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Do you get an ABS fault by running a 150/70/r17 rear tire?

TheIronWarrior

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ABS sensors use tone rings to count (fractions of) revolutions of the wheel over time. The output from the ABS sensor is a measure of the wheel rotational speed.
Converting between rotational speed and linear speed is done through the radius, not diameter (v=rw). The difference in unloaded radius of the tires is 0.35". The effect of this radial difference on effective rolling radius is not known to me, but I think we can assume that the effective rolling radius would be impacted by approximately the same 0.35". In general, treadwear can reduce the unloaded radius by 0.25" or so (varies with tire).
I wonder what the dimensional tolerance of tires generally is. Things coming out of a mold are usually pretty consistent, but there will still be some amount of variance. How much larger or smaller than nominal does the tire have to be before the manufacturer considers it a reject?
I think the DCT uses front wheel speed (rotational, from ABS sensor) and the vehicle speed sensor (same as speedometer, from the transmission out) so a rear wheel change might not even matter to the DCT at all as far as shift RPMs. ABS would compare the two wheel speeds to determine a wheel slip, so out of the two systems if I had to guess, a rear wheel size change would affect the performance of the ABS system more than the DCT, but I also think you'd have to be way outside the realm of design values.
Overall, I think there are way too many variables that can affect the readings, especially when underway (accelerating in a turn may load up the tires differently, for example) and causes of "normal" wheel speed variances (elevation changes, for one) that Honda would have designed their ABS system to spazz out because of a less than 3% change in tire size.
If I was asked to come up with the ABS algorithm, I'd just look to see if during braking one wheel was below a certain low speed value (maybe 2km/h equivalent rotation, or even fully stopped) and the other was above another certain speed value (maybe 8km/h equivalent rotation) to trigger the ABS. The only case where that wouldn't work is if both wheels locked (but the bike can never tell two locked wheels from stopped anyway) or if you were slower than the higher speed value, where you're effectively stopped anyway. You'd just have to decide the slowest speed you want/need ABS to work, and the tolerance you'd want on the slower wheel to read as "locked". A wheel size mismatch would only cause "problems" around the two target speeds, and a small percent of a small number is generally insignificant. The system would still work, just might drop into the "inactive" slow speed range a little faster or slower than design.
 

TheIronWarrior

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As a follow up, the indirect method of TPMS uses the same sensors as the ABS to detect a low pressure tire by comparing the wheel rotational speeds. If the computer detects a discrepancy in the speed of a wheel, the tire pressure light clicks on. In order for that to work, TIRE PRESSURE HAS TO AFFECT THE ROLLING RADIUS OF A TIRE. If I'm not mistaken, it has to reliably detect a 25% pressure drop, so on a 32PSI tire (typical car tire), an ABS system MUST be able to measure the difference in wheel speed after losing about 8PSI. How significant an impact on rolling radius/wheel speed, I do not know, but it's significant enough for the speed sensor to read and react to it.
 

hulkss

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Yes, there is a small change in effective rolling radius, enough to detect a 25% pressure drop. On a Toyota, if you add air to your tires and press the TPMS reset button, the indirect system will measure and average data for an hour of driving at speeds above 19 mph before reactivating.
 

TheIronWarrior

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Yes, there is a small change in effective rolling radius, enough to detect a 25% pressure drop. On a Toyota, if you add air to your tires and press the TPMS reset button, the indirect system will measure and average data for an hour of driving at speeds above 19 mph before reactivating.
Right, so a 25% drop (8PSI in a 32PSI tire, 10 PSI in a 40PSI tire) is able to be read by the ABS sensor. I would guess that the sensors can read half that, or even less (5PSI or less) in order to ensure the measurement for 25% is accurate.
Remember back to the initial query, is a 3% change in rear tire OD enough to confuse ABS and DCT. The comments around treadwear and pressure aren't to say that worn tires or a few PSI will confuse the system, it's to say that wear and pressure has an effect of similar magnitude to the proposed size change and the system works just fine. The whole point is treadwear and pressure variance has a non-zero, but overall negligible impact.
 

hulkss

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The comments around treadwear and pressure aren't to say that worn tires or a few PSI will confuse the system, it's to say that wear and pressure has an effect of similar magnitude to the proposed size change and the system works just fine. The whole point is treadwear and pressure variance has a non-zero, but overall negligible impact.
Variations in effective rolling radius due to pressure variation and wear will be additive to the physical tire size change. If wear and pressure effects are of similar magnitude to the size change then the "size" induced errors are doubled.
 

TheIronWarrior

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Variations in effective rolling radius due to pressure variation and wear will be additive to the physical tire size change. If wear and pressure effects are of similar magnitude to the size change then the "size" induced errors are doubled.
Except the tire is 3% larger, so the unworn tire will have an error if similar magnitude in the "larger" direction, and reduce the error to approximately zero when worn, assuming pressure is kept at the specified value.
IE depending on how the tolerances are set, the fresh tire may exceed tolerance if the range has very little overrun. If the tolerance is, say, +/-5% you'll be fine but if the tolerance is +1%, -4% you may have problems until the tire wears out a bit. It's unlikely the tolerance would be that tight anyway, I can't imagine the sensor itself is even accurate to that small of a variance.
Note, this only applies if the system requires an accurate (small tolerance) measurement of the actual value. I posit the system is looking at at low-speed high and low limit values, where one tire exceeds the high value and the other is below the low limit value. In this case, any rotational speeds that aren't close to these limit values are going to be completely unaffected by tire size.
I suspect unless a Honda Engineer who worked on the ABS system chimes in or someone does extensive testing, we will not know for sure though.
 

Darkw

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This new learning amazes me Sir Bedevere. Explain to me again how sheep's bladders may be employed to prevent earthquakes
 
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