Special vs. standard RPM

[jetmechmarty]

I played some tachometer games with JeffH on our two day ride we just completed. His is a XS1100E, and mine is a XS1100SG. My speedometer runs faster than his and I believe strongly that it is because his Avon tires run larger than my Pirelli tires. Both bikes wear 100/90-19 up front. The E rear tire is 130/90-17. The SG rear is 130/90-16.

Where we had a straight stretch of road, we matched tachometers at 4000 rpm. In 2nd, 3rd, and 4th gear he creeps away from me. It's hardly noticeable. In 5th gear, he walks past me. No faster than that. With Jeff at 4000 rpm and me pacing his bike, my Special is turning about 4200 rpm and certainly no more than 4250.

I understand that there could be errors in these ancient tachometers and there are possible errors in the tire sizes. Just the same, I have come to believe the different size wheels on the Special and the standard are of little to no consequence. I went into this experiment believing otherwise. After seeing these results, I have changed my mind.

[Arthur_of_Hex]

That's it right there. All things being equal- transmission gear ratios, final drive ratios...the only thing left to change is tire size. With the smaller 16 on the special, the exact same rotation of engine, driveshaft, and hub will produce less forward movement with each rotation of the wheel due to the smaller wheel having smaller circumference or rollout. The special's 16 will rev a bit more to keep up with the 17. Looks to me like things are working perfectly normal.

[Arthur_of_Hex]

Just ran some quick numbers. 17 rollout is 82.3 inches. 16 is 79.1 inches. 60 mph = 1 mpm = 5280 ft/min = 62496 inches /min. Divide...at 60mph the 17 turns 759.3 times to 790 for the 16. I didn't research the final drive ratios but if you divide 4000 rpm by (calculated wheel rpm) 759.3 you get 5.26. The special 4200 calculated 5.30. Granted these aren't true final drive numbers, but...in doing math from the information given, it shows that most likely the trans gears and final drives are the same, leaving only tire size to give us the difference.

[jetmechmarty]

Matt,

Transmissions and final drives are identical. The only difference is the back wheel. It doesn't make as much difference as I thought it did.

[BA80]

The Standard with stock FD and the 17 inch wheel gear ratio is exactly the same as the Special with 750/850 FD and the 16 inch wheel.

With the stock FD more power gets to the rear wheel no matter which tire.

My special with the 850 FD will do 115 - 120 easily in 4th gear but if I shift into 5th I lose speed and top speed is about 97mph. The power just doesn't get to the ground.

[Arthur_of_Hex]

True, the difference is marginal but there, if my math is anywhere near accurate then it shows the difference. Funny you mentioned comparing tachs at different speeds, my best friend and I have been lining up speedometer needles and tachs forever. Every single time we ride, one of us will pull ahead and display speed with hand motions, the other will match and take note of variations for the conversation at the next stopping point.

[crazy steve]

When the bikes were new there was more difference....

This is because the standard was originally fitted with a 4.50H17, which was about 1" taller than the currently-available 130/90-17. The Special always came with the smaller 130/90-16, so it's tire size hasn't changed.

http://www.xs11.com/forum/showthread.php?t=36698

It's also interesting to note that by DOT rules, speedometers can read up to 10% fast but cannot read slow.

[jetmechmarty]

It's also interesting to note that by DOT rules, speedometers can read up to 10% fast but cannot read slow.

I wonder how they're supposed to get that right when tire sizes vary so much. I have several 100/90-19 tires mounted here, and they measure from less than 95 mm wide to 105 mm wide. I expect all the difference between the E and SG speedometers is in the difference in the front tires.

[BA80]

I expect all the difference between the E and SG speedometers is in the difference in the front tires.

The fronts are exactly the same size.

With the 110/90/19 on the front of mine the speedometer is pretty much dead nuts on.

[jetmechmarty]

The fronts are exactly the same size.

No, they are not! I have failed to get that point across. Both bikes have 100/90-19 front tires. On that we agree. They are not the same size. I was unable to measure the Pirelli on the E as I don't carry a caliper on my bike. The Pirelli on the SG is 95 mm wide. 100/90-19 means the width is 100 mm and the aspect ratio is 90%. The tire falls short. Looking at the way the Avon fills the fender on the E, I am sure it is bigger than the Pirelli.

With the 110/90/19 on the front of mine the speedometer is pretty much dead nuts on.

Greg, measure your front tire. I'm willing to bet it falls short of 110 mm wide. I bet it's 105 mm or less.

Does anyone on here run a stock fender and a 110 Metzeler on the front? No, they don't, because it won't fit.

[jetmechmarty]

I just got the word. The Avon 100/90-19 front tire is 105 mm wide. There lies our speedo error.

[BA80]

Why do you think width would cause a speedometer error?

Diameter would be the only thing that would affect the speedometer.

[jetmechmarty]

Why do you think width would cause a speedometer error?

Diameter would be the only thing that would affect the speedometer.

If the aspect ratio is true to 90%, then the wider tire will have a taller sidewall and larger diameter. Are you with me now?

My point is, these tires are rarely true to size. I contend that if you try to put a 110/90-19 Avon or Metzeler on your bike, it won't fit. Unless of course, the manufacturer makes them the same size as the 100/90-19 and simply changes only the sidewall markings. That would not surprise me.

[BA80]

I understand that part Marty but if one isn't correct how can you expect the other to be?

I think Tod may have run a Metzler or Avon on the front at one time or another. Except for an occasional balance issue (like now) I run strictly Shinko 712s on the front.

[jetmechmarty]

I think Tod may have run a Metzler or Avon on the front at one time or another.

Yes, but not 110's. Any further explanation will have to come from someone in the tire manufacturing business. I have simply pointed out that there are sizing inaccuracies. 100/90-19 tires will always fit on the front. Some of them fill the front sender and some don't. The one's that don't, make the speedometer read faster and I witnessed that over the last couple of days.

Bottom line; a Special engine turns around 200 rpm faster at legal highway speeds than a standard due to the wheel size difference. That number will vary slightly with different tire brands due to sizing inaccuracies. All this assumes stock drive line components.