Topic: rpm at 70 mph

[dusterdude]

would someone smarter than me be kind enough to tell me what gearing i need to achieve 4000 or so rpms at 70 mph on a 750 with a 25 inch rear tire.thanks

[Bodi]

The rear wheel RPM should be fairly easy to calculate. I think this is correct:
A 25 inch diameter gives a 78.5 inch (6.54 feet) circumference, you could measure the roll length with you on the bike to be more exact - your weight compresses the tire a bit and shortens the roll length.
At 5,280 feet/mile that's 807 wheel revs per mile. So 60 MPH (1 mile per minute) would mean 807 rear wheel RPM. at 70MPH you should have 942 RPM (807 x 7 / 6).
The puzzler is that the primary ratio and 5th gear ratio were changed during 750-4 production. It looks to me like stock gearing will give you quite close to 4000 RPM, and if you have a sprocket set (any) to try you can find the change % you want and calculate the sprockets you need.
Knowing the year and model would help generate a sprocket size recommendation.

[dusterdude]

if this helps,the bike is a late 71 build k1,i rode it earlier today and im getting roughly 4000 rpm at 60,i would like to change the gearing to get 4000 rpm at 70 mph if possible.

[Bodi]

If you have the stock 18/48 sprockets, using a 41 rear and the 18 front would do it and you could cut the chain to fit.
I'm not sure how large a sprocket will fit in the engine, but a 21 would be close to 4K at 70 with the 48 rear.
No idea if these sizes of sprockets are readily available, but they can be made on special order.

[trism18]

I have also been interested in lower the RPM at higher speeds for my CB750 K1.  So, you can lower ther RPM by changing out the front and rear spockets, right?  Will this affect the quickness of the bike, or have any negative affect on stock performance?

[Bodi]

 trism18 -
Yes, there's an effect on performance. Taller gearing to reduce highway RPM will reduce acceleration. The opposite is possible too, you can have snappier acceleration if you set it up so the highway RPMs are higher. If you go too tall the clutch tends to wear quiclky because you have to slip it a lot to get moving with the higher final ratio in 1st gear.
Messing with the transmission ratios is also possible and you could have normal ratios for 1-4 and a higher overdrive 5th gear for cruising, but because the gears are in the engine case and special ratio gears would have to be custom made that's a rare modification.
The old CB900 Custom has a high/low primary gear selector, low for tearing around town and high for freeway cruising.

[kghost]

A 40 tooth rear with an 18 front gives a 4300 rpm cruise at 70 (K2).

[dusterdude]

thanks everyone for the help,i think ill try to scare up a 41 or so tooth sprocket and give it a try.thanks again.

[KB02]

new sprokets are readily aviavable through Dennis Kirk (www.denniskirk.com). I just got a 40 tooth rear for my bike from them. They have various sizes and teeth available for most models.

[mrbreeze]

I just put on new Sunstar sprockets--- 42 rear and 18 front. I am cruising right at 4k doing 70.

[dusterdude]

ok guys more info,i rode to work this morning and i am turning about 4500 at 70.i would like to turn 4000 at 70,so would i still need the 40 tooth to drop the revs the necessary 500 rpm.thanks