T120R Vs. T120RV - Final Drive Ratio

[NM Bonny]

Shortly after buying my bike, I found out that there were early '71 and'72 five speeds with a V designation added to the end of the engine number. However, I then found threads on various forums, more than one of which linked to this article.

According to the above article, "With the exception of the stamping on the engine case, there were no external differences between the 5- and 4-speeds. They had the same final drive ratios and sprockets, and the 5-speeds were remarkedly quicker." [emphasis added]

The latter statement would seem to jibe with the performance data in the original shop manual:
T120R - low-speed pass = 363 ft. @ 7.5 seconds; high-speed pass = 1,015 ft. @ 10.3 seconds
T120RV - low-speed pass = 375 ft. @ 8.0 seconds; high-speed pass = 940 ft. @ 9.4 seconds

Can anyone confirm that, indeed, the final drive ratios of the R and RV bikes are the same? If so, it would seem that the only benefit to having the latter is a closer-ratio gear box, and not a true overdrive as I had imagined--and I must admit, fretted when I found out about after I made my purchase.

 

[Rudie]

were early '71 and'72 five speeds with a V designation added to the end of the engine number
linked to this article.

Has some nuggets in it but a lot of it is nonsense. The only 71 model year 5-speed T120's were the 200 built in June 1971 to homologate specifically the 650 with 5-speed for AMA Class C racing.

Not noted in the article (unless after I stopped reading) but shown in a photo. is the 71 "V" is larger than the "T120R" - Triumph used "block stamps" (one tap with a hammer stamped the complete model code) but no one thought it was worth making "T120RV" stamps just for 200 engines and frames.

According to the above article, "With the exception of the stamping on the engine case, there were no external differences between the 5- and 4-speeds. They had the same final drive ratios and sprockets.

Different rear wheel sprockets are not listed for 4- and 5-speed twins; otoh, Triumph listed everything from 15 to 20 teeth gearbox sprockets irrespective of the number of gearbox speeds; otoh2, triples had 50 and 53 teeth rear wheel sprockets.

the 5-speeds were remarkedly quicker."
The latter statement would seem to jibe with the performance data in the original shop manual:
T120R - low-speed pass = 363 ft. @ 7.5 seconds; high-speed pass = 1,015 ft. @ 10.3 seconds
T120RV - low-speed pass = 375 ft. @ 8.0 seconds; high-speed pass = 940 ft. @ 9.4 seconds

Depends how things are being measured.

The main thing to remember is 5-speed first gear is a lower ratio than 4-speed first, then the gaps between intermediate 5-speeds are smaller than between intermediate 4-speeds.

"performance data in the original shop manual" - were the "passes" made in top gear or did the rider change down - as you would in real life - before accelerating?

not a true overdrive

Correct; top is 1:1 in both gearboxes.

 

[NM Bonny]

Has some nuggets in it but a lot of it is nonsense. The only 71 model year 5-speed T120's were the 200 built in June 1971 to homologate specifically the 650 with 5-speed for AMA Class C racing.

Not noted in the article (unless after I stopped reading) but shown in a photo. is the 71 "V" is larger than the "T120R" - Triumph used "block stamps" (one tap with a hammer stamped the complete model code) but no one thought it was worth making "T120RV" stamps just for 200 engines and frames.


Different rear wheel sprockets are not listed for 4- and 5-speed twins; otoh, Triumph listed everything from 15 to 20 teeth gearbox sprockets irrespective of the number of gearbox speeds; otoh2, triples had 50 and 53 teeth rear wheel sprockets.


Depends how things are being measured.

The main thing to remember is 5-speed first gear is a lower ratio than 4-speed first, then the gaps between intermediate 5-speeds are smaller than between intermediate 4-speeds.

"performance data in the original shop manual" - were the "passes" made in top gear or did the rider change down - as you would in real life - before accelerating?


Correct; top is 1:1 in both gearboxes.

Last point first - excellent! I installed a Datsun 5-speed in my '67 MG Midget 'test mule' for my side business not merely due to the full-synchro 1st gear but for the true overdrive ratio 5th gear. So, would've been bummed out if a true overdrive option was out there and I'd missed it due to inattention at purchase.

On the rear sprockets: have you had experience with fitting a smaller rear sprocket on the Unit 650s, rather than going with a larger front sprocket? I bought a 20-tooth front sprocket to go with the standard rear sprocket after reading several threads on that combination--but wonder about fitting a slightly smaller rear sprocket for better highway cruising.

BTW, I have stock K70s fitted now, and when they wear out will be mounting stock-size Battlaxes. I need to measure the circumference of the Battlax rear tire to compare with the K70's circumference...

 

[Rudie]

have you had experience with fitting a smaller rear sprocket on the Unit 650s

No. Plus the 47 tooth rear on unit 650 and '73/'74 750 twins is already the smallest of the three Triumph made available.

Fwiw, standard T160 final drive gearing is 19:50 but many have 19:47 using a converter to mount the standard rear conical sprocket in place of the T160's special dished sprocket.

wonder about fitting a slightly smaller rear sprocket for better highway cruising.

Might not be possible to have smaller than 47 tooth rear because of the space between chain, sprocket mounting fasteners and brake plate?

I need to measure the circumference of the Battlax rear tire to compare with the K70's circumference...

4.00x18(?) K70 o.d. is 26.73" (679 mm), from "SPEC CHART" almost at the bottom of Dunlop US K70 webpage.

4.00x18(?) BT46(?) o.d. is 673 mm from Result | Tire Size | Motorcycle Tires | Bridgestone Corporation, or search other tyres' o.d. at Motorcycle Tires | Bridgestone Corporation

If you are concerned about speedometer accuracy when changing rear tyres, Triumph fitted the same ratio speedometer drive gearbox to US 650 (4.00x18 rear tyres), UK & General Export 650 (3.50x18) and Trident (4.10x19). Other model ranges had different drive ratios only because they had heads with different speed ranges - triples, 750 and 650 twins had 150 mph heads, 500/350 twins had 120 mph heads, 250 singles, Tiger Cubs, Bantams had heads with even smaller speed ranges.

 

[NM Bonny]

No. Plus the 47 tooth rear on unit 650 and '73/'74 750 twins is already the smallest of the three Triumph made available.

Fwiw, standard T160 final drive gearing is 19:50 but many have 19:47 using a converter to mount the standard rear conical sprocket in place of the T160's special dished sprocket.


Might not be possible to have smaller than 47 tooth rear because of the space between chain, sprocket mounting fasteners and brake plate?


4.00x18(?) K70 o.d. is 26.73" (679 mm), from "SPEC CHART" almost at the bottom of Dunlop US K70 webpage.

4.00x18(?) BT46(?) o.d. is 673 mm from Result | Tire Size | Motorcycle Tires | Bridgestone Corporation, or search other tyres' o.d. at Motorcycle Tires | Bridgestone Corporation

If you are concerned about speedometer accuracy when changing rear tyres, Triumph fitted the same ratio speedometer drive gearbox to US 650 (4.00x18 rear tyres), UK & General Export 650 (3.50x18) and Trident (4.10x19). Other model ranges had different drive ratios only because they had heads with different speed ranges - triples, 750 and 650 twins had 150 mph heads, 500/350 twins had 120 mph heads, 250 singles, Tiger Cubs, Bantams had heads with even smaller speed ranges.

Oh, okay - thanks. It seems unlikely that 6mm in O.D. will make a huge difference--though 6mm in O.D. translates to a slightly more meaningful difference in circumference. I think we get:

K70: 679mm O.D. = 2,113.14mm circumference, or 83.982"
BT46: 673mm O.D. = 2,114.29mm circumference, or 83.2398"

83.982"
- 83.2398'
= .7422" difference in circumference. Not sure what percentage of error that would net at 60 mph, and of course odometer reading difference. But 3/4 of an inch would presumably net something discernible...?

At any rate, whenever I wear out the K70s (or sell them right off the bat in favor of the BT46s), if I am irritated by the slight error in reading, I can always install an inline ratio correction box from this guy. He's got plenty of ratios to cover even much larger differences, so such a small correction would be duck soup--and the compact box can be hidden away nicely on this bike.