[idrivearocket]

Originally Posted by Thejoyofdriving

Hmmm, the way I look at it is that car manufactures do the best they can on a budget. The OE tires may very well be the best tires to put on the STI in the price range that the manufacturer has in mind. I think there are much much better tires out there, but, the reason that they didnt use these better tires is, either that they have too much road noise for the common consumer, their tread wear is too fast or the ride comfort may be too poor . Think about it. They wouldnt put the best grippiest tires on the car stock because it would A) cost to much or B) the consumer might not be willing to trade grip for tread wear or discomfort. For me, i really dont care how loud the tires are or how bad the ride is. I am a die hard grip/handling guy, unlike the average consumer i am willing to trade tread life, ride comfort, and noise for some more grip. So the RE50's may very well diliver the best grip with out being too loud, uncomfortable, and tread life good enough so that there will still be tires left on the car when you drive the car home from the dealer

I totally agree with your comments about OE tires, but on the other hand, it has to be really expensive to put such stiff sidewalls on a tire. My uneducated guess would be that putting sticky rubber on a tire is much cheaper than putting stiffer sidewalls. If they really wanted to put cheap tires on the STi they could have used anything other than the RE070.

[MGizzle]

The RE070's are sick tires. Very grippy yet very loud, stiff uncomfortable and not the best in wet. STi did not go cheap on this guys. I have one RE070 in my basement, one Azeni 615 that is burned up, and a Micheline tire. The RE070 is rock hard on the sidewalls, which is great for lateral grip but not so good for ride. The Azenis are stiff too while the michelins are very very soft.

Anyhow, as far as the slip percentage, do a bit of a serach on google and go to images. You will see a crap load of tire curves. You will notice then that at certain slip angles or slip % the tire TOPS in friction force. Once that is passed the tires falls off drastically and before that it drops off significantly too. There is a sweet spot and I am sure you have felt it when you pushed your car.

This is the big reason why when you drive your STi and your car UNDERSTEERS you need to turn the wheel back some (towards the outside of the turn) and not give it more steering input. By turning back some your are climbing back to the peak greap and your car will actually UNDERSTEER LESS. This is because you are REDUCING the slip angle and when you look at those tire curves you will see that decreasing the slip angle will bring you back up to the peak once you passed it (passing it will put you in understeer on a stock STi most of the time). Same goes with longitudinal forces. There is a sweetspot and very good racers get it by modulating their brakes. However, for us the ABS unit does all that work and unless it is Activated you are probably a bit away from the peak grip.

[Thejoyofdriving]

Im just thing to understand in terms of static and kenetic friction

[Flycaster]

Originally Posted by MGizzle

The RE070's are sick tires. Very grippy yet very loud, stiff uncomfortable and not the best in wet...

Accurate summary - the best dry grip and turn-in tire street I've ever used. Those of who have switched to different rubber did not do so for a dry performance shortcoming: there isn't any.

[Mykl]

Originally Posted by Flycaster

Accurate summary - the best dry grip and turn-in tire street I've ever used. Those of who have switched to different rubber did not do so for a dry performance shortcoming: there isn't any.

Yeah, Azenis feel like mushy Lincoln tires in comparison, and surprisingly the Hankook R-S2's feel even worse.

I would use nothing but RE070's, but they're more than I would like to spend on an evaporative street tire.

I can say with some certainty that the RE-01R's certainly feel like they have more stick than the RE070's though, but the initial turn characteristics don't seem to be as crisp.






...as far as braking mods, I will never again own an Impreza that doesn't have a brake master cylinder brace. The difference is only noticeable when you're braking really hard, but there is a difference. At the limit modulation accuracy is greatly improved.

[seattle944t]

Originally Posted by MGizzle

Ok. Let me give this another shot. Take a look at a tire slip curve (not lateral one but its similar). It gives you friction force vs. slip %. You will notice that the peak grip of any tire is around 15-30% slip. Before and after that it drops of significantly. This doesn't mean the tire is "slipping" entirely. Your ABS unit is calibrated for this. Hence, if you slap better tires, or worst tires on it, the ABS unit will still use the values it was calibrated for with the stock tires. Now try to imagine how this can go wrong. here is an example. The ABS unit uses speed sensors to determine percent slip. Now if you slap a better tire on it will use the same speed differences BUT your new tire needs to slip more or less to achieve better grip (depending on tire slip curve). Hence, the ABS unit will not allow it to go to that ideal slip % and will release it sooner or later. That is bad and will reduce your distance. Look at it this way. When you put a bigger turbo on your engine you need to TUNE IT, calibrate it for the new turbos specs. It is simple as that. Same thing with tires.

Your pads and rotors will not mess this calibration up and if so insignificantly. Now, what about calipers??? Hmmmm.....listen up. Bigger calipers are OK if and only if the P-V (pressure vs. volume) curves of those calipers match the stock ones pretty good or close enough. What is this in english? The caliper fluid volume has to be same or similar to the stock ones because this is what the ABS unit is calibrated for AND MECHANICALLY ENGINEERED FOR. Hence, if you get bigger calipers with bigger pistons you will need more fluid to generate the same force/pressure and your ABS unit WILL take longer to do this, hence your time to lock up etc. is longer and effected. This can throw off your distance some. Hence why companies that know what they are doing design their calipers bigger but with smaller piston sizes etc. to try and match the stock P-V curves.
<snip>

Don't take this the wrong way - I really do want to know the answer, but I may come across the wrong way.

I don't see how better tires can make the braking worse, better tires should always improve braking distances even if the ABS system isn't calibrated for that tire. Please explain to me the flaws in my thinking here.
The ABS, as you said, can't tell what tires you have. It can only tell wheel speed differences. So - you have tire A)RE070 and tire B) Hosier DOT racing tire. For the sake of discussion lets say that the 070 wants 15% slip angle, and the Hosier wants 20%.
To keep things simple, braking in a straight line with the same car - just changing the tires.
Pass on the 070s threshold braking - the tires get up to the best slip angle and then one exeeds that level and begins to lock up (slowing down at a rate greater than the others) and the ABS kicks in to keep its deceleration rate within the defined percentage. Right? All the tires are then at about this 15% slip angle. Right?
Pass on the Hosier same scenerio - threshold braking gets up to best slip angle, tires are all declerating at the same rate so the ABS system doesn't think its lockup even though the rate of deceleration of the whole car is now greater than the other tires. Same tire exceeds its slip threshold - ABS kicks in on that tire only, we are dealing with a 4 channel ABS so each wheel is independantly controlled, so the ABS keeps the deceleration rate of that one wheel within the calibrated value. The other 3 tires are all still at maximum slip rate - and the locking one will be near that same slip rate because its speed is kept relational to the other tires. So even if the target slip rate is 15% the other tires are all still exceeding that, to keep the ABS speed target happy it is going to base it on the rotational speed of the other sensors. Worst case you have 3 tires at a 20% rate and one at a 15% rate until that tire ends up rotating faster than the other tires and the ABS releases.

Honestly, I don't see how braking could get worse in our case, maybe on cars that have different sized tires front and rear it makes things much more complex, but with a 4 channel system dealing with 4 tires of the same size - the system only has the values of the other wheels as inputs. So if those inputs change the system is somewhat self-calibrating. Would it be optimal? No. Would it be worse? I can't see how.

And FYI - sport compact car did a story on tuner WRX's some years ago - guess which one had the shortest braking distance? Thats right, the only car in the test using DOT R-compounds (Kumo).

[MGizzle]

You are right in assuming the sensor differences but your R compound tires are probably nowhere near your street tires as far as peak force at some certain slip %. All tires are different and 5% makes a big difference depending on the other parameters. Hence, your street tires might be at 15% but your R compunds might be at 30%. How do you know where it is at?? You don't. You can do trial and error and determine "yeap, I stop shorter." Great, but could've you stopped even shorter than that? Very likely. Hence, if you slap R compunds on the car, your ABS unit will see 15% difference in speed between, let us say front left and front right, and release brakes on that tire WHEN you are actually creeping up to the peak force at 30%. This means your ABS is releasing before you hit peak grip. Does that make sense??

HOWEVER, you could get a slick tire that reachs max force at 12%, hence yes you will stop better than your street tire because you are overshooting SLIGHTLY and you will float between the max force during your entire event with a higher Mju due to the slicks. Calibration is key. Consider the engine forum and you will realize how detrimental it is to calibrate things in order to optimize your system.

Anyways, as in my previous posts, I stated YOU COULD lose distance with slicks if your peak percent is significantly different than what your ABS unit is calibrated to. And yes, it can go the other way too as I explained above. In the end though, switching to R compound tires WILL require you to upgrade to better pads and rotors possibly to achieve the same brake output, hence pads and rotors being the best upgrade in my opinion no matter what tire.

Also, the Europeans don't calibrate their ABS units to tires a lot because they have such good tire compounds that all of them are almost the same and you can slap whatever you want on your car. With R compounds though I doubt that they get similar numbers from the stree tires as far as slip% to generate peak force.

We actually just completed calibrating units for the Mustang 500 club racer cars and they are significantly different as compared to the Shelby 500 due to the parameters metioned in the subject of this thread. Everything plays together and everything messes with your optimization of the system.

[seattle944t]

Originally Posted by MGizzle

You are right in assuming the sensor differences but your R compound tires are probably nowhere near your street tires as far as peak force at some certain slip %. All tires are different and 5% makes a big difference depending on the other parameters. Hence, your street tires might be at 15% but your R compunds might be at 30%. How do you know where it is at?? You don't. You can do trial and error and determine "yeap, I stop shorter." Great, but could've you stopped even shorter than that? Very likely. Hence, if you slap R compunds on the car, your ABS unit will see 15% difference in speed between, let us say front left and front right, and release brakes on that tire WHEN you are actually creeping up to the peak force at 30%. This means your ABS is releasing before you hit peak grip. Does that make sense??

Yes - this does make sense - but I must still be missing something. Because if one wheel is slowing down at a greater rate than the other, it has to have exceeded the slip rate of the tire - which in your example both tires would still reach the 30% mark before slippage is detected between the two front tires.
So if say the 15% tires will generate 2g's of deceleration, and the 30% ones generate 3g's - you will still hit 3g's of deceleration and its not until you are over that threshold that the ABS will kick in, and drop you down to somewhere below 3g's depending on how many and which wheels are locking.

But even with the programmed target of a percentage difference to a set pulsing frequency - at some point if the tire that was slipping is not given enough brake force its rotation rate will start to match, and then exceed the wheel that was not locked because the tire will not be decelerating at the peak rate like the other tire. At this point I see one of three things happening:
1) ABS shuts off, its work is done, resume normal pressure -tire will go back to peak slip rate
2) ABS keeps pulsing the tire that was locked up until pedal pressure is released or the car stops
3) ABS now sees the other tire(s) as locking up because its rate of deceleration is now greater than the one being pulsed.

Only the last case is one for concern, and I would hope the ABS system is smarter than that.

And yes, I know this is all thinking in a "perfect" environment, and things like road surface, turning, and suspension travel all work against a simple wheel speed differential calculation, and there has to be some additional logic (I hope) to account for that. So, if anything, this would be what messes up the simple wheel speed differential calculation.

But it still seems to me that you should be able to nearly always reach the peak slip angle of the tires before getting into the ABS, and thus still stop shorter with better tires.

[Flycaster]

MGizzle, could you please explain the need for a M/C brace? I hadn't heard this before, and I'm trying to wrap my head around the issue. Lines, pads, rotors, fluid, I'm good with all that, but the M/C brace just took me by ignorant surprise. Thanks.

[FHItoSTI]

I would say it all depends on the type of breaking you are looking for. I can see tires being most important in either emergency stopping or hard driving situations. There are different routes you can go for both.

Emergency could be Tires, Calipers, Pads and Lines.

Hard Driving could be Tires, Calipers, Lines, Pads, Rotors (slotted, dimpled, j hook, drilled etc). That is one thing everyone seems to overlook. The stronger the caliper the faster the brakes heat up and gasses get trapped. If the rotors arent drilled or slotted they cant cool fast enough thus resulting in loss of braking power. Regardless of what other mods you have done to the brake system. So dont overlook upgrading rotors.