Found this gear ratio chart in an old magazine. NOTE: Please keep in mind that this chart refers to gear ratios for HARLEY XL's.

Does anyone have a similar chart for Triumph 650's ? Or even 500's & 750's ?

The chart illustrates something that I've seen elsewhere on these boards about dropping 1 tooth on the m/s sprocket is like dropping 2 teeth from the rear sprocket. From the magazine: "Remember, as the numbers get smaller on gear ratio, cruising will be improved and acceleration will suffer. And, of course, the opposite is true as the numbers get bigger".

M/S SPRKT..18.....19.....20.....21.....22.....23 REAR SPROCKET 49.............4.72..4.48..4.25..4.05..3.86..3.70 50.............4.82..4.57..4.34..4.13..3.94..3.77 51.............4.92..4.66..4.43..4.21..4.02..3.85

I developed an excel spreadsheet sometime ago to help me quickly calculate road speeds using varying combinations of internal gear ratios, engine,countershaft and rearwheel sprockets as well as wheel/tire sizes. It is based upon 4 and 5 speed Triumphs but can be adapted to other bikes quite easily providing the gearbox internal ratios are known. I have tried to simplfy input as much as possible. Find it to be quite usefull myself.

Anyway,will email the spreadsheet to anyone who maybe interested.

Thanx guys, the info you provided is much appreciated. Was thinking that even a "ballpark chart" (kind of a misnomer, LOL!) would help people to decide whether to change m/s sprocket or rear sprocket to arrive at an acceptable gearing for their own bikes.

Panic, above the chart in the magazine it says: "34T Motor Sprocket, 59T Clutch Sprocket"

"dropping 1 tooth on the m/s sprocket is like adding 2 teeth to the rear sprocket"?...right & thanx for the correction.

Belated p.s.....Brian, Thanx very much for that pdf. file. I can rough-sketch a chart from there, using 18-20 m/s sprockets/44-48 rear sprockets. Just what I was hoping for! Thanx again!

Remember, as the numbers get smaller on gear ratio, cruising will be improved and acceleration will suffer. And, of course, the opposite is true as the numbers get bigger.

If interested, I can compute theoretical road speeds in mph for the various sproket combinations you have tabulated using my spreadsheet if you would also provide the outside diameter of your rear tire and desired RPM.

(stock engine and clutch sprockets assumed unless otherwise advised)

Hey, that'd be great! Was thinking that an easy reference chart might be helpful for folks to decide which way they wanted to go with their M/S & rear sprockets for general street applications. Theoretical road speeds would be an added plus for those experimenting with different sprockets.

Using the pdf. file that Brian provided & figuring that 46 & 47 seem to be the most common rear sprocket used for street, also included 2 teeth down & up in either direction.

I'm running a 16" rear wheel, but figures for 16" & 18" rear wheels could be very helpful since it would apply to both choppers & stockers.

Desired RPM ? Well...I don't run a speedo or a tach (one of those primitive chopper, loosey-goosey kinda things, LMAO!) so whatever RPM ranges you determine to be standard would be cool.

Based upon 25.6" diameter tire and 6700rpm (max BHP point for T120) in top gear (1:1)theoretical road speeds corresponding to highest and lowest ratios taken from table are as follows:

For ratios in betweeen just multiply road speed corresponding to ratio above by ratio used in calculation and divide by new ratio e.g.:

88.3*5.76/5.18 = 98.186 mph

For different tire dia just multiply by new tire diameter and divide by tire diameter used in the sample calculation above e.g for 23.6" diameter tire result would be:

98.186*23.6/25.6 = 90.515mph

And, of course, for different engine speed it would again be a simple matter of calculating the corresponding road speed e.g. for 3000 rpm:

90.515*3000/6700 = 40.529mph (more in line with wot you chopper guys like :p ):

Hope I've got everything right else might get pounced on

Thanx once again very much, Britbodger, for taking the time to post that info. And thanx to Panic as well for the intermediate gearing calculations. Will print these out & will have to study 'em a bit. I can do basic arithmetic...addition, subtraction, multiplication & division, so should be able to figure it out (In high school geometry, the teacher told us to "plot an equilateral triangle"...I misunderstood & got thrown out of class for conspiring to plot against a mathematical equation )

Between this & the "pdf. file" (try sayin' THAT 10 times fast & watch the Child Protective Service people get excited ) there should be enough to keep me busy until sane riding weather.

"90.515*3000/6700 = 40.529mph (more in line with wot you chopper guys like ):"......ROTFLMAO!

Thanks for the thanks Riff-Raff. Only too pleased to be able to help.

Personally, I think that the stock gearing using 19/46 ratio is just right for the T120. With an 18" 120/90 (26.5" outside diameter) rear tire this equates to a top speed of 109mph at 6700rpm. When my T120 was in road trim I was able to achieve 108mph (recorded on my cycle speedometer). Admittedly this was under favourable conditions (slight tail wind and slight downgrade).

My stock Triumph loved to rev - maybe because it had been balanced properly at the factory. And when I took it down after years of what a lot of riders would consider abuse the bottom end was in unbeliveably good shape - as was the rest of the engine for that matter. The T120 engine will take a lot of beating and likes it .

P.S. Thanks for taking my chopper digs graciously . Shows you have a sense of humor.

Thoughts... 1) These charts aren't seen very much any more because anyone with a computer and spreadsheet program (Excel) can generate one 30 pages long in minutes. As was pointed out, the “sacred ratios” are nothing more than the result of dividing the larger sprocket tooth count by the smaller.

The ratio does 2 things: a) It explains what 1 tooth equals on the other sprocket, and b) lets you easily compare an 18/49 to, say, a 19/52. (Answer: not much!)

2) The m/s (mainshaft) sprocket is the clutch basket! The rear wheel chain on a Triumph runs to the countershaft sprocket. A sprocket, oddly enough, named for a shaft that no longer exists!

3) On a pre-unit you can also "re-gear" the primary drive by changing the engine sprocket, since the primary uses a single row #428 chain that can easily be extended or shortened. Unit constructions are not normally re-geared at the primary drive since changes to a double row endless chain are beyond the financial capability of most owners.

On newer bikes you'll see huge rear sprockets at the dirt track, but you usually see more normal sized rear sprockets on pre-units. This is because pre-unit racers usually re-gear at the engine sprocket. 1 tooth at the engine then equals 5 or 6 at the rear wheel.

4) Ratios are all fine and well, but don't get too caught up in the numbers. The reason for this is chain pitch. Ratios are simply an easy way to comprehend what happens when you change one or both sprockets, but stay with the SAME chain pitch. However, if you decided to convert to a chain with a different pitch then the comparison doesn't hold. Why? Because what you are REALLY comparing is the ratio of sprocket DIAMETERS.

This is not such a concern on a Triumph because you can only get 5/8" pitch chains (#530 & #525). But if you were re-gearing a Jap bike and went from a #428 to #525 then you'd be in trouble. You may have to mentally wrestle a while with this, but you'll finally realize that an 18T sprocket for #428 is NOT the same diameter as an 18T for #530.

5) As was pointed out, going from a 4.00x18 K70 to a metric tire will also change your rear drive ratio simply because of the change in effective diameters. The K70 is a std height tire and the metric series are usually lower. On a "120/90" for instance, the "90" means the tire is only 90% the height of a regular old school tire.

Heck, for that matter changing the air pressure in your rear tire will have the same effect. It is, after all, the radial distance from the axle to the ground with the rider's big butt parked on the saddle that determines the last of the final drive ratios. So your 110 lb wife might be faster on the same bike simply because she doesn't depress the rear tire as much as your lardly, beer sodden arse does.

Mornin' All, Good to see more good info that many can benefit from & utilize. I respect that Britbodger, Panic, Brian & RF enjoy the deeper aspects of mechanical science which is commendable & an obvious bonus for the site overall. My knowledge isn't as extensive & I try to absorb what I can while keeping it simple, fun & enlightening. Was mainly interested in a basic reference chart for general use, but the additional info generated in this thread is excellent and, again, helpful for others seeking a deeper understanding of these items.

Britbodger, I also run the 19T/46T on the TR6 & am pleased with that combo. Even with the 16" on the rear, it feels right & as you pointed out, these Triumphs LIKE to run hard. Heck, this chopper even gets allowed to stretch it's legs on occasion...sometimes up to a whopping 45 mph *ahem*

Panic, By going from 19T to 21T on the transmission sprocket, are you saying that it'll not only be slower off the line, but also reduce top speed ? Or will top speed simply take longer to achieve but be essentially the same as before ? (Guess I should be doing the previous math, but still working on the 1st cup of coffee, LOL!)

Messing with your gearing can be a double edge sword, depending on your particular application. A 1969 "Motorcycle Mechanics" road test of a bone stock T100T gave a top speed of 105 mph in top gear with a 19 tooth gearbox (U.K. spec) and 46 wheel sprocket. The same bike was measured at 116 mph with a 20 tooth gearbox and 45 wheel @ approx. 8,000rpm. For their liking and purposes, they considered the bike to be still under-geared. I've changed my gear box from an 18 (U.S. spec)to a 20 tooth and find that, for my purposes, it is much more enjoyable to ride. At least now I can get across an intersection from a red light without having to change gears. Of course I don't win any stop light Grand Prix, but I also find cruising at 60mph for 300 miles much less tiring.

Perhaps he may have benefited by using a close ratio box instead. Then his shift from third to fourth wouldn't have resulted in such a large drop in rpm which his engine wasn't strong enough to make up at 120mph. Having said that however, you still need the horsepower to pull a high top gear.