What RPM do you all get in 4th gear at 70MPH? And what sprockets are you running in the front and rear?

 

SK.......are you calculating your final gear drive for your machine?
My XLCH is set up like this:
engine sprocket= 34T
clutch sprocket= 59T
c/s sprocket= 24T
rear drum= 51T

Take the clutch sprocket #teeth divided by the engine sprocket #teeth=_______ (Write down the result)
Take the rear drum #teeth divided by the counter shaft #teeth= _____________ (Write down the result)
Multiply the two results to give you the final gear ratio. Round up to 2 places.
In my case this would look like this:
59 divided by 34= 1.735
51 divided by 24= 2.125
1.735 X 2.125= 3.69 final gear ratio. This means on my engine every 3.69 rpms my rear wheel turns one complete turn. All of the sprockets on all Sportsters have a fixed number of teeth, excepting the counter shaft sprocket. The c/s is the only one that you can change. A 1971 XLCH came with a 19T. The result for that is 4.66:1 final ratio, as so stated in the manual. This is really low gear for the street. Better for drag racing or red light to red light.
Conversely, the 24T is the largest available, and I just recently installed this on mine. 3.69 is a really tall gear. I wish it were a half tooth lower (not possible) but the trade off is worth it to me because the engine lopes along on the freeway with less rpms. At 3500 rpms I am going 60 mph on my speedometer. Perhaps you can use this figure to compare to yours.

You can figure your gear ratio on your machine and then take it further if you want. Your engine turns at ___rpm and your gear ratio is _____, therefore you know how many turns the rear wheel makes relative to the engine rpm. Your rear tire is ______inches in circumference. 60 minutes in an hour, and there is 63,384 inches in a mile etc, etc. The result is that you can calculate the miles per hour for a given rpm. The only unknown is the slippage that occurs. We used to calculate this as being 6%.

 

I am currently running a 24t front and a 43t rear. I have a 16" rear wheel. Assuming the engine and clutch are stock can you calculate what my tach should read at 70mph?

 

SK.....I think I could come up with something close, but there is one piece of info I need from you before I can do it.
What I need is the circumference of your rear wheel. I know you have a 16 inch, but that is the rim size. What I need is the outside. The easiest way I know to get this is to put the bike on a stand and run a string around the tire. Mark where it overlaps and then lay it out on the ground and make a direct measurement of this string. Post that and I will see what I can do.

Edit remark: SK......This is an ironhead Sportster, correct? With that small rear sprocket of 43T and a 24T on the c/s means your final ratio is 3.11:1 That is an unbelievable high gear! If the machine is not an ironhead, then the engine sprocket is probably not 34T, but smaller than that. I do not think an ironhead could pull such a high gear (3.11:1) Something is not correct and I find that I am confused.

 

I will measure it tomorrow. In the mean time, I was wondering if you can list your RPMs in 4th gear for 50, 60, and 70 MPH. I was running the same sprocket set your are running now a few months ago. Perhaps I can compare your RPM to mine and see how far off it is.

 

The ratio is correct. I changed out the rear sprocket for a 43T in order to use my 4th gear as an overdrive for better gas mileage. I plan to swap out my first gear for a 2.66 wide ratio first to compensate a bit on the take off so I do not burn out my clutch. On the highway, I get an indicated 3000RPM at 60MPH and 3500RPM at 70MPH in fourth gear. I use 1-3 in the city and leave 4th for Highway. I do about 3000 RPM at 45MPH in third.

But as stated in other threads, I suspect my tach is off. Which is why I want to compare everyones tach readings and what gear ratios they are running to determine how far off my tach is. What RPM do you run at doing 45mph in 3rd?

 

SK........So that's correct then, you are pulling a 311 gear. Wow, talk about an overdrive! As I said about mine, using 3.69 to 1 requires me to slip the clutch a little taking off from a dead stop. Around here it has been raining all week, but as soon as the road dries I will take my Sportster out and record the rpm level at the different speeds and post it back to you so that you can compare.
Since the tach is a mechanical drive, any error that happens must be in the tach head itself. If I recall correctly, and it's been a while, the tach drive is a 2:1 ratio to the engine. Of course a tach head that is 2:1 is also used so that correct readings are given. I am not 100% sure that I am correct in stating this as fact though, so don't hold me to it.
Thinking back to my days at Daytona on the big track, I rode a KR and the gear ratio was 3.96:1. That was the most gear that the KR could pull and it resulted in flat out runs of 6800 rpm maxxed out. The old 45 cu inch flathead was just about equal to an ironhead in power and speed. If I stacked higher gear than the 3.96 on the bike, it just dropped the top rpm, and of course the speed as a result.
Ok, I will look for some sunshine and get back with you.

 

Just get an SE 6 spd and forgettttaboutit.

 

I got it up to 90mph in 4th today with the 3.11 ratio. 4500 RPM indicated. I suspect it was closer to 4000RPM in reality if the amount of tach error I estimate is correct.

 

You ain't around old iron much, eh?