rake and trail specs

by **madass140** » Thu Jan 12, 2012 4:41 pm

can some one list the rake and trail for the following
Early Commando which uses the Atlas yokes
later 750 Commando 70-73
850 Commando

I want to determine what would be an ideal rake when using the early Atlas type yokes, which have less offset than later
common yokes 70-73

by **Rohan** » Thu Jan 12, 2012 5:13 pm

The Workshop Manual lists those later 750 as 63 degrees, and the 850 as 62 degrees in the frame checking diagrams. Not seen the trail listed. Never met those early forks.

Note - amended which way around they are written.

by **madass140** » Thu Jan 12, 2012 7:35 pm

Thanks

by **madass140** » Thu Jan 12, 2012 8:43 pm

ok, thanks, its really 28 degrees for the 850 and 27 for the 750
I need to know the trail of each which is more important for me.
, its not good to put the 750 yokes on the 850, but I'm trying to determine whether fitting the very early Commando yokes which were Atlas yokes would be ok, they have less of an offset compared to the later 750's
I have to do some more research but my initial finding which could be wrong is that it would be ok, If I can find out the rake and trail of the 3 models then I'll fit some early yokes to an 850 frame and take some measurements for comparison.

by **rpatton** » Fri Jan 13, 2012 9:22 am

January '74 test in CycleWorld has 28deg rake and 4.36" trail on 4.10 tires listed for the 850.

by **Matt Spencer** » Fri Jan 13, 2012 5:53 pm

You could always measure it , if you have the differant ones . Axle location ( distance ) frame glued to a box ( Beer Crate ) as long as the frame / datum is not varied , axle posn will relate directly .

Somewhat of a fuss is made of it all , the tyres themselves have varying requirements ( per Type ) Trigonics liked ample ( 4 in trail plus ) others required less . 3.00 KR76 on front got light steering.
Trail figures published on the real ( early ) clamps would be with the 3.00 ribbed tyre , fitting 3.25 or 3.50 would increase trail , and raise front , so marginally increae rake , unless you fitted a bigger
tyre to the rear , raiseing it , which , if more , would decrease them . :lol:

Then theres tyre profile , for ' roll steer ' , compounds , and . . . .

String and chalk , and a plumb bob ., a big square ( builders ) tape measure , and flat ground .

A ' big square thing ' that catches consistantly under the frame , so can measure below to axle is fine , or backed against a wall , indoors . :lol:

( or the other way round , Nose to wall , chock under frame for consistancy of hight , measure to footrests or sim , a half in more means its a half in more .)

COMPARATIVE positions . The steering axis inclination ( rake ) is fixed . ( the steering head ) . so AXLE position fwd = more trail , aft = less .

The angle / inclination of the fork legs is a seperate other issue .

by **madass140** » Fri Jan 13, 2012 6:54 pm

Thanks, obviously Norton has printed the specs as Bob has detailed the 850 spec.
I thought these measurements would be readily available and known by the experts here on this forum, apparently not.
If I had the 3 bikes in question then I would be taking my own measurements,

by **Rohan** » Fri Jan 13, 2012 10:09 pm

CycleWorld possibly measured the trail themselves ?

Its not listed in the WorkShop Manual, and doesn't seem to be listed anywhere else in anything Norton. Easy to find for the 961 though, (24.5 degrees and 99mm)

When it was pointed out some years ago on the NOC list that the forktubes are slanted 'backwards' at about a degree and a half (?) in the 850 yokes, was greeted with much derision.
And accident damage mentioned. Until folks actually went and measured it themselves !!
Took 30 years for that info to come to light in print.
Try finding that info anywhere from Nortons. ?

by **Stephen Hill** » Fri Jan 13, 2012 10:40 pm

I am also looking at putting some early Commando (Atlas style) yokes on an 850 frame. And also wondered if there would be any issues with the geometry. I plan on using early headlight ears as well, and wondered if how this would work without gaiters in terms of look, etc.

Stephen

by **Matt Spencer** » Fri Jan 13, 2012 10:48 pm

Took 30 years for that info to come to light in print.

Woulde been quicker to go and get a tape measure .

by **madass140** » Fri Jan 13, 2012 11:35 pm

Stephen, when you think that the offset of the Atlas type yokes is less than the "normal" 750 Commando yokes then on the face of it you would think that by fitting the Atlas type yokes to an 850 frame you would be achieving a similar result as using the 850 angled back yokes,
I may have to gather up some bits and do measuring .

by **Rohan** » Fri Jan 13, 2012 11:53 pm

Matt Spencer wrote:Took 30 years for that info to come to light in print.

Woulde been quicker to go and get a tape measure .

It doesn't really show up with simple measurements.
Most folks were not even aware that the forks tubes angled 'backwards' until it was pointed out to them. Some took a lot of convincing....

by **Cheesy** » Fri Jan 13, 2012 11:59 pm

madass140 wrote:Thanks, obviously Norton has printed the specs as Bob has detailed the 850 spec.
I thought these measurements would be readily available and known by the experts here on this forum, apparently not.
If I had the 3 bikes in question then I would be taking my own measurements,

I think they are around somewhere, I found the 750 trail to make sure my drawing was correct, I think it was around 90mm

by **Rohan** » Sat Jan 14, 2012 12:05 am

Which year of 750 did you draw ?

by **Matt Spencer** » Sat Jan 14, 2012 12:18 am

It doesn't really show up with simple measurements.
Most folks were not even aware that the forks tubes angled 'backwards'

HIDEING SOMEWHERE , Is It . Complicated measurements might do it then . !

Plumb Bob off Axle .Mark. Straight Edge off Steering head , minus half diameter .
Theres at least two bolts holding the headlamp on , and a piece of 4 x 2 would be required for clearance, a extra pair of hands, and its done . :shock:

Only issue is if its static or loaded , so try measureing both , an inclinometer would say if the rakes changed.

The Angle of the stauchions , while affecting suspension action , is irrelevant to figures .

Incidently , Id think the Gaiters would match the looks , cant see the point in exposed staunchions , unless you donr ride it .

rake and trail specs

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