850 yoke offset… why…?

another option was to crate the bike up at factory and launch the crate out of a truck as it arrives in N America, at which point the front frame tubes bend, as bike was secured at the front wheel only.... see Norman White’s book...quickens up the steering very neatly!!
 
MichaelB said:
So, using 750 trees will lessen the trail, thus quicken the steering on an 850?

Have other manufacturers use off set trees?
Click to expand...
Yes. AMC/Matchless did this extensively, sadly without an "ANG" marking.
 
I asked someone who raced and still breathing - the 'fake rake' was introduced as the 750 would go light on the front end when powering out of corners, to over come it they changed it to improve front end stability. Pushing down A roads following a 750 they seem to turn in far quicker and effortlessly than an 850.
 
My guess is that they wanted to alter the rake and trail without changing the wheel base of the bike. Most likely a lot of the factory and shipping logistics was based around a bike of a fixed length, and knowing how short money was back during the Commando production days, re-tooling anything was likely stepping over the line....

Been there done that in our own plant at work...

FWIW
 
Have you ridden it yet with the yokes swapped? If so, did you feel the "why they did it" difference?
 
Derek Wilson said:
My guess is that they wanted to alter the rake and trail without changing the wheel base of the bike. Most likely a lot of the factory and shipping logistics was based around a bike of a fixed length, and knowing how short money was back during the Commando production days, re-tooling anything was likely stepping over the line....

Been there done that in our own plant at work...

FWIW
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And then they retooled nearly everything for the MK3.
Last gasp I guess.

Glen
 
IMO, they used angled yokes because it was the cheapest way to achieve the desired trail,
after they changed the rake.
Rake is less important than trail.

Interesting read:
motochassis.com

Experiments with steering geometry » Motochassis by Tony Foale

© Tony Foale. 2000-2018. (The experiments were conducted around 1982/83.) Rake The author’s ideas mentioned in other articles in relation to the angle of the steering axis (rake) were subsequently put to the test by modifying a readily available standard production machine – a BMW R75/5. There...
motochassis.com motochassis.com
 
Madnorton said:
I asked someone who raced and still breathing - the 'fake rake' was introduced as the 750 would go light on the front end when powering out of corners, to over come it they changed it to improve front end stability. Pushing down A roads following a 750 they seem to turn in far quicker and effortlessly than an 850.
Click to expand...
Yes, the 750 Fastback I raced did that......Coram (as it was in '75) at Snetterton being the most entertaining with the front wheel bouncing off the bumps. I found the most effective solution was to relax your grip on the bars and ignore it!
 
ludwig said:
IMO, they used angled yokes because it was the cheapest way to achieve the desired trail,
after they changed the rake.
Rake is less important than trail.

Interesting read:
motochassis.com

Experiments with steering geometry » Motochassis by Tony Foale

© Tony Foale. 2000-2018. (The experiments were conducted around 1982/83.) Rake The author’s ideas mentioned in other articles in relation to the angle of the steering axis (rake) were subsequently put to the test by modifying a readily available standard production machine – a BMW R75/5. There...
motochassis.com motochassis.com
Click to expand...
But why didn’t they simply change the rake enough to achieve the desired trail? Presumably the required rake increase had underside side effects?

After my test ride today I’m totally convinced that Norton did what they did intentionally as a far more seriously thought out and deliberate intent than I had perhaps given them credit for. But I still do not understand the science behind how the two aspects of fork rake and trail changes interact, which was why I asked the original question.

Nevertheless, even without the desired knowledge, I tested the bike today on a route with a great little stretch for testing purposes, involving a fast stretch of road, with some mild inclined and declined curves with bumps and irregularities in the middle of these curves.

If Glen has a Dyno Hill… this is my Chassis Dyno…

The bike was very noticeably more stable. I did several runs trying different angles of attack and acceleration rates, plus steering damper settings, ending up with it totally backed off. Both during acceleration and sustained speed, even nudging 7,000rpm in top, it was totally stable and confidence inspiring. Previously, although this bike never ‘slapped’ it could be made to feel like it was on the verge of doing so (I know, totally subjective, but I could still ‘feel’ it).

I also fitted new clip ons that give approx 3” more total leverage. I mention this because I could not detect ANY increase in weight / feel to the steering at lower speeds at all. In fact I could not detect any negative effect whatsoever. My wife might suggest this is due to my insensitive nature (and she may have a point), but perhaps the increased leverage of the new clip ons countered any increased weight / force required for steering?

Whatever the science, (which I’d still like to know more of, so I’m currently reading John Robinsons book on chassis design) the clear conclusion from actual testing is that I will be sticking with the offset yokes.
 
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Fast Eddie said:
Whatever the science, (which I’d still like to know more of, so I’m currently reading John Robinsons book on chassis design) the clear conclusion from actual testing is that I will be sticking with the offset yokes.
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Real world testing beats theoretical talk every time in the world I live in.

Are you using stock forks with stock valving, springs, and preload?

Next you'll be putting 32mm Amal Premiers on a stock small port head and saying it's a big improvement over your ported head and the FCRs. Or have you already done that. ;)
 
Schwany said:
Are you using stock forks with stock valving, springs, and preload?

Next you'll be putting 32mm Amal Premiers on a stock small port head and saying it's a big improvement over your ported head and the FCRs. Or have you already done that. ;)
Click to expand...

Not quite stock forks, I have adjustable Maxton cartridges in the stock fork legs and stanchions, with spring rates set for my weight etc. Can’t realistically get much better in that dept and anyway, they’re way better than me !

If I tried 32mm Amal’s on my head (Maney stage 3) they’d drop into the ports …
 
I have CBR600RR Showa 29mm inserts in my 850 front forks and get no handling issues but even with stock dampers my 850 never shook its head. Worst it has ever handled was when I bought it and the TT100 white lined and later when a badly worn rear tyre gave a vague feeling above 50 mph.
 
Fast Eddie said:
Not quite stock forks, I have adjustable Maxton cartridges in the stock fork legs and stanchions, with spring rates set for my weight etc. Can’t realistically get much better in that dept and anyway, they’re way better than me !

If I tried 32mm Amal’s on my head (Maney stage 3) they’d drop into the ports …
Click to expand...
I'll bet your fork setup helps a lot with stability. I have yet to meet anyone that puts in the time you do with your toys that can't ride. I'm sure you can hold your own.
 
Fast Eddie said:
But why didn’t they simply change the rake enough to achieve the desired trail? Presumably the required rake increase had underside side effects?
Click to expand...
Nigel,
I’m no expert but I think by increasing the rake and then reducing the triple tree angle puts the front axle behind the center line of the headstock. Sort of like changing the caster on a car, (think shopping cart wheel). Perhaps that adds to stability? Just spit balling here.
Pete
 
I've never measured or seen posted what the actual trail numbers are but a rough measure is around 3" offset for the top clamp. The bottom clamp bore is obviously offset for the angle change in the same old casting. The trail must be very short on the old bikes so if some say their unstable at speed if you hit the wrong bumps I believe them. I think that yes it was a clever way to get more trail.
Eddie, how much offset do your aftermarket clamps have? On a bike with 27 degree steering head and 17" tires 1.5" offset gives about 4" trail. I haven't drawn up numbers for a bike with 19" tires and it may be a bit different than what i'm thinking
 
Deets55 said:
Nigel,
I’m no expert but I think by increasing the rake and then reducing the triple tree angle puts the front axle behind the center line of the headstock. Sort of like changing the caster on a car, (think shopping cart wheel). Perhaps that adds to stability? Just spit balling here.
Pete
Click to expand...

I’m not sure where you’re measuring in your explanation Pete? Measurements are normally taken on the floor.

Thus, trail is the figure taken from a centre line through the front spindle (on the floor) to the center line from the head stock (on the floor). In most (all?) the wheel spindle centreline is behind the headstock centre line when done like this.

So on my bike the rake remained unchanged (headstock angle not changed), but the fork angle was steepened by 1.5 degree.

What the actual trail measurement was before and after I can’t say, but the front wheel spindle centreline did move backwards just over 18mm when measured on the floor.
 
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edgefinder said:
I've never measured or seen posted what the actual trail numbers are but a rough measure is around 3" offset for the top clamp. The bottom clamp bore is obviously offset for the angle change in the same old casting. The trail must be very short on the old bikes so if some say their unstable at speed if you hit the wrong bumps I believe them. I think that yes it was a clever way to get more trail.
Eddie, how much offset do your aftermarket clamps have? On a bike with 27 degree steering head and 17" tires 1.5" offset gives about 4" trail. I haven't drawn up numbers for a bike with 19" tires and it may be a bit different than what i'm thinking
Click to expand...

Perhaps we need to be clear when we talk offset. There is the ’fork offset‘ ie the measurement between the steering stem and the fork stanchions. Then there are ‘offset yokes’ ie yokes that do not hold the forks parallel to the steering stem / headstock.

I can’t recall the exact fork offset but IIRC the offset of the top yoke is the same in 750 and 850 yokes, it’s only the bottom at differed on the 850s ‘offset yokes’ as it kicks the fork angle back in.

Earlier posts state the 850 kicked out the rake by one degree, and the yoke offset kicked the fork angle back by one degree (edit: corrected by Ludwig as 1.5 degrees).

Theres lots written about rake and trail, but not a lot about changes to fork angle via offset yokes. But fork angle dictated by yokes clearly makes a big difference (in my tests at least).
 
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Trail ( with std 19" wheels)
750 till '70 : 81 mm
750 '71 -'73 : 82 mm
850 : 106 mm
The angle in the lower yoke is 1.5 °
I made one with 1°, because my yokes are set further apart.
I also have an 18" front wheel, which reduces trail.

A difference of 25 mm in trail is huge, which explains the increased stability and slower steering.
 
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