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standard tires on 2.5

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I don't know why but always thought you weren't supposed to mix different kinds of tires for front and back. Does it make zero difference in terms of safety then? Seems like you found a good alternative but would be nice to have all 4 tires the same. AD07s and the AD048 I think are the only official options though. Thanks for looking into this.
 
dsm363,

I would not say it makes zero difference in terms of safety.

Yea making a change from manufacturers recommendation is a risk. When you make this change, the car will not perform the same way as it was configured by the manufacturer. Since I have been around cars, and have some understanding, experience, and a desire to make things better, I decided to see what I could do.

It was clear that there was something that could be done about the handling of the car. Safety is definitely a big consideration. I did try to think carefully about the change.

In my opinion, the biggest risk is over-steer. So anyone who tries it should understand that it is different from what Tesla may have intended, and you should exercise some care and caution. So far I am convinced that, with the exception of pushing hard maneuvers at high speed, (still unknown territory) the car performs admirably with these tires.

So lets be careful, safe, and enjoy the cars, -that we are so fortunate to be able to drive.
It is probably a bad idea to let youthful drivers play with a roadster with these tires on the back!
 
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Is there traction control on the front tires of the Roadster? I didn't think so....

I am sure there are modern wheel speed sensors on all four wheels, that are used as required signal inputs to the system that provides ABS/traction control safety features of the Roadster.

In normal driving conditions all wheels would have (nearly) the same speed. The system would be looking for one or more wheels to suddenly change speed in relation to the other wheels. This is how the tire-slip condition is detected by the system.

Gradual differences in thermal expansion is not likely to be of concern IMO, indeed, the changes I have made (that significantly change the way the car handles) is much more significant than slight (if any) differences in thermal expansion of the tires.
 
Gradual differences in thermal expansion is not likely to be of concern IMO, indeed, the changes I have made (that significantly change the way the car handles) is much more significant than slight (if any) differences in thermal expansion of the tires.

Yes, but the "New Tires" function calibrates out the tire change, leaving only the thermal expansion differences.

OK, I'll believe it uses front wheel inputs, but I have a hard time believing that differing tire expansion rates front and rear (which even if same brand and model are sized differently to begin with) matter.

Yes, I am also a bit skeptical... I'm just repeating what my Ranger told me.
 
You put on the same rear tires and have AD07s (stock size) on the front? I may have to try this but have a few thousand more miles on my new rear AD07s.

Yes, the front tires are now different from the rears. But they aren't Yokos... they're the Kumhos that I didn't like on the rear. This is my fourth set of rear tires, and I'm on the second half of my second fronts. I did, at one time, have Michelin (not sure exactly which) on the rears with AD048s on the front, and it was just fine except for the road noise from the front.
 
Yes, but the "New Tires" function calibrates out the tire change, leaving only the thermal expansion differences.



Yes, I am also a bit skeptical... I'm just repeating what my Ranger told me.


There are other differences: structural, profile, mix of rubber compound, tread design, etc.
Tire manufacturers pride themselves on creating new differences to improve performance.

"New Tires" does not calibrate out the significant changes in the way the car handles with these tires. (fortunately)
:)

I am not opposed to matching tire types, I'm just saying there are much bigger things to worry about than thermal expansion differences.

I don't doubt that is what the ranger said though.
 
OK, I'll believe it uses front wheel inputs, but I have a hard time believing that differing tire expansion rates front and rear (which even if same brand and model are sized differently to begin with) matter.

Here's a blast from the past link from the engineer that developed the algorithm: The Spin Stops Here


Thanks for posting that link.
Such fun, it must have been!
 
I don't know why but always thought you weren't supposed to mix different kinds of tires for front and back. Does it make zero difference in terms of safety then? Seems like you found a good alternative but would be nice to have all 4 tires the same.
What I was always told is that you want new tires all around, as opposed to mixing some old tires with new tires. Considering how much traction differs as a tire ages, this makes total sense. I always buy tires in sets of four (and try to stay away from nails as long as possible).

However, it's not always possible to put the exact same tire on front and back when the wheels are different sizes. You might have the same model on front and back, but the precise performance of a 16" version of a given model is not going to exactly match the 17" version, not to mention widths and contact area. I've never heard anyone say not to mix different models front to rear, so long as they're all new and you do your homework.

Then there's the weight distribution. The Honda S-2000 has a 50/50 weight distribution (worst case 49%/51%), making it easier to match front and rear. However, it's incredibly rare for any car to be that balanced. Even with the same tires on all four wheels, many factors affect the traction ratios.
 
Even with the same tires on front and rear each tire and each axel is going to have different thermal expansion rates. So that causing an issue is nonsense. The tire that has the most break applied or the most slippage will heat up more than the other tires. However you should have matched tires on the same axel with the same tread depth. Unmatched tires left to right will cause the car to corner differently left to right, this can result in driver error especially when recovering from a skid/slide and resulting in the driver over correcting and saving the car to immediately spin out in the opposite direction.
 
Trying to explain this: You may have heard the expression that a sports car corners "like it is on rails" as in a train track. This is not literally possible because rubber tires have a deflection to the side (radially outward) in a circular turn for example. So as a result, there is an outward radial vector introduced to the actual path of the wheels (and the car) in a turn with significant speed and cornering force. So although the tires are applying a given degree of "turn" to the path of the car, the wheels will always take a slightly larger turning radius than would be indicated by the physical orientation of the wheels. So if you can follow me this far in the explanation, keep going a little further.

Under-steer behavior results when the front tire deflection path(radius) is greater than the deflection of the rear tires. The reverse of this would be a dangerous over steer that we all want to avoid. So if I succeeded in explaining this, you can see the reason for wanting a bit more rubber on the rear tires, to allow a bit more rear tire deflection in turns, and thus less under-steer. Only, not so much as to cause over-steer!

I'm trying to understand your reasoning. First let me explain my understanding of understeer/oversteer:

Friction forces depend on the contact area, and therefore the vertical force on the tire. There's a static weight on each tire due to the weight of the vehicle (and any aerodynamic downforce). During cornering the weight shifts to the outside. During acceleration it shifts to the back, and during deceleration it shifts to the front. This changes the forces available at each tire.

Tires achieve maximum friction forces (grip) when they are slipping slightly. Race car drivers try to keep their tires "at the limit" in order to maximize their speed through corners. In racing, ideally all four tires would be at the limit throughout the corner, but this is not really achievable in practice. One of the tires is going to hit the limit first, and will really start sliding first if you overdo it. The tire that slips first depends on the car setup (alignment, suspension, tires, tire pressures, etc.), but also on the conditions at the corner in question, and how the driver handles the car.

If the rear wheels slip first, the car turns more than the steering input. The back end swings around, and if the driver can't handle it the car goes into a spin. This is called oversteer.

If the front wheels slip first, the car turns less than the steering input. The car plows; you can't get it to turn more by increasing the steering angle. This is called understeer.

Race car drivers like their cars balanced. In practice the car will behave a little differently on each corner; it's not uncommon for a race car to understeer into a corner and oversteer out of it. Clearly handling a car like that requires a very experienced driver. But that is great for the race car driver because it maximizes the speed through the corner - all the tires are close to their limit throughout, so you get the maximum cornering force.

Understeer is considered much safer for street driving, so pretty much all production cars are dialed in for understeer. The reason is that an inexperienced driver - meaning everyone who has never had high-performance driving instruction and on-track experience - is much more likely to be able to deal with plowing as opposed to spinning. And this is why the stock Roadster comes with quite a bit of understeer dialed in.

So, as I understand your objective, you intend to reduce traction on the rear wheels, to lower the limit so it's closer to that of the front wheels. Yes, that will work, but I'm not certain it's the best approach. Increasing the grip in the front, if possible, would perhaps be better.

Probably the best way is to use the adjustable suspension. I don't have a Sport but I ordered my car with the adjustable suspension. I've got my front sway bar in the loose position, which helps the front tires hold a bit more in a corner. I've still got some understeer, but it's safer that way. I could tighten the rear bar as well, but for street use that might be unwise. Twiddling the tire pressures is commonly done for modest adjustments in the field, and of course you can adjust the shocks if you have the adjustable suspension. More advanced stuff includes changing the wheel alignment... this stuff starts getting pretty technical...
 
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One more thing... the reason why the Roadster has so much understeer, and why Tesla put so much work into the traction control, is the EV drive train. Two reasons:

1. Power oversteer - stomp on the accelerator, all the weight goes to the back, your rears are at the traction limit, and your front tires have little weight on them. You run the risk of the rears breaking loose, and you have a lot less steering control because the fronts aren't loaded.

2. Snap oversteer - you lift off going into the corner, regen braking shifts the weight forward, and your rear tires break loose. This is nasty because the best corrective action is probably to press the accelerator, and few drivers will do that.

So if you do dial back your understeer, be aware of these things. And be very cautious about turning off TC.
 
Here's a blast from the past link from the engineer that developed the algorithm: The Spin Stops Here
Aside -- glad to see that Drew (the author of that blog entry, as well as The Evolution of Motor Control: From Waves to Bits) is still with Tesla, now as "Powertrain Systems Architect":

Drew Bagliano | LinkedIn

I really enjoyed those two blog entries. Provided some great insight into the technical excellence that Tesla was striving for in EV powertrain design, all the way back in late 2006/early 2007.
 
One more thing... the reason why the Roadster has so much understeer, and why Tesla put so much work into the traction control, is the EV drive train. Two reasons:

1. Power oversteer - stomp on the accelerator, all the weight goes to the back, your rears are at the traction limit, and your front tires have little weight on them. You run the risk of the rears breaking loose, and you have a lot less steering control because the fronts aren't loaded.

2. Snap oversteer - you lift off going into the corner, regen braking shifts the weight forward, and your rear tires break loose. This is nasty because the best corrective action is probably to press the accelerator, and few drivers will do that.

So if you do dial back your understeer, be aware of these things. And be very cautious about turning off TC.


I mostly agree with the ideas of this post Doug_G.
That last line there is very good advice for folks.
I would add to be especially cautious if you drive at higher speeds (even with traction control active), since oversteer and its negative consequences are a serious risk at speed.


In the previous post you go into other topics that are not really a part of understanding what understeer is and what I have done. Some of what you discuss are consequences of over-steer, and is what I would call loosing control of the car and spinning out. As I have mentioned in my earlier posts, we all want to avoid over-steer behavior! -the possible loss of control, of course, is why. I am not particularly interested in race track driving the Roadster. That is probably a subject for another thread.

- -

Doug_G, in reference to this previous statement you made, it shows that you do not yet understand the change I have made, or why it does what it does:

"So, as I understand your objective, you intend to reduce traction on the rear wheels, to lower the limit so it's closer to that of the front wheels. Yes, that will work, but I'm not certain it's the best approach. Increasing the grip in the front, if possible, would perhaps be better. "

-the first line is just wrong.

1) My objective is NOT to reduce traction in any way, and it is VERY different from adjusting limits of traction.
2) I have accomplished my goal quite effectively, and am delighted with the results.
3) If you have different ideas on how to approach this, you can try that instead! Let us know how it goes, if you do.

- -

Roadster drivers experience understeer pretty much all the time on curves and when taking turns on public roads, just as I did, before I selected different tires for the back of the car.

I am not saying everyone should do this. If you do not understand what is going on, and the risk of tuning the car in this way, I suggest you keep the car configured as it was delivered.

-Richard
 
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1) My objective is NOT to reduce traction in any way, and it is VERY different from adjusting limits of traction.

Okay, you're going to have to explain what you mean.

The maximum friction/grip a tire can provide happens when it's just starting to slip slightly. Push beyond that point and traction starts going down. Fall short of that limit and you're not getting the maximum possible force. That peak is the limit of traction.

How can you change that limit (by changing the tire)? There are two things you can do: you can make the limit higher or lower, or you can make it more/less progressive.

Are you doing one of these things, or something else altogether?
 
Okay, you're going to have to explain what you mean.

The maximum friction/grip a tire can provide happens when it's just starting to slip slightly. Push beyond that point and traction starts going down. Fall short of that limit and you're not getting the maximum possible force. That peak is the limit of traction.

How can you change that limit (by changing the tire)? There are two things you can do: you can make the limit higher or lower, or you can make it more/less progressive.

Are you doing one of these things, or something else altogether?


Understeer
The subject is tricky to understand, and will require more time thinking before you can really "get it".

First you will have to forget about everything you state in that post.
These things do not relate in any way to what under-steer is, and are not what causes it.

Can you do that?

If so, then go ahead and forget about that stuff, and try this:

Read my shortcut explanation again (at least a few times) at the end of this page:
standard tires on 2.5

Spend a little time thinking about how the ideas I have presented relate to different degrees of change in the "orientation of the car" while taking turns with different cornering forces.

If you don't get an "aha!" from that, more explanation may help.

Try the parts of this article that talk about under steer:
Understeer and oversteer - Wikipedia, the free encyclopedia
-though that is way more detailed and tedious to read.

Keep in mind the behavior I am addressing does happen at normal driving speeds and conditions while cornering.