I killed my performance

[OrlandoGS]

I recently had to get a new tire after 7500 on the original. I was dumb and got a 130/80 tire instead of a 130/70. When I picked it back up from the shop, it took me a minute to realize why it was so slow. Then I noticed that the rpm's were nearly 500 lower at a given speed than it used to be and I realized that I essentially geared my bike slower with the slighty bigger tire. Is a new front sprocket the answer? Should I go with a 15 or 14 tooth? Please help.

[weaselnoze]

i could be wrong but... that sounds like a load of crap. something else might have happened.  i moved from a 130/70 to a 130/80 as well and noticed nothing like that...

[Rema1000]

130mm * 0.7 = 91mm tire height.
130mm * 0.8 = 104mm tire height.
Wheel radius = 17/2 inches, or 215mm
So you went from a wheel with circumference 2 * 3.14159 * (91 + 215) mm (or 1923mm) to a wheel with circumference 2 * 3.14159 * (104 * 215) mm (or 2004mm).  At a given speed, I'd expect your RPMs to be 96% of what they were on the old tire.  Speed at 5K rpm on the old tire would only need 4800 RPM on the new tire.

...theoretically... if I have the numbers right.

[john]

130mm * 0.7 = 91mm tire height.
130mm * 0.8 = 104mm tire height.
Wheel radius = 17/2 inches, or 215mm
So you went from a wheel with circumference 2 * 3.14159 * (91 + 215) mm (or 1923mm) to a wheel with circumference 2 * 3.14159 * (104 * 215) mm (or 2004mm).  At a given speed, I'd expect your RPMs to be 96% of what they were on the old tire.  Speed at 5K rpm on the old tire would only need 4800 RPM on the new tire.

...theoretically... if I have the numbers right.

Smart ass    :mrgreen:

[Faxxxy]

130mm * 0.7 = 91mm tire height.
130mm * 0.8 = 104mm tire height.
Wheel radius = 17/2 inches, or 215mm
So you went from a wheel with circumference 2 * 3.14159 * (91 + 215) mm (or 1923mm) to a wheel with circumference 2 * 3.14159 * (104 * 215) mm (or 2004mm).  At a given speed, I'd expect your RPMs to be 96% of what they were on the old tire.  Speed at 5K rpm on the old tire would only need 4800 RPM on the new tire.

...theoretically... if I have the numbers right.

larger tires equal more speed..
Not to be confused with more power..

Your speedo is probably 3 Mph slower then you are actually going now that you have the larger tire..

[Rema1000]

Your speedo is probably 3 Mph slower then you are actually going now that you have the larger tire..

Speedo sensor is on the front wheel.  He replaced the rear tire.  
Not that the speedo was ever accurate to begin with .

[Faxxxy]

Your speedo is probably 3 Mph slower then you are actually going now that you have the larger tire..

Speedo sensor is on the front wheel.  He replaced the rear tire.  
Not that the speedo was ever accurate to begin with .

I knew that..
My bad..  

[OrlandoGS]

well, i used to be at 8000 in 6th, going 100 , now i'm at 7500. That is just the speed that I remembered the difference in the tach. It feels sluggish all over. I also had my 7500 mile service done at the same time. Could the dealership have screwed something up?

[Kerry]

A very similar thing happened when I replaced my stock rear tire with a 130/90 ... except that I dropped 1000 RPM in a given gear at a given speed.  It took me a while to figure out (duh!) but since the tire was the only change....  

The fact that some have seen no change could be explained by different manufacturer sizing, etc.  But your results match exactly with mine!  

I haven't worked out the math, but I bet that dropping one sprocket tooth in the front will roughly make up for the larger tire in the rear.

[Rema1000]

well, i used to be at 8000 in 6th, going 100 , now i'm at 7500.

That's about right.  96% of 8000 is 7680, and as Kerry writes, there is some difference in tire height between manufacturers.  So you may actually have gone from 8000 RPM to 7500 RPM (93.75% of old RPM).

As Kerry writes, going from 16T to 15T in the front should raise RPMs by 16/15 (+6.7%).  And   ((16/15) * (7500/8000)) = 1 , so a 15 tooth front gear should put your 7500RPM exactly back up to 8000 RPM.

[scratch]

Don't feel bad, I just accidentally ordered a 110/70 Avon front, and rode in the rain this morning on it. Brand new tire not broken in yet + rain=  Skittish! I really need to take out another preload washer (too stiff, pushes the front).