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#837292 01/19/21 5:45 pm
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I am trying to determine what engine and rear wheel sprocket combination to use for the '70 TR6R I'm working on.

I plan to install a 5-speed gear-set. I have listings stating the numbers of teeth on FOUR-SPEED gears,
but I can't find any information on the numbers of teeth on each of the FIVE-SPEED gears.

This information would be of help in calculating overall gear ratios at the rear wheel when the bike is in each of the five gears.

Does anyone know where I can find five-speed gear teeth numbers?

Thanks.

Last edited by Irish Swede; 01/19/21 5:46 pm. Reason: I was missing the word "anyone" in last sentence
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The parts book lists all the gear teeth #'s

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John, I have a 1976-77 750cc T140 parts book.

It lists the gear PART numbers, but not the number of TEETH on each gear.

It's the number of teeth on each gear that I need.

The older T120 parts books with 4-speed gear boxes DO list the number or teeth on each gear.

Is there a service bulletin, or any other source, that can provide that information?

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Isn't it just the gearbox ratios that are needed here?

Rather than the teeth numbers of the cogs.

Both 4 and 5 speed boxes have a top gear ratio of 1:1.

The 5-speed ratios are 1, 1.19, 1.40, 1.837, 2.585

The 4-speed ratios are 1, 1.19, 1.69, 2.44

Last edited by koan58; 01/20/21 7:37 pm. Reason: typo2.44 rather than 2.64
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Mainshaft (Top): 21T, 20T, 18T, 16T, 13T.(1st)

Layshaft (Top): 15T, 17T, 18T, 21T, 24T.(1st)

Sleeve gear/layshaft ratio (21/15) = 1.4:1

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THANKS, L.A.B.

It's what I needed.

I hope it wasn't too much trouble for you to get this information.

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Hi Irish Swede, Thanks Koan for posting ratios. So top gear & one gear down are the same. The real difference when riding is 1st gear feels lower & 3rd gear is perfect for canyons with sharper curves 20-35 mph. This speed is just a killer for 4 speed as motor is spinning silly in 2nd, yet bogs in 3rd.

I would recommend 20t in most cases. I've changed 19 to 20t on several 650. It really helps rpm at freeway speeds. Even on faster 2 lane roads. I've had anyone want to put 19 back on. I even put 20t on my TR6C which I commuted on as well as much off road riding. Took a little extra clutch slipping in dirt over the 18 it came with, but still very good on dirt. 18 at 65 mph motor is spinning crazy fast.

With the lower first on a 5 speed you won't notice a difficult take off even riding two up or taking off up hill. Super steep sharp switch backs are no problem either with 5 speed in first. With 20t on 4 speed the same switch back requires a fair amount of clutch slipping.

I've ridden both many times on same roads. I'll take 5 speed any time. Also going into first is much better. The dogs just engage without grinding like a 4 speed can do when gears are not quite stopped yet. 5 speed gears are steeply back cut so the lock together tightly under load. So always fully pull clutch when shifting. However, even slightest clutch drag, getting neutral at stop light can be difficult. Blip throttle lightly. Just as revs begin to fall gears will be unloaded & neutral is easy to find every time. Takes only a little practice to master this. If I had 4 speed that needed much work, going to 5 speed is what I'd do every time. I can't think of downside.
Don


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Originally Posted by Irish Swede
THANKS, L.A.B.

It's what I needed.

I hope it wasn't too much trouble for you to get this information.

thumbsup
No, it didn't take long.

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A quick summary:

[Linked Image from hermit.cc]

Additions? Corrections? ping me.


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Interesting that the 5 speed bottom gear is higher than the 4 speed - never realised that! Poster edited, 5 speed is indeed lower!

Last edited by TinkererToo; 01/21/21 12:13 pm. Reason: Poster edited entry
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Here is what I have used to calc gearing:
gearing.xls

Replace the individual gear ratios in the pink column. The pink column is where you put your bike's information in.

Hope this helps

Last edited by John Healy; 01/20/21 8:42 pm.
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Hermit,
I see what you’re getting at but the left column is upside down.

It is top (ie 4 th or 5 th gear) that is 1:1.

The layshaft/mainshaft teeth don’t matter in top of course, because the mainshaft is locked to the sleeve gear. The layshaft is freewheeling.
So teeth numbers don’t matter in top.

They only matter in the lower 3 gears of 4-speed, and the lower 4 gears of 5-speed.

I don’t understand why anyone would be so concerned about the numbers of teeth on cogs, when the only thing that matters is actual ratios from 1:1 upwards.

The only things you can easily change on a unit Triumph is gearbox sprocket, rear wheel sprocket and rear tyre OD.
Those things can be easily calculated for.

edit: the 4 speed 1st gear ratio is 2.44, not 2.64

Last edited by koan58; 01/20/21 7:31 pm. Reason: omission
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That's better!

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I asked for the numbers of teeth on the stock five-speed gears because some after-market gears with different numbers of teeth are available to provide additional gear-ratio options.

With this information for comparison, my local math expert (Wife, Masters degree in mathematics) can work up final gear-ratios at rear wheel using different gearbox final drive chain sprockets.

This is a typical wintertime, "snow-day, stay-at-home" project. I'll share her results whenever this project is completed.

Thanks to everyone who provided information in response to my request.

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Didn't triumph change 4 speed 2nd ratio around 1969?
'The year of the gearbox' in triumph history. (loads of mods)

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L.A.B., Koan58 - thanks for the corrections! With an F5 Refresh everyone should see update with corrections. Thanks again.


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NickL - yes - but it was the 3rd gear ratio that changed according to Triumph service Manual 329: I'll let the table above reflect that.



[Linked Image from hermit.cc]


Bruce Miller
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Have you use the gearing.xls spread sheet it works the treat. You can change sprockets tire radius etc. and get instant answer.

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Somewhere I can recall reading that the number of teeth on layshaft and mainshaft total the same in the various gears.

This applies to the 4 speed box (36T total), but in the 5 speed the sum varies between 37T and 36T (3 of the gears total 37T).

But according to a recent post by Hermit, the 3 rd of the 4-speed changed in 69 or so to a22/23 ratio. So there’s an odd sum.

I’m not enough of an engineer to know what this means, if it means anything at all. Just thought I’d mention it for the more serious engineers amongst us.

Just an observation is that the very durable 4 speeds (IMHO) have 30-16 teeth on their gears.

To fit the 5 speed in the same case obviously required some compromises, the gears range from 24-13 teeth.

It would be reasonable to expect about 25% extra wear on the 5 speed gears, just based on the number of tooth surfaces.
I’m sure there’s much more to it than that, some of the gears are narrower for example.

However, for all of it wonderful simplicity, the Triumph box, 4 or 5 speed is a great box. I’ve often had folk saying “why don’t you have a Norton box in your triton, they’re much stronger?”. My answer is that I’ve used this Triumph box since the original build in 1980, its been 2000 miles round Europe then 4000 miles to Africa and back, has your gearbox done that yet? And still strong.

I’m rambling!

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The dog arrangement on the 5 speed is far superior to the 4.
More like the Norton/quaife type, that makes 'em last longer and
able to handle more torque.
I raced against blokes with imp engine outfits using them. Some
were producing 95+ BHP etc admittedly most used the close quaife
gears but the box was the same. Stick an outrigger on 'em to help
the case and top gear a little and they are very good.

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Hi, Other than the early factory 5 speed, I don’t see durability issues with 5 speeds.

Factory fitted the reinforced 5 speed 1973. They improved gear on main shaft in ‘76.

I’ve now covered 36k miles on my 73 Tiger. Never had problem with trans. Never even had outer cover off. I did sprocket seal at about 30k miles.

Just today rode the canyons with John & his ‘69 Bonnie. Pondering all this I stand by there is no downside to installing 5 speed. John & I kick this around often. The $1300 is the only hold up for him.
Don


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I have 2 complete (now) '76 on clusters waiting to go into pre-unit shells that cost me complete £706, roughly $480 each. These were bought piecemeal on fleabay with new parts to bring them up to scratch, and include all of the selector parts etc, only bearings to get, so the stuff is around if you know what you're looking for. I've done at least 8 boxes, some using the slighly earlier cluster. $1300 is buying everything new top dollar from a Harris dealer.
Mick.

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WOW! I never expected such a wealth of information to become available because of my question!

The after-market gear options I mentioned come from a 2000 bulletin from TRIPLE CYCLES in the U.K. who offered conversion of the stock Triumph
five-speed box to close ratio, using their 20T lay-shaft 2nd gear, and 22T 1st gear.

The overall ratio of 2nd gear changes from stock 1.84 to a new ratio of 1.75.

The overall ratio of 1st gear changes from stock 2.58 to a new ratio of 2.37.

Whether these ratios are suitable for street use on a long-stroke TR6R conversion to 750cc, with a lone rider weighing 170 lbs. is yet to be determined.

Again, THANKS to all involved, for this information.

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Hi Irish Swede, my good friend has ‘70 TR6R with Morgo 750 kit, Pistons it came with, Boyer, Premier carb, 20t trans sprocket. 4 speed. Has proven powerful & durable.

I’ve ridden this bike. Feels like a very powerful 650. Thinking of close ratio gear box if you’re not yet put out, I would recommend standard ratio 5 speed. The spread of 1,2,3,4 are just right for general street use & sporty canyon riding where the switch backs are steep & tight going into short fast straights & winding curves. Very dangerous to exceed 45mph in the twisties. Speed signs say slow to 15mph. Switch backs are 5mph. Scary steep & sharp. You need the lower gears to keep rpm up as road will not allow high speeds. Many motorcycle deaths, crashes. Redwood Rd., Pinehurst rd. Oakland California, to Castro Valley. Street view on google maps makes it look flat & easy. Following speed limit of 25mph it is.

Again was thinking about this yesterday on ride. Whoever selected ratios at factory must have actually ridden bikes. It’s pretty good for real life riding in all road conditions on public roads. Race track I don’t know.
Don


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Correction.,. The ‘76 gear improvement is on lay shaft, not main shaft. Reinforced 4th gear positively positioned with snap ring. Gear is shaped differently so visual ID is easy.

Yes, $1300 is new LF Harris conversion kit. You still need sprocket & a few things to install it that are not included. Good used not too easy to find hear. Worn ones no problem.
Don


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