as patch size is concerned.

There may not be a huge difference, but I don't think the entire reason behind why high performance cars have wide tires is the cool factor.

Quote

Differences in tire construction would have a greater affect; the number and composition of belts used, side wall stiffness and perhaps tread pattern.

That was my idea, if the side wall stiffness is similar from the narrow and wider tires, then each tire will be allowed to compress a little bit, forming the contact patch. As the narrower tire gets more weight on it, it still can only compress so much without damage, so the tire doesn't handle the increased load as well. A wider tire can compress (flatten) the same amount, and will allow a larger patch because of the increased width. I have no data to back this up, just a guess on how the physics might work.

Quote

Either way, the coolness factor is there and a wide surefooted stance has the psychological affect of making the care seem faster or better handling.

There is definitely a handling benefit, because the wider tire with the shorter/wider contact patch has greater lateral grip, so it grips better under hard cornering without rolling over. The misconception is when people stuff huge tires in there thinking they're getting a larger contact patch when they're not. But it does help in some aspects of performance, which is why wider (and stickier) tires help in autoX, where top speed is not high, but lateral grip must be optimized.

Cab
1990 325i(s)
2004 325XiT
January 19, 2009 12:34PM
Quote
Dave_G
we haven't yet reached the limit of fatness -- fatter is always faster.
There is a limit though...


3-2-1-Go! Drop the clutch, and bang. Sheared all the wheel studs and both rear wheels took off without the car.
January 19, 2009 02:00PM
I still don't understand why a narrow tire and a wide tire with the same sized patch would have different grip...in any direction. It's all about friction and if I have 7 sq in of patch area for example, the grip should be the same since the same physical laws are acting on both. Why should a square patch have more friction than a rectangular on with the same surface area?

It must be something other than patch size that makes the difference. The only thing that my tiny brain can think of is that the patch size changes during cornering with the different tires. The wider tire with the stiffer side walls may keep more of the patch in contact with the road than the taller softer side walled narrower tire.

Bear with me, I'm just trying to wrap my head around this winking smiley



Edited 1 time(s). Last edit at 01/19/2009 02:00PM by Archeo-peteriX.
January 19, 2009 02:49PM
Take a cardboard cereal box standing up on end. Push on it on the wide side, it falls over very easily. Push on it on the narrow side, it requires a pressure point higher up on the side of the box to get it to tip over. The force is different even though the "contact patch" area is the same. The cereal boxes contact patch is more stable when a force is put on it from different directions. Same idea with tires. A wider (even though it's narrower) contact patch will provide more solid grip when cornering before the tire begins to fold over and funky things happen with the tires.

Cab
1990 325i(s)
2004 325XiT
January 19, 2009 03:25PM
since i have felt like i've indirectly been taking some flack for having bigger brakes on my car, i would like to reiterate the fact that, when doing the e36 5-lug conversion, one must use the brakes from the e36 m3. Since they are already on the car, I might as well keep them. I don't think there is much arguing that they are superior to other e36 brakes regardless of the weight of the car. Maybe they are overkill on an e30, but if less braking force is needed to stop with the e36 m3 brakes, then theoretically they should last a bit longer anyway. and as i said before, they look cool B)-

and i am glad that my desire to have wider tires on my car has sparked such an interesting discussion about tires. to me it makes sense that, when moving laterally, wider tires will provide better grip.


January 19, 2009 04:41PM
Quote
Cab Treadway
Take a cardboard cereal box standing up on end. Push on it on the wide side, it falls over very easily. Push on it on the narrow side, it requires a pressure point higher up on the side of the box to get it to tip over. The force is different even though the "contact patch" area is the same. The cereal boxes contact patch is more stable when a force is put on it from different directions. Same idea with tires. A wider (even though it's narrower) contact patch will provide more solid grip when cornering before the tire begins to fold over and funky things happen with the tires.

So the key here is that the wider stiffer tire will retain a larger portion if it's contact area than the narrower tire during 'fold over'...that makes sense. I think smiling smileyo
January 19, 2009 04:44PM
Quote
daniel
since i have felt like i've indirectly been taking some flack for having bigger brakes on my car, i would like to reiterate the fact that, when doing the e36 5-lug conversion, one must use the brakes from the e36 m3. Since they are already on the car, I might as well keep them. I don't think there is much arguing that they are superior to other e36 brakes regardless of the weight of the car. Maybe they are overkill on an e30, but if less braking force is needed to stop with the e36 m3 brakes, then theoretically they should last a bit longer anyway. and as i said before, they look cool B)-

and i am glad that my desire to have wider tires on my car has sparked such an interesting discussion about tires. to me it makes sense that, when moving laterally, wider tires will provide better grip.

No flack Daniel, we're just having a nice thinking session about the merits of wider tires and bigger brakes. There is nothing wrong with your setup and certainly no need to change any of it. It's cool and as you can see, others here might like to have the same or similar setup at sometime down the road B)-
January 20, 2009 06:19AM
There is no intended flak from my viewpoint, I apologize if I've come across that way. In motorsports, bigger brakes are, in general, better than smaller brakes. It's certainly not something I would get rid of if my car came that way. I'm just saying that for my intended purposes, I don't think a BBK is necessary, or even desired. I would like to retain my 14' and 15" wheels, I feel that the stopping power of the stock E30 brakes is more than adequate, and the one negative thing that a BBK adds is more unsprung weight. I don't think that the benefits of a BBK would outweigh (no pun intended, of course) the extra weight. Plus, stopping distances at street and autoX speeds are more dependent on the tires than on the brakes. The main benefit of a BBK is to resist brake fade and overheating on a track. I'm not a track junkie, and don't intend to be, so for me, a BBK doesn't seem worth the extra $1k.

All that said, there's nothing at all wrong with big brakes, and I certainly wouldn't eliminate them if my car already came equipped. I'm glad you are enjoying your car! They certainly do look cool, I know I've taken many pictures of brakes at car shows, a nice brake package really looks great.

Cab
1990 325i(s)
2004 325XiT
January 20, 2009 06:49AM
Quote
Archeo-peteriX
Quote
Cab Treadway
Take a cardboard cereal box standing up on end. Push on it on the wide side, it falls over very easily. Push on it on the narrow side, it requires a pressure point higher up on the side of the box to get it to tip over. The force is different even though the "contact patch" area is the same. The cereal boxes contact patch is more stable when a force is put on it from different directions. Same idea with tires. A wider (even though it's narrower) contact patch will provide more solid grip when cornering before the tire begins to fold over and funky things happen with the tires.

So the key here is that the wider stiffer tire will retain a larger portion if it's contact area than the narrower tire during 'fold over'...that makes sense. I think smiling smileyo

The other thing with the shape of the patch has to do with how the force is applied. The tire is attached to the wheel, which is attached to the car at the hub. Basically all the force is transmitted from the road to the car at the hub. So you have a focused force opposite in direction from the way the car is turning that keeps the car from sliding. The force can be represented by a vector (think of an arrow pointed right at the middle of the contact patch). If the vector is centered in the long side of a long and narrow patch, there is less rubber directly in line with the vector, and the force can push the middle of the contact patch more easily than if it is pushing against a shorter but wider patch.

When you're going forward, the shape of the contact patch is less important (unless you're dealing with driving through snow/rain, in which case a narrower tire is better b/c it has less snow/water to have to displace), but for lateral grip, wider gives more resistance to the sideways motion.

Cab
1990 325i(s)
2004 325XiT
January 20, 2009 07:40AM
Quote
Cab Treadway
When you're going forward, the shape of the contact patch is less important
Your explanation makes perfect sense from a physics theory perspective. However, does this imply that a monstrously powerful car like a 500-HP Corvette Z06 should accelerate in a straight line just as quickly with skinny 185s as it does with the 325s that it comes with? Experience would suggest otherwise, i.e. that more powerful cars need wider tires to put the power down to the road. If this is the case, then either the wider tires have a greater contact patch, or the shape of the contact patch is still important in a straight line.

__________
Dave
'91 325iX
January 20, 2009 08:03AM
Quote
Dave_G
Your explanation makes perfect sense from a physics theory perspective. However, does this imply that a monstrously powerful car like a 500-HP Corvette Z06 should accelerate in a straight line just as quickly with skinny 185s as it does with the 325s that it comes with? Experience would suggest otherwise, i.e. that more powerful cars need wider tires to put the power down to the road. If this is the case, then either the wider tires have a greater contact patch, or the shape of the contact patch is still important in a straight line.

I don't know for sure, but my intuition (which is often wrong) tells me that a high HP car like that is going to have substantial weight transfer to the rear under hard acceleration, which will cause a 185 tire to squat so much that it would be extremely deformed, and this probably has an effect on the ability to put down power. It's not just the weight of the car, the acceleration will put more force on the rear wheels than just the car's weight, so the tire has to be strong enough and wide enough to create enough of a contact patch without overwhelming the tire's capabilities...

I don't know...

Cab
1990 325i(s)
2004 325XiT
January 20, 2009 10:41AM
Quote
Cab Treadway
When you're going forward, the shape of the contact patch is less important (unless you're dealing with driving through snow/rain, in which case a narrower tire is better b/c it has less snow/water to have to displace)
That's an important factor. A narrow tire will cut through snow, whereas a wide tire will tend to float up on the snow losing contact with the road surface.

Quote
Cab Treadway
If the vector is centered in the long side of a long and narrow patch, there is less rubber directly in line with the vector, and the force can push the middle of the contact patch more easily than if it is pushing against a shorter but wider patch.
Now we're making progress.

How much a tire will deform under side loading depends on many different factors like sidewall stiffness, tire pressure, camber angle, etc.. When I first started parking lot pylon racing, my tires used to fold over so far that I was grinding the lettering off the sidewalls. Someone suggested pumping the pressure up to 45 psi and that made a huge difference!

But, assuming the two tires otherwise have equal characteristics, the only difference being one has a long skinny contact patch and the other one has a short wide patch. With the car sitting still in your driveway, have someone big and strong to lean against the side of your car. The car will move slightly sideways, trying to stretch the tire off the rim. Given the same sidewall stiffnesses etc, I'm thinking either tire should deflect pretty much the same amount. The big difference is how the tires behave under side loading while the tire is rolling.

The tire is stretched sideways at the contact patch, while the rest of the tire doesn't change shape. So, when the tire is rolling forward under side loading, the new patch of rubber comes straight down, is planted on the road, and subsequently stretched sideways. Even though the rim and the rest of the tire is pointed straight ahead the whole time, because of the way the rubber moves as it comes down, is planted, then stretched, the tire actually rolls forward at a slight angle to the direction it's pointing in. That's called the "drift angle".

Note, do not confuse "drift angle" with the currently fashionable sport of "drifting", which actually involves breaking traction completely and sliding the tires. "Drift angle" occurs any time there is a side load on the tires while they are rolling while planted firmly with grip. Even though the tire is pointed straight ahead, it's actually crabbing off at an angle away from where it's pointed.

A tall skinny tire with a long narrow contact patch will have a higher "drift angle" than a wide tire with a short wide contact patch.

This photo of Jack Brabham at Zandvoort in 1966 shows the extremely high drift angles of the tall treaded tires they used back in those days. The tires are not "sliding". At the very limit of grip, this is the extreme crab angle at which the tires worked best. Wider low profile tires, having a shorter wider contact patch, operate at much smaller drift angles giving the appearance that modern cars are operating on rails.

The advantage of tires that operate best at smaller drift angles is in the response time when transitioning from turning one way to another. Looking at the photo of the 1966 Brabham F1 car, you can imagine there's quite a lag time between having the car set at maximum right hand cornering as shown, and then pitching the car the other way into maximum left hand cornering, whereas a modern car with very small drift angle tires will react instantly to left/right transitions.


January 20, 2009 02:39PM
Thanks Ferd...that was what was missing. I can actually visualize this happening.
Funny how that which seems so obvious can be somewhat incorrect if all the pieces of the puzzle aren't there grinning smiley
Sorry, only registered users may post in this forum.

Click here to login

Online Users

Guests: 5
Record Number of Users: 3 on September 29, 2015
Record Number of Guests: 109 on June 08, 2017