Tire Tech, lots of useful info

[Brandon J]

Here is a general tech about tires. Take information you want. Ignore information you don't want. I tried to keep things simple so everyone can understand. Lets see if I am clear enough for you guys.

Ever wonder why e34s work so well with 16in and 17in tires? I am sharing my knowledge about tires with the bimmer.info community.

I am going to break it down to 3 categories:
1. General info
2. Tire size and contact patch
3. Performance threshold and Roads

This information was generated on bimmer.info, for bimmer.info, and bimmer.info e34 owners/contributors.members. I made the quick drawings and thought they would help in the understanding of tires.

General Info
The purpose of the tire is to do work and to make contact with the road. Tires are designed to work just like a shock absorber. It take-up the bumps, irregularities, and shocks to the wheels so the tire itself stays on the ground. If you look at the shoulders of a tire, you will see how it is round. This design allows that cushioning effect. So, a more square-ish shoulder may in fact restrict the tire from doing the work it is suppose to do.

We should all know tire speed ratings and so forth. If not, look on tirerack's website for more general tire information.

Tire Size and Contact Patch
The most common upgrade for tire performance is to up size your wheels and tires. So, a 15in 205/65 goes to 225/55 R16 for plus 1 and a 235/45 R17 plus 2. What does this do, well the tire width is greater and there is less tire shoulder.



Here is a hands on approach. Look at figure 1. Take 2 pencils. The first pencil has all the original eraser. Now move the upper wooden part side to side with the eraser on a piece of sandpaper...making sure not to let the eraser slip. What do you notice? The eraser moves or flexes. This is like the tire wall on a turn.
Take the second pencil with about half of the eraser shaved off and do the same thing. What do you notice? There is less flex in the eraser. This is what accounts for that quick turn-in feeling.

Since there is less tire wall on the plus size tires, there is also less tire to do work. Now lets look at the contact patch also.

So, you bought wider 18in tires and swear that it was the best upgrade for looks and for handling. Well, you do get more grip because you probably went with a stickier tire, but you did not necessarily gain more contact area. Look at figure 2 above for what the different contact patches look like for a right side tire. If you remember how the second pencil eraser had less give/flex in it, the lesser tire wall means the tire has a harder time conforming to the road. So, the contact patch actually moves towards the center of the e34, not necessarily gains in area. The tire cannot flex as easily to accomodate for the road or the suspension.

Once you begin removing the amount of tire wall, suspension & suspension angle become more critical and sensitive. This is why on an e34 street car, going to an 18 or 19in size wheel/tire combo can actually hurt the suspension. So, the tirewall size that is optimized for the e34 suspension lies in the 16 and 17in sizes.

Isn't this information great. See the type of information we talk about during the bimmer.info e34 meets.

Performance Threshold and Roads
Now which tire should I get? Well, lets look at the performance sweet spot and the performance threshold. The stock tires are pretty much progressive. They begin to lose grip in a linear fashion and they tell you when they are losing grip because the squeals get louder. In a performance tire, the progression is not linear and the squeals are not as helpful as we are already close or at our breaking point (threshold).



If you look at figure 4, you will see the progression of grip and the sweet spot. The diagram represents how much the tire can grip as far as side force and how long it takes to get there. So, an all season tire does grip better when warm, but it takes some time to get to its sweet spot. The performance tire can quickly get to its sweet spot. The drawback with the preformance tire is there is less predictability before losing its sweet spot and thus the tires break loose. Since the sweet spot is larger, it is more difficult to know where you are in the sweet spot and predict the breaking point. That's how the tires have to be designed for the performance tire. Those with better compounds and testing can better engineer these aspects. Weird huh. That's what you pay for. Imagine your e34 getting to the sweet spot faster and you feel comfortable so you continue to push the turn. What also happens is your e34 is also arriving to the breaking-point faster without us knowing it. Keep on reading.

Tire performance around corners is also dynamic. So, the actual breaking point threshold might be the same on performance and non performance tires. How is this so? I thought performance tires have a higher threshold for gripping. Well, think how you don't get to the the apex (sharpest point of a turn) right away in a turn. It is usually around the middle. So, the performance tires grip and work the lateral forces within its sweet spot before reaching the threshold (the apex) and afterwards. Thus, collectively, the overall grip throughout a turn is greater with the performance tire. It's that sweet spot.

So, ever wonder how those people on the videos with so-so stock tires can keep up with the same car in plus 2 size wheels and tires? Well, pehaps it is easier to drive with the stock tires as the driver knows where the sweet spot is and can balance the sweet spot and breaking the tires loose. The tires are more predictable.

What does this all mean....performance tire grip thresholds and sweet spots should be balanced with the suspension of the e34. This is critical to get the sweet spot that works in a balanced fashion to the long wheelbase, the suspension upgrades, the suspension angle, etc. We want balanced work in the tires & suspension, we want both predictable sweet spot & break loose threshold, we want the tires to stay on the ground and to get good grip, we also want all this for safety.

Now to the roads....
You drive on them everytime you are in a moving car. So they are very important to learn about. If you look at figure 3, you will see cross sections of different roads types. Roads are engineered for grip, longevity, and water drain. There is a lot more that go into them, but we are concerned about these.



Asphault generally uses bigger pebbles (in its mixture when compared to concrete). The larger the pebbles and its mixture, the more grooves there are for water to escape. Now there is myth that temperature does a lot in reducing tire life. So, if you live in a warmer climate, then your tires will wear out faster. There is some truth, but temp does not play as big of a role as you think. The road surface does.

We have asphault with the big pebbles. We have concrete with typically smaller pebbles or mixture. Then we have the worn concrete that is polished. As cars and trucks drive on concrete, they wear it down smoother and smoother. What is used in different mixtures is usually based on location. In Hawaii, they use the porous lava rock in their asphault. In Florida, you can find sharp seashells in their concrete. What do these local resources do to the road surface? They increase the grip and grooves for water passage. Isn't the information on bimmer.info great! Almost finished.

Pro race car drivers make going into a turn at 150mph with 3 different road surfaces look easy. Asphault wears out faster than concrete so race tracks like to replace the turns that use asphault (the place where the most force and wear happens) with concrete. There could be patches in a race track and even wear grooves. The racers and tire techs have to learn them for grip. Now you have just learned some of the same important information to keep your grip and keep you safe.


Thoughts
So, I hope this tire tech info session helps you with your BMW e34. Our tires work and we need them to do work. They have to grip, shock absorb, conform to the road, they talk to us by squealing, they help our e34s perform, they help us drive in a safe manner.

My personal experience and my theory: I use the larger front tires 245/45-R17 to absorb shock, have a wider contact patch, along with the suspension upgrades & angle, I can run a softer tire pressure so now the tires can conform to the road better. I do raise the pressure a little at the track only. I chose Michelin because of the progressiveness, grip, longevity, tire wall design, engineering and the Michelin quality. I balanced the size so all four corners can break loose predictably. I can push the front, I can break the rears loose, in a controlled and predictable manner.

I think that my tires have 25k miles on them and can get another 5k miles because of the balance of tire preformance to suspension and suspension angle, as well as the roads. Here in Chicago, I accomodated for the potholes with a larger tire wall, but the roads can be engineered to have less water grooves because of the big road crown and organized sewer system that rain the water. Also, with Chicago being a transportation hub, there are a lot of trucks that cross through. They wear down the concrete so it is more polished. Smoother road, less tire wear, but also less grip.

So you ask what tire is the best. It depends what you need for your balance. Hope this info helps all you bimmer.info e34 members.

[attack eagle]

you forgot that increasing the tire width changed the shape of the contact patch from being biased fore and aft, giving good straight line stability to shaped left right and increasing drag and decreasing stability.

addin extra caster or slightly more toe in can compensate for the reduction in wheel stability caused b the change in contact patch shape, but as indicated above these were designed in based upon the original tire sizing.

it is not th tire sidewall but the contact patch shape that affects stability, and it isn't so much the sidewall height as the unsprung weight vs. spring/shock combo that affects ride quality negatively.

[bimmerd00d]

This info doesn't apply only to the E34, i've talked with BrandonJ a bit and this stuff applies directly to the E39's, and i'm sure every other vehicle out there.

Back when I had my '93 525i 5-spd, I went through an assortment of wheels and tires for it. The original setup were 235/45 17" Mille Miglia wheels with some Kumhos on them. Sure it looked sweet with them, but it was very unpredictable on the backroads. As soon as i got my 16" E31 turbines, and stuck some decent BF Goodrich's on them the car completely changed. It felt much more predictable, and to me, it handled better. It may not actually gripped any better, but I felt much more in control and didn't feel i had to back off as much through the corners. With the 17's, I would take a corner and then bam, snap oversteer. The 16's produced a much more progressive slippage at the limit, the tires talked to me. The same thing applies to my E39 540i right now, the el cheapo Kumhos that came on my new wheels are doing the same thing. The BFG's that were on my old wheels were MUCH better for my driving style.

[Brandon J]

I am glad you added. Yeah, I left a lot out because this much information is already difficult to grasp and to remember. Also, I wanted to focus more on the e34. I actually learned a lot of this from the tire tech guru at Tirerack. He does articles for Grassroots, and the other notable car magazines.

you forgot that increasing the tire width changed the shape of the contact patch from being biased fore and aft, giving good straight line stability to shaped left right and increasing drag and decreasing stability.

addin extra caster or slightly more toe in can compensate for the reduction in wheel stability caused b the change in contact patch shape, but as indicated above these were designed in based upon the original tire sizing.

[Jon K]

So, you bought wider 18in tires and swear that it was the best upgrade for looks and for handling. Well, you do get more grip because you probably went with a stickier tire, but you did not necessarily gain more contact area. Look at figure 2 above for what the different contact patches look like for a right side tire. If you remember how the second pencil eraser had less give/flex in it, the lesser tire wall means the tire has a harder time conforming to the road. So, the contact patch actually moves towards the center of the e34, not necessarily gains in area. The tire cannot flex as easily to accomodate for the road or the suspension.

This is why a lot of people get a kick out of "spinning tire" to confirm their power output. "Oh yeah? My M30 w/ ______________________ mods can spin 2nd and chirp third" and then you get people wondering why theirs can't and they fear something is wrong with the car.

WRONG! I can spin the CRAP out of my 255/40/17 but I get surpringly nice traction with 205/65/15's with 28 psi in them. How can that make sense? Contact patch. The 255/40/17 are wider yes, but it's a lower profile tire without much deformation and thus lesser in-line contact patch. This is why you see guys struggling for traction at a drag strip with 23-24" tall slicks 11" wide. Switching to a 26 - 28" tall slick with 9" width will yield a solid chunk more of traction promised!

[repenttokyo]

A friend of mine can't hook up at all with his Mini. Formerly powered by a civic SI engine, he had 10 or 12 inch (diameter) slicks on the car, but there was nowhere near enough sidewall to get any traction at the strip. For some cars, there really aren't any definite solutions due to design. Even finding slick tires for that size wheel was difficult. Most of what is available is designed for go kart applications, which have a much lower mass to move.

[Barney Paull-Edwards]

Bloody good thread, the only thing I would add is to check the date stamp on new tyres,if they were not wrapped then they will degrade in open air in about six months. UV degradation affects rubber badly,check out window rubbers.To my suprise BFG`s came out top in a recent UK survey,eagle F1`s grip good enough to frighten me!

[bimmerd00d]

Bloody good thread, the only thing I would add is to check the date stamp on new tyres,if they were not wrapped then they will degrade in open air in about six months. UV degradation affects rubber badly,check out window rubbers.To my suprise BFG`s came out top in a recent UK survey,eagle F1`s grip good enough to frighten me!

This is interesting, if you find any sites that confirm this please post them. That's VERY good info to have if it's true.

[rob101]

I think also people underestimate how hard it is to get the correct tyre pressure the only way to work that out definitely is with a pyrometer this could be another contributing factor. this makes a huge difference to contact patch.

Another thing to point out is. going to a larger wheel actually increases the unsprung mass. not hugely but it does. This is another reason why the ride of lower profile tyres is harsher than the higher profile the other reason being that you're replacing some of the rubber sidewall with steel/alloy aluminium which obviously is going to stiffen the whole wheel tyre assembly up in bump.

On a side note its good to see some interesting technical discussion here, i mean apart from Jon's powder coating and turbo forays.

One not of warning though. don't try and deduce effects from things that are anologies. a rubber does not behave as a pneumatic tyre an eraser has most of its stiffness in its centre whereas a tyre has most of its stiffness on the outsides where the sidewall is. This is one major difference there certainly are similarities, but they aren't the same and people should not be under that illusion.

[attack eagle]

verytrue, the weight of the car is not held up of the ground by the squish of the lowersidewall, but HANGS from the upper sidewall.

if you don't have a pyrometer, you can chek for drastily off tire pressures by driving across a strip of wet concrete and reading the "foot print" it leaves.

About the tire's degrading immediately once manufactured... absolutely true. ozone and uv affect the rubber, so a set of 5 or 6 year old tires, even if new or low miles are worn out.

http://farmindustrynews.com/news/Dat...-tires-110904/

wrapping might help postpone it, but it will happen nonetheless.