Early Commando tach drive ratios

Joined
Oct 28, 2009
Messages
693
Country flag
Someone has a Smiths tach drive BG1508/06 for sale. Says it is a 1:1 ratio and correct for early Commando.

I thought that the BG1508/06 is correct for the early Commando, with a 2:1 ratio.

I thought the BG1508/05 is correct for Atlas, with a 1:1 ratio.

On this site, in archives, one post says that the BG1508/06 is 2:1, another post says that BG1508/06 is 1:1.

Can somebody please shed some light on this?

Stephen Hill
Victoria, BC
 

L.A.B.

Moderator
VIP MEMBER
Joined
Nov 20, 2004
Messages
15,691
Country flag
The BG1508/06 tach drive gearbox for the '68 Commando is 1:1 ratio (Reversing) according to the Smiths list, however, the tach listed for that year is RSM 3003/10 4:1 ratio which would require a 2:1 drive gearbox.
 
Joined
Dec 4, 2009
Messages
273
I have a smiths motor cycle equipment book 1962 - 1975 It shows a pic of the gear box Below it states BG 1508 Series (Replacing 70549 Series)

These gear boxes reverse the rotation of the shaft to which thay are attached
The rotation is determined by looking immediately on the end of the engine
spindle with which the gear box or flexible drive is ingaged

Plain phosphor bronze bearings Ratios 1:1 and 2:1

Phil
 
Joined
Oct 28, 2009
Messages
693
Country flag
I'm still confused.
If the correct tach drive for the '68 Commando is a BG1508/06 1:1 ratio
And the 4:1 tach calls for a 2:1 drive,
WTF.
Stephen Hill
Victoria, BC
 

L.A.B.

Moderator
VIP MEMBER
Joined
Nov 20, 2004
Messages
15,691
Country flag
Stephen Hill said:
I'm still confused.
If the correct tach drive for the '68 Commando is a BG1508/06 1:1 ratio
And the 4:1 tach calls for a 2:1 drive,
WTF.
There is the possiblity that the BG15806/06 ratio was listed incorrectly as 1:1 (I think the RSM3003/10 is 4:1)?

Perhaps our 20M3 Commando owning members can add some information?

Is your tach (RSM3003/10?) marked 4:1?

Edit:
http://www.burtonbikebits.net/Rev-Count ... rboxes.htm
BG1508/06 2:1 Reversing
http://www.britishonly.com/webcatalog/15.txt
BG1508/06 GEARBOX,TACH DRIVE,2:1 REVERSE
 
Joined
Oct 7, 2005
Messages
1,170
On the two prototypes we had trouble with the wrong gearing for the speedometer drives, but the tachometers were fine. The speedos showed 1.6x actual road speed, so were off the top end a lot of the time we were riding (actual 75 mph showing as 120).
 

Ron L

VIP MEMBER
Joined
Feb 27, 2004
Messages
3,005
Country flag
All Commando tachometers are 4:1 ratio. All P11/N15/G15/Atlas tachometers are 2:1 ratio. BG1508-06 tach drive is 2:1 and used for 4:1 tachometer. BG1508-05 is 1:1 and used for all 2:1 tachometers.
Tachdrives are driven at cam speed (1/2 crank speed) so final ratio must be 2:1, thus a 4:1 tach needs to be reduce by a 2:1 drive, but a 2:1 tach is driven by a 1:1 drive.
 
Joined
Mar 12, 2012
Messages
4
Five years later...restoring my 1968 Commando, I found that the tach was reading very high. The drive gear on the bike was the BG1508/05, a 1:1 drive, while the tach is a 4:1. I got the correct 2:1 drive gear from Andover Norton and now it reads correctly.
 
Joined
Jun 30, 2012
Messages
9,795
Country flag
If the Tacho read-out is 4:1 the drive ratio off the camshaft must be 2:1, because the ration between the crank and the timing gear is 2:1.
I recently got involved in a debacle over a tacho for a racing Triumph. I offered my friend a 4:1 tacho off an RG250 Suzuki which is what I use with my 850 motor. There are two ratio Triumph tacho drives - the one which fits the generator hole is 1:1, the one in the '59 Bonneville timing cover is 2:1.
So two different ratio tacho read-outs are used at the handle bars. My friend wanted to use the drive in the generator hole, but he needed the 2:1 drive. An answer would have been to fit an early Commando 2:1 drive onto a standard timing chest , driven off the exhaust cam to use the 4:1 read-out, but it became all too expensive.
All he has done is buy a cheap 2:1 read-out and a flexible mount intended for a manx Norton - problem solved !
 
Joined
Jun 30, 2012
Messages
9,795
Country flag
If you are changing the tacho read-out. the direction of rotation is important. The advantages of the RG250 Suzuki tacho are that the direction of rotation can be reversed by simply undoing two screws on the back and turning the helical drive over, and vibration won't kill it. The tacho read out is usually much cheaper to change than the drive mechanism at the motor.

 

Top