Hi All,

I need some advice on gearing as I’m trying to figure out what size rear sprocket I need.

What is a reasonable cruising RPM for a wr250 dirtbike with road tyres?

Currently my WR has stock gearing and geared stupidly low for off-road work – at the moment I’m doing 5000rpm @ 60kph and 8250rpm @100kph (Calcs below).

I was thinking in the order of 6000rpm when cruising at 100kph……

This would mean I need a 14 front and ~38 rear sprocket.

Can anyone see an error in my calcs (if you can figure out what I have done)?

WR250F

* Primary Drive Ratio = 57/17 (3.353)

* 1st gear ratio = 31/13 (2.385)

* 2nd gear ratio = 28/16 (1.750)

* 3rd gear ratio = 28/21 (1.333)

* 4th gear ratio = 25/24 (1.042)

* 5th gear ratio = 0.814

* Front sprocket (stock) = 14

* Rear sprocket (stock) = 52

Tyre - 120/80-18 dunlop

= 18” + (80% of 120) = 457 + 96 + 96 = 649 div 2 ……radius = 325mm.

Circumference - 2 PI r = 2 * 3.1415 * .325m = 2.04m per revolution.

Calcs

Speed (kph) = RPM *60 (revs/hour) / 5th gear ratio / primary ratio * chain sprocket ratio * wheel circumference /1000 (kms)

60kph = XXX*60 / 0.814 /3.353 * 14/52 *2.04m/1000

XXX = 4970rpm. @60kph.

100kph = YYY*60 / 0.814 /3.353 * 14/52 *2.04m/1000

YYY = 8280rpm. @100kph.

110kph = ZZZ*60 / 0.814 /3.353 * 14/52 *2.04m/1000 =

ZZZ = 9110rpm. @110kph.

I want to be riding at roughly ~ 6000rpm @100kph.

Therefore

XXX = 6000rpm. @100kph.

100kph = XXX*60 / 0.814 /3.353 * AA/ZZ *2.04m/1000

AA/ZZ= 0.3716 - ie 14/38

I need some advice on gearing as I’m trying to figure out what size rear sprocket I need.

What is a reasonable cruising RPM for a wr250 dirtbike with road tyres?

Currently my WR has stock gearing and geared stupidly low for off-road work – at the moment I’m doing 5000rpm @ 60kph and 8250rpm @100kph (Calcs below).

I was thinking in the order of 6000rpm when cruising at 100kph……

This would mean I need a 14 front and ~38 rear sprocket.

Can anyone see an error in my calcs (if you can figure out what I have done)?

WR250F

* Primary Drive Ratio = 57/17 (3.353)

* 1st gear ratio = 31/13 (2.385)

* 2nd gear ratio = 28/16 (1.750)

* 3rd gear ratio = 28/21 (1.333)

* 4th gear ratio = 25/24 (1.042)

* 5th gear ratio = 0.814

* Front sprocket (stock) = 14

* Rear sprocket (stock) = 52

Tyre - 120/80-18 dunlop

= 18” + (80% of 120) = 457 + 96 + 96 = 649 div 2 ……radius = 325mm.

Circumference - 2 PI r = 2 * 3.1415 * .325m = 2.04m per revolution.

Calcs

Speed (kph) = RPM *60 (revs/hour) / 5th gear ratio / primary ratio * chain sprocket ratio * wheel circumference /1000 (kms)

60kph = XXX*60 / 0.814 /3.353 * 14/52 *2.04m/1000

XXX = 4970rpm. @60kph.

100kph = YYY*60 / 0.814 /3.353 * 14/52 *2.04m/1000

YYY = 8280rpm. @100kph.

110kph = ZZZ*60 / 0.814 /3.353 * 14/52 *2.04m/1000 =

ZZZ = 9110rpm. @110kph.

I want to be riding at roughly ~ 6000rpm @100kph.

Therefore

XXX = 6000rpm. @100kph.

100kph = XXX*60 / 0.814 /3.353 * AA/ZZ *2.04m/1000

AA/ZZ= 0.3716 - ie 14/38

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