Does anyone know the primary reduction ratio in a WR400?
Posted February 13, 2001 - 03:46 PM
Posted February 13, 2001 - 04:15 PM
Primary reduction ratio: 62/21 (2.952)
Secondary reduction ratio: 50/14 (3.571)
Gear ratio -
1st 29/12 (2.416)
2nd 26/15 (1.733)
3rd 21/16 (1.312)
4th 21/20 (1.050)
5th 21/25 (0.840)
Hope that helps.
Posted February 14, 2001 - 08:22 AM
Oops! If I use that ratio I get a top speed of 103 (14/50, 110/100-18 tire) at 11,500 rpm, and almost 99 at 11,000 rpm. If your bike won’t go over 90 then maybe you DO have a problem. Are you still running 14/50 gearing? It would take 4 additional teeth out back or one less up front to bring it down to 90.
Has anyone else tried to calculate this? I get 93 mph for a YZ which also seems high. I must be using too large a number for the tire diameter (26.7 inches). I calculated this based on the size (110/100-18) but maybe the only way is to actually measure it.
Posted February 14, 2001 - 09:14 AM
I wonder if things would work out per your calculations on a dyno…
Posted February 14, 2001 - 12:43 PM
Posted February 14, 2001 - 03:17 PM
I’m just thinking you can’t arrive at a real-world diameter using the tire size. I think I’ll take an actual measurement tonight to see what is going on…
Posted February 14, 2001 - 03:26 PM
2.952 Primary Reduction Ratio
3,896 Transmission primary RPMs
0.84 5th gear ratio
4,638 Front sprocket RPMs 5th gear
14 Front Sprocket
50 Rear Sprocket
3.5714 Secondary Reduction Ratio
1,299 Tire RPMs
25 Dupont tires 27" diameter worn to 25"
101,988 Tire inches traveled / minute
6,119,295 Tire inches / hour
509,941 Tire feet / hour
97 Tire Miles / hour
This leads me to believe that the REV limiter is not cutting in at 11,500. It would be more like 11,000 unless Taffy's 5% tire slippage is accurate. Take 5% from 97 and you get right at 92. That is about what I clocked myself at last night with the GPS after putting the 180 main back in.
Posted February 14, 2001 - 04:09 PM
And what kind of surface were you riding on?
Posted February 15, 2001 - 09:06 AM
I was riding on hard asphalt.
Also, when you sit on the bike you compress the tire. Even though the tire may be XX inches in circumference, the actual inches traveled would be Pi times the length from the center of the rear axle to the road surface when you are sitting on the bike.
The only two factors which would change this would be centrifugal force pushing the tire out and slippage on the road. Between these two I don't see how they could make more than a couple of percent difference.
Posted February 15, 2001 - 09:45 AM
I have noticed this phenomenon on both roadrace and dirttrack and ice bikes... When a tire that it is 3/8" or so away from a part of the bike while sitting still, will rub on it at high speed.
I guess you are just doing this out of curiosity... because the only way to know for sure on the RPM is with an accurate tach.