Fork Yokes (Triple Trees)

Both Andover Norton and Norvil mention on their websites that early Commando bottom yoke 060344 will fit featherbed frames but they don't say that the offset is the same. I would suggest that a forum member who owns a pre 1971 Commando measures the offset.

OK I Have a Early 69 fast back, a 70 S and a 66 650SS Where do you want the maesurements taken from?
Al

possm said:

OK I Have a Early 69 fast back, a 70 S and a 66 650SS Where do you want the maesurements taken from?
Al

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Good question Al, I asked the same in an earlier post on this thread.
I would assume you measure center to center, so from the center of the yoke spindle to the center line of the stanchion holes. Don't worry it doesn't need to be too accurate, it's just for a comparison.
Many Thanks for you help

Webby

Very hard to measure with the yoke fitted to the bike but measure i get is 2 1/4" on all 3.
Al

possm said:

Very hard to measure with the yoke fitted to the bike but measure i get is 2 1/4" on all 3.
Al

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Thanks for taking the time and having a look Al
2 1/4" sounds correct, it must be the 850s only that are fitted with 2 13/16", even with them being hard to measure I'm sure you would have noticed a difference like that.

So I'm looking for a pair of Featherbed or 750 Commando Yokes :mrgreen:

Thanks again

Webby

Well, I've bitten the bullet and bought a set of 750 Commando yokes off our old mate and forum member Tim ( username: kraakevik) It's amazing the price of some secondhand parts (and bikes) over here in Europe, even with the postage from the US to France Tim's triple tree's were still cheaper than anything I could find on flea bay UK!
So I'll have a good measure when I receive them and put an end to this thread

Webby

Some yokes also have an inbuilt castor angle, which is the difference between the fork stanchion angle, and that of the steering stem. A change to this was made to Commandos in 1972, after it was found that works race bikes with this mod lapped the IOM 1/2 minute faster.

Carbonfibre said:

Some yokes also have an inbuilt castor angle, which is the difference between the fork stanchion angle, and that of the steering stem. A change to this was made to Commandos in 1972, after it was found that works race bikes with this mod lapped the IOM 1/2 minute faster.

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Do you know what the different angles were Carbonfibre?
I've got 1°difference in my head for some reason, I've no idea where that came from?

Webby

No idea, but the mod was mentioned by Peter Williams in an article published in this months "Classic Bike Guide". There is a follow up to this article next month, and I would think more info will be provided then.

For anyone seriously interested in bike chassis design its well worth getting hold of a copy of Tony Foales excellent book on the subject, which covers almost everything.

Thanks for the info Carbon,
I've only ever bought Classic Bike Guide once last year when I was in the UK, it seemed pretty good. Do you think it's worth the price of the subscription? I might consider it, at the moment the only mag I subscribe to is Classic Bike.

Webby

Classic Bike Guide wasnt very much good up to a couple of issues ago, but now it is very good and well worth a subscription. Not quite up to the standard of "Classic Bike" but pretty good effort bearing in mind that most Mortons titles dont have any staff writers as such, whereas CB does have good writers on the staff.

Thanks Carbon,
I had a look on their website (something that Classic Bike doe's not have!) and it looks pretty good.
Especially, the subscription of £52 delivered to France seems good value.

Webby

The two important features of a motorcycle front end are "rake" and "trail". If you put have a laser beam going through the center of the steering head it will make a spot on the ground a certain distance in front of the tire contact patch, this certain distance is the trail.

The more trail you have, or the more the steering head leads the tire patch, the more stable the bike is at high speed, but will also take more effort and time to steer. So motorcycle chassis designers try to juggle the angle of the steering head and the fork tubes in relation to the steering head to get the best handling characteristics for the intended use of the bike.

You can see that if you increase the angle the steering head kicks out the front forks then it puts the laser pointer further ahead of the tire patch for more stability. If you decrease the rake it pulls the laser point back towards the tire patch until you get the the point where they are vertical, the laser point and the tire patch are centered on each other and there is no stability at all.

You can alter the trail and keep the same head angle by having different "offset" in the lower and upper fork clamps.

So for whatever reason they changed the head angle on the 1973 and later Commandos, they had to alter the fork clamps to keep the trail at a certain dimension for stability. This is why if you mix parts from 72' and earlier with fork parts from 73'-on Commando forks, you can get into some real trouble because you are going to drastically increase or decrease the amount of trail the chassis has outside the range where it is either safe and stable or performing well.

The simplest day-to-day example of this in use is on the wheels of your shopping cart at the grocery store. You can see that though the pivot of the carts front wheels is vertical, it is offset backwards so that the vertical line through the pivot is in front of the wheels contact patch, so when you push the cart forwards the wheel for the most part steers in the direction in which you push. If the wheel had no offset backwards so it was directly under the pivot, then the wheel would stay turned or sideways when you changed direction and you would have to skid it where you wanted to go leaving black marks on the floor.

Hi Beng,
I'm well aware of the the effects on handling with regards to trail and rake.
As I mentioned in one of my earlier posts, when i receive the yokes then all the measurements can start, if the yokes have the incorrect offset or trail when compared to standard featherbed specs they will be sold or put on the shelf for another project. I decided to take the plunge with these as a) I want to know the definite final answer as to weather early Commando and Featherbed Yokes are interchangeable and b) They were cheap enough to take the risk and try.
So in a week or two, I will have learned something and possibly have a set of yokes for my bike

Webby

Pretty much all modern sports bikes are running 24 degree head angles, and I would imagine an old Brit is going to be about 28-29 degrees, with maybe 1.5 degrees in the fork yokes to increase stability, and possibly to provide some self centering effect?

Steeper angles provide much sharper handling, but maybe not that great an idea on something like a Commando, and certainly not a modification that everyone is going to like in any case.

Hi Carbon,
You're right about the rake, a modern Commando is running a rake of 24.5° and a trail of 3.9".
All I need to find now are the specs for a featherbed Norton, I've just spent two hours searching the web and have found nothing. The next step is my stack of old bike mags.

Webby

Using early yokes on late frames will not be catastrophic, you could do just as much change just by switching to different tires in the front or the rear :roll:

Jean

Webby03 said:

Hi Carbon,
You're right about the rake, a modern Commando is running a rake of 24.5° and a trail of 3.9".
All I need to find now are the specs for a featherbed Norton, I've just spent two hours searching the web and have found nothing. The next step is my stack of old bike mags.

Webby

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You need to check whether your yokes have any castor effect designed into them, as it was very common to use something like 1.5 degrees here. One of the helpful effects of steeper head angles is that weight on the front tyre is increased, and this provides better handling. However altering the geometry is not something that should be undertaken lightly, and on an older bike is really going to need modern tyres, suspension, and brakes to provide the best results.

Jeandr said:

Using early yokes on late frames will not be catastrophic, you could do just as much change just by switching to different tires in the front or the rear :roll:

Jean

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I don't think there would be enough difference between yokes that you would really notice, you would need a drastic change in castor angle to make any significant difference. I've just been reading an article about a modified Bonneville, the guy who built it reduced the rake to 24° to run modern tires and suspension, although to compensate he also lengthened the swingarm.

I found the specs for my 94 Speed Triple, it has a huge (for a modern bike) rake of 27°, I believe this is due to the fact that the bike is top heavy, the increased rake would add to the stability.

Now, to continue looking for those featherbed specs

Webby

Carbonfibre said:

Webby03 said:

Hi Carbon,
You're right about the rake, a modern Commando is running a rake of 24.5° and a trail of 3.9".
All I need to find now are the specs for a featherbed Norton, I've just spent two hours searching the web and have found nothing. The next step is my stack of old bike mags.

Webby

Click to expand...

You need to check whether your yokes have any castor effect designed into them, as it was very common to use something like 1.5 degrees here. One of the helpful effects of steeper head angles is that weight on the front tyre is increased, and this provides better handling. However altering the geometry is not something that should be undertaken lightly, and on an older bike is really going to need modern tyres, suspension, and brakes to provide the best results.

Click to expand...

That's the plan Carbon!
I will be running modern tyres and upgraded suspension, however you don't want to decrease the rake to the point where the bike turns into a tank slapping monster. As part of my research I'm also going to dig out an pair of OIF Bonnie Yokes I've got in my spares box and measure those up, I know the offset is less than a Norton but I've seen them fitted to Tritons in the past.

Webby