[pikcachu]

245s in the front would help.....
higher psi at front also although that will have less effect than 245s

[Shovelman]

DOH you are right, the tires are Continental DWS 06, 205/50/17

Understeer is not often a problem on dry pavement. The car corners like it is on rails and unless I get into trouble I won't be worrying about it there. It is on wet or icy or dusty pavement, and on dirt that it is annoying.

Yes applying a touch of braking or a moment of excess steering input should make a RWD car get a little tail happy. But this car just plows. A little excess throttle should do it but the nannies, even in sport mode, prevent it.

[dradernh]

Originally Posted by Shovelman

DOH you are right, the tires are Continental DWS 06, 205/50/17

You have a tiny contact patch for a car this heavy and with this much power. My take is the tire's greatest strength is likely to be hypermiling – pump them up, and you can probably get high-30s on the highway, maybe even 40 mpg.

FWIW, my wife's Mazda 3 came with 215/45-18s all around.

[Sportstick]

Originally Posted by Shovelman

DOH you are right, the tires are Continental DWS 06, 205/50/17.

Did you pick that narrow size because they stay on all year and you saw some prior threads about winter tires performing better when narrower than usual? I don’t know if any website that would suggest that size for a standard usage. How did you wind up with that?

[Apo Zubulthums]

I have an M-Sport with staggered wheels and tried going 36 psi all around. It seems to have eliminated the front end push I was experiencing.

[albertw]

Originally Posted by Shovelman

Understeer is not often a problem on dry pavement. The car corners like it is on rails and unless I get into trouble I won't be worrying about it there. It is on wet or icy or dusty pavement, and on dirt that it is annoying.

I'm certainly not an expert, but my interpretation is that on wet pavement the front tires push some water to the side so the rears have better traction, which leads to understeer.

On icy or dusty pavement it is easy to turn the wheel farther than the traction available can handle, so the front wheels start to slide before the car can load the rears. That feels like understeer but I think there should be a different word for it.

[Shovelman]

Those are the standard size that came on the car new. The contact patch is similar to lower profile tires on larger rims. (50, 60, etc is a ratio) 17" rims are best for rough streets. A bigger contact area is not a good thing on wet or snowy roads. These are good tires for a DD.

[Sportstick]

Originally Posted by Shovelman

Those are the standard size that came on the car new. The contact patch is similar to lower profile tires on larger rims. (50, 60, etc is a ratio) 17" rims are best for rough streets. A bigger contact area is not a good thing on wet or snowy roads. These are good tires for a DD.

I had to go back and check the owner's manual and I was surprised to learn that there was a 205 tire in the OEM 17" line-up. You are absolutely correct. My 17s are 225/45-17, also an OE size, The 205 has usually been reserved in our chats here for getting skinny for winter. Was your car originally a Luxury model versus Sportline?

[aerobod]

Originally Posted by Sportstick

I had to go back and check the owner's manual and I was surprised to learn that there was a 205 tire in the OEM 17" line-up. You are absolutely correct. My 17s are 225/45-17, also an OE size, The 205 has usually been reserved in our chats here for getting skinny for winter. Was your car originally a Luxury model versus Sportline?

In some markets, 205/50-17 summer tyres are specified, for example the Bridgestone Potenza S001: https://www.realoem.com/bmw/enUS/sho...diagId=36_2136

[Maynard]

FWIW, contrary to the marketing hype (and about 3/4 of the posts on here) the contact patch SIZE is always the same (at the same pressure and car weight) regardless of whether it is a 205 or a 255 - it just gets longer and narrower on a narrow tire (best for heavy rain/snow). Wider tires don't put more rubber on the road, just on the wheel.

And if the problem only occurs on very wet or muddy roads, then it is natural physics, not a question of handling balance. This is like the poster case for why the automakers try to dial in just a touch of understeer, because it is soooo much better that you bitch about a little plowing that is easy to control, rather than joining the 'where can I find a good body shop' club.

[Shovelman]

Hi Maynard
Interesting idea 'same size patch'. Well it points out a trend in similar tires but at the extremes it wanders off. Tires are more than balloons. The sidewall carries some of the weight and the center of the tread carries some. So in very narrow tires the sidewalls carry more and on very wide tires the tread carries more. Still, the trend is instructive.

205/50/17 is about the same contact patch as 225/45/18 so these are not really narrow tires. I could go to 225/17 on the front but that would make understeer worse on wet, snowy, dusty pavement and tire rotation would be compromised.

[dradernh]

Originally Posted by Maynard

FWIW, contrary to the marketing hype (and about 3/4 of the posts on here) the contact patch SIZE is always the same (at the same pressure and car weight) regardless of whether it is a 205 or a 255 - it just gets longer and narrower on a narrow tire (best for heavy rain/snow). Wider tires don't put more rubber on the road, just on the wheel.

I've come across this before, and for some darn reason I never remember it. Maybe that's because I'm all about wider being better!

I went to a remarkably comprehensive tire calculator page to find the contact patch sizes for the 245/35-18 that came on the rear of my car and the 205/50-17 tires that came on OP's car. The difference was 2.2%, or virtually nil for our purposes.

The calculator is on this page: http://bndtechsource.ucoz.com/index/...alculator/0-20,

and this is its input form:


and these are its static and dynamic output forms:

[Maynard]

Originally Posted by dradernh

I've come across this before, and for some darn reason I never remember it. Maybe that's because I'm all about wider being better!

I went to a remarkably comprehensive tire calculator page to find the contact patch sizes for the 245/35-18 that came on the rear of my car and the 205/50-17 tires that came on OP's car. The difference was 2.2%, or virtually nil for our purposes.

The calculator is on this page: http://bndtechsource.ucoz.com/index/...alculator/0-20,

and this is its input form:



and these are its static and dynamic output forms:

That is a really cool page - serious competition for the paperwork sitting in front of me for sure. I think they are getting down into the real nitty-gritty of geometric angles, but I learned this based off of the simplistic physics: contact patch is weight/air pressure, so a tire loaded with 800lbs at 40psi will have a patch that is 20 square inches. It is perfect with a balloon, but I'm sure the thickness of the tread adds a trig function in there to account for the extra depth, probably also some factor from the sidewall rather than air pressure too.

[aerobod]

The other thing to remember is that friction is proportional to friction coefficient and load, so for the same load and same rubber compound with the same mu, the friction (grip) will be the same. The tyre compound is much more important than the width - the width, profile and tread variations will just allow a softer compound to be used before destroying it.

[albertw]

Originally Posted by Maynard

FWIW, contrary to the marketing hype (and about 3/4 of the posts on here) the contact patch SIZE is always the same (at the same pressure and car weight) regardless of whether it is a 205 or a 255 - it just gets longer and narrower on a narrow tire (best for heavy rain/snow). Wider tires don't put more rubber on the road, just on the wheel.
.

I applied the tire calculator page provided by draderh to a few more realistic comparisons, and I didn't get results that agree with this claim.

For example, for the default sizes that come up on the web page, 245/45ZR17 vs 275/40ZR18 the wider tire has a 23% larger contact area.

For the tires sizes on our cars, 225/40ZR18 and 245/35ZR18 I get an 18% larger contact area for the 245s. (That's using wheel sizes of 7.5 and 8, which I think are correct. The results are quite insensitive to wheel width. I used a load index number of 95. I couldn't find those numbers, but if they are the same then the actual number makes little difference.)

I'm calling BS on this claim that tire width makes no difference to contact patch area.

[albertw]

Originally Posted by aerobod

The other thing to remember is that friction is proportional to friction coefficient and load, so for the same load and same rubber compound with the same mu, the friction (grip) will be the same..

I have some trouble applying this law of friction to tires.

If you change to tires with a larger contact patch (for example comparing the 225s and 245s that are available on our cars, which have an 18% difference in contact patch area according to the calculator provided by draderh) the weight per square inch of the contact patch should decrease exactly as much as the contact patch area increases, so the total friction generated by the contact patch should be the same. Practical experience indicates this is not true - otherwise there would be no reason to use wider tires.

My hand-waving argument is that the law of friction applies to smooth surfaces, where only microscopic irregularities cause friction. Road surfaces have macroscopic irregularities that distort the contact area and it takes extra force to move the tire over those irregularities. Thus tires with a larger contact area can produce more friction since they cover more macroscopic irregularities.

It would be great if someone could provide a better explanation of why we're not using bicycle tires on cars since by the basic theory of friction they should produce just as much cornering force.

[aerobod]

I would take that tyre characteristics calculator with a grain of salt without being able to look at their input equations. If you put in the same parameters for wheel diameter, tyre load index and adjust the profile to give the same tyre diameter, then recalculate with the same tyre pressure and corner weight, it gives exactly the same contact patch length for a 205/50-18 tyre vs a 275/35-18 one. This goes against all tyre manufacturer information showing contact patch photos with different tyre width. The contact patch area is just a simple calculation of length x width in the application, this is unlikely to be true for any tyre.

It just doesn't pass any reasonable analysis as being accurate, but may overall give a general indication of trends. Most likely the equations used are very basic and not based on real-world materials values and characteristics or dynamic tyre properties.

[Moflow]

Originally Posted by albertw

I have some trouble applying this law of friction to tires.

If you change to tires with a larger contact patch (for example comparing the 225s and 245s that are available on our cars, which have an 18% difference in contact patch area according to the calculator provided by draderh) the load per square inch of the contact patch should decrease exactly as much as the contact patch area increases, so the total friction generated by the contact patch should be the same. Practical experience indicates this is not true - otherwise there would be no reason to use wider tires.

My hand-waving argument is that the law of friction applies to smooth surfaces, where only microscopic irregularities cause friction. Road surfaces have macroscopic irregularities that distort the contact area and it takes extra force to move the tire over those irregularities. Thus tires with a larger contact area can produce more friction since they cover more macroscopic irregularities.

It would be great if someone could provide a better explanation of why we're not using bicycle tires on cars since by the basic theory of friction they should produce just as much cornering force.

Pretty much this. I'm not going to question the math, but in the real world, wider tires end up having noticeably more grip. I drove the same car for almost 14 years with no mods other than trying out different brands/sizes of tires. Switching from a 20mm stagger to a square setup made the car much more prone to oversteer. I understand that the actual contact patch width may or may not have been the thing that changed the handling balance, but practically speaking, wider tires = more grip assuming you're not going crazy with other variables.

Which is not to say other variables won't have as big of an effect, my POS Lemons race car went from "neutral" to "insane oversteer" when i swapped the front knuckles, slightly changing the front suspension geometry while leaving the tires completely untouched.

[aerobod]

Originally Posted by albertw

It would be great if someone could provide a better explanation of why we're not using bicycle tires on cars since by the basic theory of friction they should produce just as much cornering force.

The rubber would not work properly over such a vast range. The tyre also has to be operated in it's optimum temperature range for maximum grip. The optimum width for a track tyre isn't always the widest, it is the tyre that can be operated at the right temperature (grip is lower when the rubber is below or above this temp). The rubber chosen also has to maintain grip without excessive shearing due to to high vertical load.

My Caterham on 195/50-15 road tyres is faster for the first 8km of track distance (At Castrol Raceway in Edmonton, which you may well know, being in Alberta) than on the slicks that have an equivalent tread width as 205/50-13 front and 235/45-13 rear tyres.

The RE71-R road tyres hit the optimum temperature for their rubber after about 5km, then give consistent grip for the next 10km or so before getting greasy due to overheating and loosing grip again. The Hoosier slicks take about 10km to get to their optimum grip, but if I have to slow down under a yellow flag, then they have less grip than the road tyres after 1km or so due to being too cool. Those I know in the UK with Caterhams find that too wide a tyre gives no more grip for the same compound, but takes longer to hit optimum temps.

For any given car there will be a point where you can't keep the rubber at the optimum temp for the conditions it is used in and the width of tyre in use. Wider isn't necessarily better for reaching and staying at that optimum point.

[TajoMan]

Originally Posted by dradernh

I've come across this before, and for some darn reason I never remember it. Maybe that's because I'm all about wider being better!

I went to a remarkably comprehensive tire calculator page to find the contact patch sizes for the 245/35-18 that came on the rear of my car and the 205/50-17 tires that came on OP's car. The difference was 2.2%, or virtually nil for our purposes.

The calculator is on this page: http://bndtechsource.ucoz.com/index/...alculator/0-20,

and this is its input form:


and these are its static and dynamic output forms:

This is very cool.

245 vs 205 tire, contact patch area difference is only 2%. (compare to ~20% tire width difference)

[TajoMan]

I have feeling that OP is simply entering a turn little too fast when wet (or any low friction situation)
Especially OP is saying the dry surface is not real issue, I bet it's just too much entry speed.

[aerobod]

Originally Posted by albertw

.....why we're not using bicycle tires on cars ....

Just another thought on bicycle tyres. My road bike with 22mm wide tyres at 8.5bar / 125PSI can corner on asphalt better than a fat bike with 140mm/5.5" wide tyres at 0.4bar / 6PSI, considering the fat bike tyre has a much larger contact patch of about 225 sqcm compared with 11sqcm for the road bike.