How many teeth on the MR2 hubs vs on the Miata?

 

MR2 Spyder front hubs have the ABS sensors built in, but I'm pretty sure the rear half shafts on an MR2 Spyder run 48 tooth ABS rings. So, to make the ABS work, I suspect some work will be required so make the front MR2 ABS hub (which presumably also works on a 48 tooth signal) work with the Miata 44 tooth rear half shaft rings.

I'm wondering whether you could just fab up some 48 tooth rings to install in the Miata rear shafts, and would that work to trick the Miata ABS system into working with the MR2 front hubs?

 

I can't imagine why this wouldn't work. The ABS system will think you're going faster than you actually are, but it works off deltas, not absolutes.

 

Maybe, just maybe, the ABS algorithm is speed sensitive, so it takes the rotational speed of the wheels into account. I wouldn't want to significantly change the number of teeth on the ABS rings. Not more than say 5-6%.

 

No way. Going from 44 to 48 teeth is the same as going from 44mph to 48mph. There's no way that significantly alters the function of the ABS. I can think of half a dozen things that would alter the ABS function more than a 4mph error in the true speed.

 

Hey, that's just me being cautious with a system that can disable my brakes.

While it shouldn't really make a difference like you said, I would still prefer to fit 44-teeth rings on the front hubs instead if possible, although by the sound of it, the hubs on the MR2 are a sealed unit so you can't just open them up and retrofit any other ring you want.

 

I believe most ABS system, the Miata included, are speed sensitive below a threshold of 5-7mph. They basically disable ABS at very low speeds because that is where the error rate in calculating vehicle speed and wheel speeds becomes significant. You don't want the system to dump pressure at 5mph because an error made the system think your wheels are locked up when in fact you're just driving slowly and do indeed want full brake pressure to stop the car. 100% slip is not that dangerous at low speeds and is actually helpful on gravel and snow. It's really at higher speeds where the ABS plays a significant role in vehicle stability.

That being said, I have to imagine that changing the tone ring to one with a different tooth count would most likely effect vehicle speed and wheel speed calculations. The big unknown is what effect that will have on brake system performance. Without testing under a variety of conditions (or talking to the Mazda development engineers) it's hard to say exactly what will happen. It seems reasonable that it could effect the threshold speed for enabling/ disabling ABS as well as wheel slip calculations. The algorithm must somehow account for people putting mismatched tires on the vehicle across an axle (e.g. temporary space saver spare tire) but it's really hard to say what the system will think or do when it senses the rear wheels are rotating 4% slower than the fronts. It might adjust for that discrepancy or it might throw the ABS light.

Did Keisler do any testing on ABS equipped cars to verify system performance?

 

Obviously, a difference in the number of teeth front-to-rear is not acceptable. The suggestion was that one could machine new 48-tooth tone rings for the rear Miata axles to match the 48-tooth rings in the MR2 hubs without any ill effect.

In any case, someone else will have to try it. My Keisler spindles will be for sale this week.

 

So with 48 tooth hubs they're speed sensitive below a threshold of 4.65-6.63 MPH. That seems like a reasonable tolerance to me. I don't often do much panic braking below idle in first, though.

 

There is also the possibility that the different number of teeth will allow for more slip which suits Rcoms and slicks better.
Or it will be worse

 

At least with the 01+ abs my sustained braking decel didnt change (withing measurement error) compared to the same setup with the abs off. So, basically the 01+ will let you brake at at least 1.1g-1.3g without engaging or ice mode.

 

The rate of deceleration is primarily a function of tire choice and road surface.

 

No ****. But some ABS systems freak out on slick tires and limit the amount of braking force and/or go into ice mode.

 

Just for the heck of it I drew 2 arcs in cad to represent a Miata driving around a curve with a radius of 100' for the outer wheel and 95' 4" for the inner.

On this fairly easy curve the outer and inner wheels are traveling at about 4.5% different rates/distance.

I suspect the ABS CPU is looking for more dramatic/sudden differences than this...

 

You would no be engaging the brakes with high line pressure. If you did, the ABS would surely modulate the inners. Most modern high performance cars will modulate the fronts more than the rears in this scenario, helping the car rotate. I dont think the NA/NB do that.

 

Understood. I didn't articulate my main idea very well.

I was looking at how the variation in the 48/44 tooth count signal might compare to other common situations.

My gut says that the system would compensate (not overreact)...

I would guess a 1-2% variation might happen just by having old/new tires on opposite ends of the car or different brands?

Best,

T


What do you think?

 

I wonder what the effect of removing 4 teeth (evenly spaced) from the rear to match the count?

I can't imagine you would end up in a ditch....


T

 

That would result in the car thinking that you were constantly unlocking and locking the rear wheels because the instantaneous wheel speed would be changing so much. It would probably end up throwing an error code.

 

You may well be right but I remember Keith had to grind off a bunch of teeth on his rally car for some sort of data computer because there was a few tens of thousands of pulses per mile, a few hundred per second.(Eight pulses/foot)

 

Damn Sav, I was hoping you'd be the one to test this. I have a 2002 with ABS system to donate to my race car, but would like to have some confidence the ABS will work with the spindles and MR2 hubs. Might be that this will have to wait until my end of year car rebuild so I have time to mess around with it (or time for Keisler or others to test and fab a solution).

More info on the ABS rings - Miata rings (at least the front rings - not sure if the rears are the same?) for 01-04 cars have 75mm ID and a 82.69mm OD with 44 teeth and are 10mm wide. We need a 48 tooth ring with similar dimensions. I have not been able to find one so far - most popular Toyota models are 48 tooth but they run a 70mm ID and only 6mm wide (OD unknown). Volvo have a 48 tooth ring with a 71.9mm OD and 8.6mm wide (to suit 850, c70 and s70)... According to this thread, the Volvo rings might work with *non* ABS axles...

https://www.miataturbo.net/suspensio...s-1-8-a-63614/

More research needed, or someone to custom fab a 48 tooth ring and test...