Are there any issues running a 21" wheel with ABS?

 

not at all.just make sure you get the abs bearings in the wheels

 

While it is true that your brakes will still work the effectiveness of your brakes and how they are originally calibrated have to come into question to some degree. Your front wheel speed changes with wheel diameter. There has to be (don't ask me what) some sort of change from the original setup. Is it detrimental ???? Based on the amount of people running them I'd say go for it if your so inclined........ Just my two cents..

 

Wouldn't the front wheel rotational speed remain the same as long as the total diameter of the wheel / tire combo remains the same, regardless of the wheel diameter? This assumes the extra wheel height is off set by a proportional reduction in the height of the tire?

 

You're right -- the wheel size is not a factor. What IS a factor is the overall tire height (OTH). A "taller" OTH will make the ABS sensor think you're going slower than you really are, relative to what it "saw" the stock wheel doing, because your sensor is calibrated for a stock ( in this case smaller) OTH.

Tire sizes are expressed in terms of tire width/aspect ratio (percentage of width)/and wheel size. An '09 Street Glide has a tire that's 130mm wide, and aspect ratio of 80 (80% as tall a sidewall as the tire is wide), and a 17-inch wheel.

Here's the formula to figure OTH. For demonstration, a calculation is run for a "stock" 130/80/17:

-Convert metric width to inches
130/25.4 = 5.12 inches

-Multiply by the aspect ratio to get tire sidewall height
5.12 X .80 = 4.10 inches

-Multiply by two, because you have two sidewalls
4.10 X 2 = 8.20

-Add the wheel diameter to the sidewall heights to get OTH
8.20 + 17 = 25.20 inches

You can plug other numbers into this to calculate other OTH values.

Bryon

 

So is there a way to recalibrate the ABS after changing to a taller 21" wheel?

 

Greetings;

I checked with one of the reputable outfits that sells tire and wheel combos. Typically, an 18-inch wheel gets a 130/70/18 tire mounted on it. If you run the calculation for OTH, that works out to almost exactly the same OTH as the stock '09 tire and wheel. The 21-inch wheel supposedly gets a 120/70/21 tire mounted on it. This works out to an OTH of 27.61 inches -- 2.4 inches taller than stock -- this will also raise the front end by 1.2 inches over stock.

Bryon

 

2.4 x 3.1416= 7.53" longer roll out difference between the 2 this is what the ABS "sees" as the change . Have no idea if plugging in this into the ABS brain will confuse it or maybe it won't matter. Find out and let us know.

 

Your correct in saying that tire height will have an effect on the distance the tire goes as opposed to width. The 08's were calibrated at a 16" height and some wheel speed based on the stock tire geometry. The 10's obviously have a 2" difference in diameter and I would bet you the MOCO calibrated their ABS on the 10"s based on that. To say the 08' s are the same as the 10's as far as calibration is concerned I think is wrong. Once again I am saying the abs is calibrated to a certain wheel speed with the computer that measures that. If you change the diameter of the tire don't you change that
calbration???? Hey I have an o8 with an 18 on it. I have found no ill effects from my standpoint. I do know my dealer has raised the question to me though and as asked me about it. I really don't have the answer here I am just looking for someone who really knows to say its really nothing to worry about or somewhere down the road it's gonna or may bite you in the A$$$.

 

A larger front wheel will make everything up front turn slower in relation to the rear sensor, bearings included. Could it be that HD built a large threshhold for error into the ABS system knowing so many owners will change the stock wheel setup. This explanation makes the most sense to me. Loosing 10% rotation speed on a larger front will just activate the ABS response to wheel lock up sooner than a smaller stock front wheel with full rotation speed on the front bearing. Maybe running the 21" rim with no tire will provide the correct OTH. .....Lol.