I don't know about smaller, but going larger doesn't affect it.

 

I think it will.

Basic theory is it compares the front wheel speed to the rear wheel speed. If the difference is greater then x% then adjust pressure to the slow end.

But I am guessing. If you test, let us know!

 

IMHO, the ABS does not know the difference between a larger front wheel and a smaller rear wheel.

 

This is correct and begs the question. No matter what size the rear tire actually is the abs will always think its the stock size. Unfortunately I don't know what the affect of changing the rear as described will actually be.

 

I guess nobody has tried it and know for sure! If increasing front doesn't affect it I assume decreasing rear would be fine as well...We'll see! Still looking for definitive answers from those braver than I.

 

We've ran 22s on all 4 for a full season on the TTB car w/ stock abs... no problems....

 

So obviously, you are getting full braking, or you would have noticed.

This is kinda weird.

On a conventional all-tires-the same car, you would think that the ABS would be looking for all the wheels to turn the same RPM.

If that is true, then you would think that the ABS on an Elise would be looking for the tire RPM ratio front-vrs-back to be the same as the ratio between the stock tires' circumference.

Does the computer "learn" the ratio from ordinary driving, then look for that ratio during driving?

If so, how long does this learning cycle take?

Would changing tires for an autocross cause it to not be able to keep up?

 

Someone could just put their wheels on backward - 17s in the front and 16s in the rear and see what happens. If my car was home I'd give that a go for you - would only take a few minutes to swap them around and see.

Are you going to a square/ish setup or dropping both front and rear sizes?

 

Plan is to go with 23" dia all around - 245 in front, 275 in back, 15" wheels. So only rear is smaller. Front stays the same.
The smaller rear diameter is to try and clear the fender. No cutting is the goal.
We'll see what happens! It'll look strange at the least.

 

Yes it will, how much will depend on the change in revolutions per kilometer. I would expect earlier dynamic rear proportioning intervention and earlier rear ABS activation with a decrease in rear diameter as the ABS thinks the rear tire is slipping more. Rear wheel slip is bad and ABS is calibrated to maintain rear wheel lateral traction capability so a car will not yaw due to excessive rear wheel slip. If you have traction control that will have issues too. Try it and see how bad it is. If you trail brake to rotate the car in corners you will reduce that ability.

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That's what I am worried about. I guess we see. Phil didn't have any noticable issues, so Ill probably be OK. Regarding TC, I always turn that off anyways.

 

The ABS on the Elige is actually an older design; IIRC it's the same 3-channel unit that was used with the Esprit. I don't think it's as sophisticated as modern designs...

My guess is that the ABS only intervenes when a wheel is nearly *locked*, i.e. one or more of the sensors is not sending a pulse within a given interval of time. If that's the case, the relative diameters of the tires make no difference at all.

EDIT: Traction Control, on the other hand, is done by the ECU... and does compare the relative rotation rates of the front and rear wheels. TC *will* be affected by tire diameter ratios that are far from OEM.

 

+1.

We haven't gotten into tuning the ABS computer, but it doesn't seem to care mush at all about tire diameter. It's pretty lenient to say the least...

 

That is partially true, usually there are multiple calculations running looking at calculated relative slip of a wheel, rate of change of slip or rotational wheel speed or both, rate of change of the rate of change (jerk) is very useful as well. Absolute calculated slip is usually there as well. Most of the calculations look at vehicle speed as an input. Typically there are multiple vehicle speed calculations being made and depending on the input the accuracy of vehicle speed can vary for different conditions. Changing the rpk of the rear wheels will effect the calculated vehicle speed used in most slip calculations and change the front to rear axle slip percentage which throws off the absolute slip. Normally there is an arbitrator program that looks at all of the indicators that could trigger an abs activation and that program decides which if any of the indicators should be reacted too. So a change in tire rpk may make no difference in straight line braking but may change lo co performance or braking in a turn performance or high speed high decel performance.

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So ever since installing the REV400 kit and smaller tire setup as described, we are now carrying much higher speeds into corners and experiencing decreased braking performance - the abs comes on on hard braking even without any tire lockup. It makes us unable to brake deep and hard and causes us to blow through corners. We pulled the ABS fuse and Voila!! problem gone, but no ABS available. So it seems the smaller rear tire diameter is confusing the ABS into premature activation. Any others out there experienced this? We are noticing this at autocross events. Have not tried open track with this as we no longer have a track...

 

I believe if you keep the front to rear ratio similar to stock then you can go up and down without trouble. It doesn't matter if you get 22" rims or 15" rims. As long as the ratio is within tolerance then the ABS won't come on. What size tires are you running?

 

See post #11. And rest of thread to get the full discussion.

 

it may just be that the smaller diameter tire is just easier to lock up than the stock diameter since the road has a lesser advantage on the rotor... The shorter tire is also likely lighter, with potentially less longitudinal grip. All 3 factors can add up to incidental lock-up, which can trigger the ABS. Additionally, we know the ABS can be falsly triggered (sometimes with ice mode) by [stomping the brake]. Perhaps you can resolve the issue just by braking smooooooooother :up:

-Phil

 

I'm positive that the car is poorly biased from the factory. There's too much front brake bias which causes the fronts to lock way before the rear gets to join in. This makes abs come on too quickly.

My car brakes about 85% better now that I've put more aggressive pads in the rear and kept the stock pads up front. Add in that I'm running 275s on the front now and it got even better.

I suspect Phil is right about the smaller tire in the rear being easier to lock up because of leverage and weight.

 

You guys are too smart. Makes sense to me, esp when disabling ABS improved the situation.
Thanks for all the insight!
:bow: