Does a 1973 750 commando frame have a different geometry than a 1974 850?

Last edited:
It would be useful to establish a checking dimension from a datum point or line, to the intersection point of a horizontal line with centerline of the steering stem.
The steering stem is hollow, so a fitting round bar threaded at one end would make measurement quite simple. One possible datum line is through centerline of lugs for the front isolastic mounting.
I don't think position of lugs ever changed? Using a water level (after ensuring the lower rails are level), taking the measurement should be a breeze.

- Knut
 
Can a 73 750 frame be used with triple trees from a 74 850? And other considerations replacing a 74 850 frame with a 73 750 frame?
 
Yes only one degree but plus the yoke change took a lot of weaving and skittishness away with the downside being slower steering.
 
When I first rode it, I was impressed with how well my mk3 850 steered. But, it was on non-standard 3.60 tyres, front and rear. The replacements were 4.10s and they have noticeably slowed down the steering.
 
I would guess you could get some of the improvement by running a lower profile rear tire and/or a higher profile front.
 
I won't be road racing the bike just some causal street riding. But I do want it to be safe. AN lists yokes and complete disc brake forks to fit bikes from 72 to 74

andover-norton.co.uk

Andover Norton - FRONT FORK ASSEMBLY COMPLETE (1972-74 Commando Disc brake) - 06.7317

FRONT FORK ASSEMBLY COMPLETE (1972-74 Commando Disc brake)
andover-norton.co.uk

andover-norton.co.uk

Andover Norton - UPPER YOKE C/W STEM 850 (

UPPER YOKE C/W STEM 850 (
andover-norton.co.uk

andover-norton.co.uk

Andover Norton - LOWER YOKE 850 (

LOWER YOKE 850 (
andover-norton.co.uk

but lower yoke 750 only
andover-norton.co.uk

Andover Norton - LOWER YOKE (750 ONLY) - 06.1916

LOWER YOKE (750 ONLY)
andover-norton.co.uk

and the same frame is listed for bikes from 72 to 74

https://andover-norton.co.uk/en/shop-details-2/16508/main-frame-assy-not-mkiii-painted
 
AN have made the decision to standardise on the 850 frame and yokes for 72 onwards, it's not a reflection of what left the factory but a reduction in the parts they stock and standardising on the better option.
 
Mart UK said:
When I first rode it, I was impressed with how well my mk3 850 steered. But, it was on non-standard 3.60 tyres, front and rear. The replacements were 4.10s and they have noticeably slowed down the steering.
Click to expand...
hhh said:
I would guess you could get some of the improvement by running a lower profile rear tire and/or a higher profile front.
Click to expand...
Actually, you have that backwards...

lowering the rear wheel center and raising the front wheel center would have the effect of creating MORE trail which makes the bike more stable in a straight line, and take a little more effort to turn the bike... You can google this kind of thing to confirm what I am saying.... or you can read 100 posts here where a single persistent poster here swears everyone is wrong....
 
o0norton0o said:
Actually, you have that backwards...

lowering the rear wheel center and raising the front wheel center would have the effect of creating MORE trail which makes the bike more stable in a straight line, and take a little more effort to turn the bike... You can google this kind of thing to confirm what I am saying.... or you can read 100 posts here where a single persistent poster here swears everyone is wrong....
Click to expand...
I agree with you and I thought that's what I said. "Lower profile rear" meaning a lower wheel center in the rear, giving more trail like the 850 frame's rake of 62 degrees vs. the 750's 63.
 
I thought you meant low profile tires, which have less sidewall height in order to reduce excessive sidewall flex, which feels like sloppy handling. I was under the impression that those low profile tires generally have smaller diameters because of that.. just trying to keep the record straight..
 
When you measure the rake, it is normal to measure it from the vertical, not the horizontal. A Manx has 24.5 degrees with about 30mm of yoke offset and 19 inch wheels. From what you have said, a Commando has either 27 or 28 degrees of rake, and probably about the same amount of yoke offset as a Manx, with 19 inch wheels. But the weight distribution also comes into the equation. Presuming a consequence, is not as good as testing. It is whether the bike oversteers or understeers when you gas it or brake when on a lean, which matters. From what has been said on this forum, you often need to counter-steer a commando as you enter a corner - that is understeer, and more predictable. When a bike oversteers, you need to anticipate where it will go, when you accelerate, but you can ride faster. When you have changed steering, it is important to note what the bike does the first time you ride it, too much understeer is dangerous. The problem can come from nowhere when you are braking. If the bike feels stiff as you brake for corners on a lean, be careful, and don't brake suddenly.
 
acotrel said:
When you measure the rake, it is normal to measure it from the vertical, not the horizontal. A Manx has 24.5 degrees with about 30mm of yoke offset and 19 inch wheels. From what you have said, a Commando has either 27 or 28 degrees of rake, and probably about the same amount of yoke offset as a Manx, with 19 inch wheels. But the weight distribution also comes into the equation. Presuming a consequence, is not as good as testing. It is whether the bike oversteers or understeers when you gas it or brake when on a lean, which matters. From what has been said on this forum, you often need to counter-steer a commando as you enter a corner - that is understeer, and more predictable. When a bike oversteers, you need to anticipate where it will go, when you accelerate, but you can ride faster. When you have changed steering, it is important to note what the bike does the first time you ride it, too much understeer is dangerous. The problem can come from nowhere when you are braking. If the bike feels stiff as you brake for corners on a lean, be careful, and don't brake suddenly.
Click to expand...
A Manx has a 26 degree head angle & a 2.250" yoke offset. The first Commandos have 27 Deg. & 2.250".
 
marshg246 said:
Yes, no problem. True from 71 with center stand on cradle to 74 850.
Click to expand...
Not sure what Greg actually replied to here. Parts fit, but that doesn't mean they are interchangeable.
It's advisable to use 750 triple trees for 750 frames, and 850 triple trees for 850 frames. Your spares list for the year in question should be your prime guidance.

- Knut
 
Am I correct in inferring that the changes to the 850 frame and 850 triple trees were made simply as a handling improvement, but not necessitated by having the 850 engine?
 


Write your reply...

Similar threads

yooper trooper
Rear tire sizes for a 1973 750 Commando.
2 3
Replies
42
Views
3,627
concours
concours
MCP
  • Locked
1971 Frame for a 1974 MK2A
Replies
8
Views
1,094
Deckard
Deckard
D
  • Locked
New here-Rebuilding a 1974 Commando 850
Replies
13
Views
1,589
Hobot
Hobot
M
  • Locked
How do I route the oil lines on a 1974 850 Commando
Replies
9
Views
1,834
hewhoistoolazytologin
H
Back
Top