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

jimbo said:

1973 750 212082

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From Norvil's site.

200000

1972

IV

750

Fastback Roadster HiRider Interstate

Breather moved to between gearbox plates behind engine - was on the left hand side of the camshaft. Engine number prefix discontinued. Optional disk brake and combat engine. Revised shape handlebar levers.

220000

1973

V

750

Roadster HiRider Interstate

Black instrument cases. Square rear light (actually fitted from 212278). Disk brake on all but Hi-Rider.

It is a pity that Norton decided for an angled triple tree to increase trail from 81 mm to 106 mm.
Probably for reasons of cost.
That 1 deg. ( or 1.5 as I measured) is by itself totally irrelevant, but apparently causes a lot of confusion.
They could have gone with a parallel tree and less offset to get the same effect.
( and this thread would be much shorter..)

MichaelB said:

........ Somewhere in the annuls of Norton writings it is said not to mix 850 (ANG) trees with 750 (steeper neck angle) frames. To be clear, I am NO expert on this. As we know, nothing is a sure thing in the world of Norton. Myself, I would run Parallel trees....Not the ANG's...

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that is contradicting marshg246;
IMHO, putting 850 triple trees on a 750 is a good thing so when I have a bad 750 triple tree, I use an 850 triple tree when I can't find a replacement 750.

........ so confusing​

Too many disagree with me so disregard all I've said on this.

ludwig said:

It is a pity that Norton decided for an angled triple tree to increase trail from 81 mm to 106 mm.
Probably for reasons of cost.
That 1 deg. ( or 1.5 as I measured) is by itself totally irrelevant, but apparently causes a lot of confusion.
They could have gone with a parallel tree and less offset to get the same effect.
( and this thread would be much shorter..)

Click to expand...

Without going to the trouble of drawing it out again & doing the trig. , I think I worked out that by using angled yokes, the wheel base was kept the same as earlier bikes. When I rebuilt me 850 MK2A I fitted Yamaha forks, & used late BMW K75s yokes. These have if I remember correctly a 57mm offset, keeping the trail more or less the same as standard, but increasing the wheel base slightly.

Matchless said:

Without going to the trouble of drawing it out again & doing the trig. , I think I worked out that by using angled yokes, the wheel base was kept the same as earlier bikes. When I rebuilt me 850 MK2A I fitted Yamaha forks, & used late BMW K75s yokes. These have if I remember correctly a 57mm offset, keeping the trail more or less the same as standard, but increasing the wheel base slightly.

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I have Yamaha forks and yokes on my 750
The yokes are around 40mm offset
The bike handles fine at all speeds
I only really noticed the increase in trail for the first 5 minutes when I fitted the front end

baz said:

I have Yamaha forks and yokes on my 750
The yokes are around 40mm offset
The bike handles fine at all speeds
I only really noticed the increase in trail for the first 5 minutes when I fitted the front end

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The other reason I wanted the larger offset, was so as not to reduce the steering lock. As it is on my bike the tank is mounted slightly further back, as I modified the frame to adjustable front tank mounts. A mate of mine has a Ducati 900 bevel fitted with modern forks & yokes, giving very little offset. It handles fine, but has so little steering lock that it's like turning the Queen Mary around.

Matchless said:

The other reason I wanted the larger offset, was so as not to reduce the steering lock. As it is on my bike the tank is mounted slightly further back, as I modified the frame to adjustable front tank mounts. A mate of mine has a Ducati 900 bevel fitted with modern forks & yokes, giving very little offset. It handles fine, but has so little steering lock that it's like turning the Queen Mary around.

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I've had to move the tank back and reduce the steering lock a little
It was ok when it was a fastback but the roadster tank is wider at the front

When I had about 94mm of trail on my Seeley 850, the bike felt fine, but a handling problem came from nowhere and tried to bite me. With 106mm of trail - the bike did not try to stay vertical as I braked, but I did not notice any other difference. However when I race, I always try to accelerate earlier when coming out of corners. That was when I found a major difference. I am fairly certain, I can brake before corners, and accelerate flat-out all the way around them. With my bike, power delivery is very smooth. The last time I raced, I was accelerating from just inside the corners. The speed differential in corners when compared with other bikes seemed dangerous, but probably wasn't. My bike turned much tighter, the other guys were all out wider.
When I first started racing, I would never have noticed the difference.

When I was young, my mate and I were aware of many of the theories. However, I do not believe that many of us knew much about machine handling. And if any of the top guys knew - they were probably not telling. I am a bit different to most people - I don't care if other guys have faster motorcycles, they still need to know how to ride them. There is only one person I will not help to ride faster - he worries too much about getting beaten.
A lot of handling improvements came out of motocross. I was involved in tuning two Bultacos, and probably should have been more aware of the effects of adjustments to suspensions.

I have the same Queen Mary turning radius because of the fairing. I actually added some metal to the steering head plate of the fairing bracket to restrict my yoke swing so I wouldn't crush my hands on the fairing. I wasn't going to bend over to use lower clip ons instead of handlebars, nor did I want to raise the fairing since it was too high to begin with...

My 70 model is 2.25" fork offset, 19" front wheel, 27 degree headstock angle ----- 104.648mm trail. - 4.12" trail

The 73 model 750 has a 2.81" offset, 19" front wheel, 27 degree headstock angle ---- 88.9mm of trail - 3.5" of trail

the 850 model has 2.81" offset, 19" front wheel, 28 degree headstock angle with a -1 degree yoke angle ---80.264mm of trail - 3.2" of trail

the 850 model has 2.81" offset, 19" front wheel, 28 degree headstock angle with a +1 degree yoke angle ---109.22mm of trail - 4.31" of trail

the 850 model has 2.81" offset, 19" front wheel, 28 degree headstock angle with a 0 degree yoke angle ---95.504mm of trail - 3.76" of trail

****************************************************************************************************************************************************

So, I see a real anomaly here in the math, so I tried the ANG rake number in both +1 and -1 directions, and also zero to see if any of them made sense.... but it doesn't actually


IF the 73 750 has 27 degree rake and the resulting trail is 3.5", It makes sense that the 73 850 with 28 degrees and no yoke rake would have a longer trail of 3.76". It would have .26" more and be more stable going straight as logic predicts

But, the 73 850 has the ANG yokes which have a -1 degree yoke rake along with the increased 28 degree headstock rake. That trail comes out to 3.2" which is less trail than the 73 750 trail of 3.5" so it should be less stable, but the change to the 850 was done to add stability. How is less trail going to make the 850 more stable???

I also computed the yoke rake as a +1 degree thinking that maybe I had the rake direction of the yoke incorrect. That trail comes out to 4.31" of trail which makes sense because it would be more stable than the 73 750 which is again 3.5" of trail,.... but the new wheel base would be longer with the + rake added and I was under the impression that the yoke rake was to shorten the wheel base after increasing the head stock angle,.... so the math of the +1 yoke rake makes sense that the bike would gain stability, but it doesnt fit with the narrative that the yoke rake was added to the 28 degree frame to maintain the shorter wheel base.... because that yoke rake would have to be negative 1, not positive 1

Anyone know how the 850 can have less trail than the 750 and claim to be more stable?.... Something isn't right or I have something backwards... Anyone?

here's the image to work out the visualization if anyone is interested.

Anyone want to explain my question above?

How can the increased headstock angle and raked yokes make the 850 a more stable bike than the 750 when that change actually reduces the trail from 3.5" to 3.2" ?? How is this more stable?? anyone??

o0norton0o said:

Anyone want to explain my question above?

How can the increased headstock angle and raked yokes make the 850 a more stable bike than the 750 when that change actually reduces the trail from 3.5" to 3.2" ?? How is this more stable?? anyone??

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You have miscalculated. All 850 models have 4.3" of trail. If early 750 yokes with 2.25" offset are fitted to a 850 the trail is more or less the same at 4.36". Both of these figures assume a 19" front/tyre wheel is fitted of approximately 26" o/d.

o0norton0o said:

How can the increased headstock angle and raked yokes make the 850 a more stable bike than the 750 when that change actually reduces the trail from 3.5" to 3.2" ?? How is this more stable?? anyone??

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The drawing appears to show you have used the same axle position for both forks to calculate trail. The offset fork axle will be further back so the line from the axle to the ground will be in a different position to the straight forks.

Matchless said:

You have miscalculated. All 850 models have 4.3" of trail. If early 750 yokes with 2.25" offset are fitted to a 850 the trail is more or less the same at 4.36". Both of these figures assume a 19" front/tyre wheel is fitted of approximately 26" o/d.

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thanks for that... I used 26.1" for the wheel OD calculation. I also figured out where I went wrong... The stupid drawing is wrong, so my visualization was based on the logic I was applying from looking at the drawing

With the forks raked backwards, the projected point of the steering yoke center on the ground doesn't change, but the wheel moves backwards so that increases the trail...

thanks LAB and matchless. I was editing the drawing to show how it's supposed to be shown when LAB posted his response.