750 Triple Tree

Fast Eddie said:

When you steepen the rake (that’s the headstock angle) you bring in the fork angle as well, and you therefore close the gap between the imaginary point on the floor taking through the headstock to the imaginary line on the floor taken vertically through the wheel spindle ie you reduce the trail.

But when you change the angle of the forks, via offset yokes, you leave the imaginary headstock point where it is, and move the wheel spindle line backwards, thereby increasing the gap ie increasing the trail.

So even though what you are doing looks almost the same, it’s having the opposite effect.

And it’s creating a different rake / trail relationship that would otherwise be possible.

So, when Norton extended the rake AND offset the yoke, WTF were they doing… and why?

Some believe that the offset yokes were a botch, a cheap way of the factory overcoming rake issues etc.

I believe the opposite, I tend to think that the seemingly contradictory combination of the extended rake and offset yoke was a deliberate move, undertaken with deliberate effects in mind, by people who knew what they were doing.

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...and what was their deliberate intent - and did they achieve it?

When they changed the head stock angle from 27 deg to 28 deg as we all know they changed the yoke offset. I think the reason they angled the yokes was to keep the wheelbase the same as previously.

robs ss said:

...and what was their deliberate intent - and did they achieve it?

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That is what I’d like to know

Matchless said:

When they changed the head stock angle from 27 deg to 28 deg as we all know they changed the yoke offset. I think the reason they angled the yokes was to keep the wheelbase the same as previously.

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The handling depends on the combination of rake and trail. Another thing is weight distribultion. If the motor is further forward, it takes more go to change the rake on the steering head to turn the bike. The Manx motor is further forward in a featherbed frame, than a Commado motor is in it's frame. A featherbed Domiracer would not handle as well as a Manx. The motor is further back.
The wheelbase of the bike is of little consequence. I have been told a shorter wheelbase bike handles differently from a long wheel base, but usually the steering geometry is different anyway.
My mate used to own the 250cc Aprilia production racer. It handled perfectly. However he could be on it in a race, going like the wind, and suddenly find himself on the deck and wondering how he got there.
The same thing p[robably applied to the first Commandos. If you lose the front end by riding over one of the cat's eyes, a road race rider would probably recover without thinking. But a novice might not.
Most motorcycle road riders and car drivers have probably never experienced their vehicle going beserk. With motorcycles, a hydraul;ic steering damper usually stops it.
When I race, I know what can happen. With that one, you need to get your hands off the bars very quickly and wait for the bike to straighten it self out. Then put youn open hands on the bars before you grab them again. If they are moving the slightest bit when you grab them, you will probably be launched. I am still carrying an injury from that.
I think with my Seeley, the Commando 850 motor might be better than the lighter Matchless G50. When I accelerate, the rear squats and the front lifts - that makes the bike oversteer in the correct direction, while staying more upright. With a lighter motor. it might happen too quickly.

If you look at a Manx - the rake is 24.5 degrees and is has 2.25 inch yoke offset. With 19 inch wheels, it oversteers when you accelerate, enough to give the rider confidence. My Seeley has a rake of 27 degrees, and 53mm yoke offset with 18 inch wheels. It oversteers a lot and stays more vertical, when I am on a lean and accelerate. A Manx does not usually lean much. My Seeley is pretty silly, it should not handle like that. When I use it, it always feels un-natural to ride it the way I do. The thing about it is, when I was riding it normally, I did not suspect I could use its handling to power through corners. Then I got into a corner too hot and had to turn it on halfway around the corner., to get around. It steered itself out of the corner.
I do not know how Colin Seeley got it right - certainly was not through theory/

acotrel said:

If you look at a Manx - the rake is 24.5 degrees and is has 2.25 inch yoke offset. With 19 inch wheels, it oversteers when you accelerate, enough to give the rider confidence. My Seeley has a rake of 27 degrees, and 53mm yoke offset with 18 inch wheels. It oversteers a lot and stays more vertical, when I am on a lean and accelerate. A Manx does not usually lean much. My Seeley is pretty silly, it should not handle like that. When I use it, it always feels un-natural to ride it the way I do. The thing about it is, when I was riding it normally, I did not suspect I could use its handling to power through corners. Then I got into a corner too hot and had to turn it on halfway around the corner., to get around. It steered itself out of the corner.
I do not know how Colin Seeley got it right - certainly was not through theory/

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Well I'm glad we've finally got it all sorted out !!!!!!

Fast Eddie said:

I agree.

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And now I agree. Both Kenny and Matt state there's are parallel. My "evidence" was the Urban Legend belief that a parallel tree on an 850 was dangerous when the opposite was true. So, parallel trees fit both and should quicken the steering of an 850.

Deckard said:

And now I agree. Both Kenny and Matt state there's are parallel. My "evidence" was the Urban Legend belief that a parallel tree on an 850 was dangerous when the opposite was true. So, parallel trees fit both and should quicken the steering of an 850.

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I'd always heard that you shouldn't the 850 type onto a 750
But 750 yokes on an 850 frame were ok

Deckard said:

From Matt. Thorough as usual:

My triple trees are parallel to the steering neck. So they do not have the negative rake like the stock trees had. Both 750 and 850 trees have a rake.

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The way I read Matt's reply is, he claims Commando 750 yokes (061915 and 061916) have a negative rake (angled vs. the steering stem).

I have a set of Commando 750 yokes here. A (negative) rake would necessarily produce different values for offset at each yoke.

I measured offset by tracing the best I could (no CMM available here) and found them to be approx. 71.5mm for each yoke, very close to the design specification of 2-13/16" (71.44mm) .

IF there is a negative rake for these, it's extremely small and of no significance. It is unlikely NV would have designed a small negative rake whose effect would not be decernible.

Hence, I conclude mentioned 750 yokes have no negative rake, i.e., stanchion bores and steering stem are parallell.

This comes as no surprise due to the fact they replaced the old style yokes inherited from the Dominator/G15 models. The old style yokes had no negative rake.

- Knut

Deckard said:

And now I agree. Both Kenny and Matt state there's are parallel. My "evidence" was the Urban Legend belief that a parallel tree on an 850 was dangerous when the opposite was true. So, parallel trees fit both and should quicken the steering of an 850.

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The interesting conclusion is, what is the resulting trail using a certain 19" or 18" tyre. Why is this important issue not stated by these manufacturers for a bike in stock trim?
Furthermore, are these yokes road legal?

- Knut

mdt-son said:

The way I read Matt's reply is, he claims Commando 750 yokes (061915 and 061916) have a negative rake (angled vs. the steering stem). I have a set of Commando 750 yokes here.

A (negative) rake would necessarily produce different values for offset at each yoke.

I measured offset by tracing the best I could (no CMM available here) and found them to be approx. 71.5mm for each yoke, very close to the design specification of 2-13/16" (71.44mm) .

IF there is a negative rake for these, it's extremely small and of no significance. It is unlikely NV would have designed a small negative rake whose effect would not be decernible.

Hence, I conclude mentioned 750 yokes have no negative rake, i.e., stanchion bores and steering stem are parallell.

- Knut

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Knut,

You are right of course. I bundled the trees together and should not have. They are parallel on the 750, negative rake on the 850. Sorry for the confusion.

That said, I have used my (parallel) triple trees on both the 750 and 850 frames with different steering neck angles since 2002 and I found that the combination works really well with either frame.

Matt

acotrel said:

Increased trail makes the bike more nimble, reduced trail makes it more stable.

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I think you have that backwards. Take it to an extreme, say a two foot trail: you'd have a hard time steering at all fighting the self correcting force. I've ridden a bike with negative trail; it wasn't fun. It felt like one wrong move and the bars would hit the tank. I told the guy to fix it before he killed himself. He actually had the whole thing apart (a Triumph hardtail) and he rechromed the frame without fixing the geometry. You can't fix stupid.

and you can’t educate pork either

Does anyone know of any other OEM that angled the fork trees?

MichaelB said:

Does anyone know of any other OEM that angled the fork trees?

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It may not count, but AMC (Matchless) did - extensively - causing a real mess to restorers.

- Knut

Interesting how it's in the same family.

Triumph T140 yokes are angled.

MichaelB said:

Does anyone know of any other OEM that angled the fork trees?

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A lot of manufacturers (triumph, Norton etc) made raked yokes for sidecar use
But the only de raked I've ever heard of on a production bike was the 850 commando?

Matchless said:

Triumph T140 yokes are angled.

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I don't think they are ?

baz said:

I'd always heard that you shouldn't the 850 type onto a 750
But 750 yokes on an 850 frame were ok

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That advice came from the NOC Commando notes, but Andover Norton list the 850 yokes in the 750 1972 parts listing so they must know differently.