Triple clamps.

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Does anyone know if there a list of dimensions for Commando triple clamps/fork yokes? I was unaware that there was varied offset between models.
 
Mr.Sparks said:
does that mean that all 850 ones are the same offset then?

I'm not entirely sure there has been a definitive answer to that question. With 850 yokes the fork legs are not parallel to the steering axis.
 
Mr.Sparks said:
Does anyone know if there a list of dimensions for Commando triple clamps/fork yokes? I was unaware that there was varied offset between models.

Data researched and computed in 1998.
Posted on various norton forums several times.
center of stem to center of fork tube
each data set is only a single sample.
no data for 68-70
early 750 commando 71-73 only
top offset 2.777"
bottom offset 2.7835"
net angle at axle +.06deg
late 850 "ANG" including MKIII
topoffset 2.851"
bottom offset 2.6895"
net angle at axle -1.342 deg
 
So later 750 and early 850 actual steering angle is 27.06 degrees and later 850 is 26.658, around a half-degree less. Seems like they went to a lot of bother to change something very little. Anyone know what the actual net trail figures are for the two setups? And what might have been their purpose for juggling the net offset, head angle and offset differences? Was it to compensate for the added weight of the 850 engine and electric starter etc?
 
On some makes of bike, when the wheel size is reduced by one inch, the rake is changed by half a degree. The Suzuki RG 250 is an example of that. The change from 18 inch wheels to 17 inch was accompanied by a half degree change in rake. You need to be very careful when changing fork yokes. The amount of rake which changes the handling is fractions of a degree, i.e. changing the amount the rear end squats can change the way the bike handles. Sometimes the situation can become dangerous. The handling depends on the combination of rake and trail. 'Make haste slowly' ! When you make a change to the yoke offset, only ride the bike relatively slowly and take note of how it feels when braking and under acceleration. If it feels stiff or self-steers a lot, you need to recognise you might have a problem.
 
Measurements given in a previous thread: well-that-won-work-anyone-have-ruler-handy-t2047-15.html#p15761

teeb said:
750 #1 Top Offset 2.781” Bottom Offset 2.781” Difference 0.000”
750 #2 Top Offset 2.776” Bottom Offset 2.776” Difference 0.000”
850 #1 Top Offset 2.759” Bottom Offset 2.685” Difference 0.074”

0.074” over 5.75” gives about 0.74 degrees (5.75” is the aprox. spacing between bearings in fork bearing races and where the offset measurements were taken).

Although the 750 and lower 850 offset dimensions basically match dynodave's data (allowing for minor variations in machining) the 850 upper (presumably pre-Mk3) dimension and therefore the angle is less?
 
Apparently the first commando had very quick steering which was changed on subsequent models because a few guys could not cope with it, so crashed. With a commando, you also have possible sideways movement in the isolastics to cope with.
 
I have been thinking about what Ken said about when you increase the yoke offset, you increase the trail. And it sort of doesn't make sense to me. As you increase the offset, you move the axle forward, so the vertical line taken trough the axle to the ground moves forward. It usually intersects with the line taken through the steering head to the ground. So increasing the offset reduces the trail. In any case, on a good handling bike such as a Drixton Aermacchi, the steering head is very vertical, and a lot of yoke offset is used. If the rake is more horizontal such as on a Seeley or a Commando, that much offset creates a major problem. I think it is also possible that in some cases the vertical line taken through the axle and the line through the steering head might not intersect before they reach the ground.
If there is somebody on this forum who better knows the relationship between rake, trail and steering stability, would they please enlighten me ?
 
Early bikes have parallel 2.250" offset yokes & 27 deg. headstock. Using a typical 100/90/19" Avon (26.4 dia.) this gives 4.2" of trail.
If the yoke sizes given by DynoDave are correct the 71 to 73 750 works out at 3.6" trail & the 850 is 4.6" of trail.
As far as I can see trail & steering angle do not seem to influence stability. If they did then why is my Buell Firebolt which has a 21 deg. head angle & 3.3" of trail rock steady at any speed. In a straight line or cranked over when hitting a bump it just doesn't move.
 
In addition to the change in the angle of the stanchion to headstock spindle, there is also a change in the frame angle on the later frames. I guess that the quick steering and slightly twitchy front end didn't suit the long distance cruise type of market that the bike was later being aimed at. Acotrel. Commandos were never made with a smaller wheel, but of course you knew that being a Commando buff
 
Sorry,
I should have said 850 had 28 degree head angle to give 4.6" of trail with the raked yokes.
 
acotrel said:
I have been thinking about what Ken said about when you increase the yoke offset, you increase the trail. And it sort of doesn't make sense to me. As you increase the offset, you move the axle forward, so the vertical line taken trough the axle to the ground moves forward. It usually intersects with the line taken through the steering head to the ground. So increasing the offset reduces the trail. In any case, on a good handling bike such as a Drixton Aermacchi, the steering head is very vertical, and a lot of yoke offset is used. If the rake is more horizontal such as on a Seeley or a Commando, that much offset creates a major problem. I think it is also possible that in some cases the vertical line taken through the axle and the line through the steering head might not intersect before they reach the ground.
If there is somebody on this forum who better knows the relationship between rake, trail and steering stability, would they please enlighten me ?

That's not what I said, Alan. I said that when you decrease the offset, you increase the trail. Just the opposite of the above. Please read more carefully.

This is the actual quote from my post - "Reduced offset in the yokes increases trail, and makes the bike more stable at speed, not less. Maybe you meant something else?"


Ken
 
I have found that less offset when used with 27 degree rake, makes the steering quicker and more towards self-steering than stability. Are you saying that the effect is due to more OR less trail ? When I measure trail, I take a line down the side of the fork leg to the ground, and a vertical through the axle to the ground, then measure that distance. I then subtract the distance between the centre of the steering spindle to the line between the fork leg centres. So in effect increasing the offset decreases the trail. With the Seeley -the more offset, the more stable the bike becomes under brakes, so tipping into a corner is difficult. As you reduce the offset, the steering becomes quicker and the bike tightens it's line coming out of corners under acceleration, however under braking the bike feels less stiff when tipping in. In fact with my bike, all you do is put your mind where you want it to be. - Dangerous for beginners.
What I don't understand is the relationship or correlations between rake and trail and stability on various bikes. The Featherbed Manx has fairly short offset and a very steep head angle. I would have expected more offset. I know most of the replica Featherbed frames use 26 degree rake instead of the standard 24.5 and handle like Suzuki two-strokes when the normal offset is used.
 
acotrel said:
I have found that less offset when used with 27 degree rake, makes the steering quicker and more towards self-steering than stability. Are you saying that the effect is due to more OR less trail ? When I measure trail, I take a line down the side of the fork leg to the ground, and a vertical through the axle to the ground, then measure that distance. I then subtract the distance between the centre of the steering spindle to the line between the fork leg centres. So in effect increasing the offset decreases the trail. With the Seeley -the more offset, the more stable the bike becomes under brakes, so tipping into a corner is difficult. As you reduce the offset, the steering becomes quicker and the bike tightens it's line coming out of corners under acceleration, however under braking the bike feels less stiff when tipping in. In fact with my bike, all you do is put your mind where you want it to be. - Dangerous for beginners.
What I don't understand is the relationship or correlations between rake and trail and stability on various bikes. The Featherbed Manx has fairly short offset and a very steep head angle. I would have expected more offset. I know most of the replica Featherbed frames use 26 degree rake instead of the standard 24.5 and handle like Suzuki two-strokes when the normal offset is used.

What I'm saying, Alan, is that the geometry of trail is really pretty simple. With conventional forks, all you need to calculate trail is the steering head angle, the offset in the yokes, and the diameter of the tire. Nothing else counts. Decreasing the fork offset moves the contact patch on the tire rearward, but the line through the steering head still intersects the ground at the same place. That increases trail. More trail means more caster effect, increasing stability at speed. It also means more force is required for the counter steer effort to initiate a turn, hence shorter trail makes the bike seem easier to flick into a corner.

Your concern seems to be more about how steering geometry affects oversteer or understeer entering and leaving corners. That depends on a lot more than just the amount of trail. Things like front and rear spring rates. tire contours, front/rear weight ratios, location of CG, swing arm angle and length, etc., all play a part. The best book I've seen on this subject is Tony Foale's "Motorcycle Handling and Chassis Design". If you're really interested in the subject, I suggest you get yourself a copy. It's great reading.

Ken

Ken
 
I have read a bit of Tony Foale's stuff. He mentioned that he had a bike jigged up to make the rake and trail adjustable. And that the two extremes are stable and self steering. Have you actually increased the trail on a bike and found it to become more stable ? When I reduced the offset on my Seeley, it moved towards self-steering. Perhaps when I did that I actually REDUCED the trail ? I've had this conversation previously with a guy who reduced the offset on his Ducati to get it to steer quicker. And I felt then that I might have got the relationship between trail and stability confused.
The effect of changing the spring rate at the rear of the bike, changes the amount the bike squats and thus the way the rake changes. I've found that my bike steers on the throttle. I also found that with a near-standard Manx Norton - feels much more positive if gassed when cranked over. I wasn't aware that the tyre size changes the trail much except when the total wheel OD is changed. What really interests me is the allowable margin of error. Fractions of change seem to have a major and sometimes dangerous effect. Another thing which affects the steering is the length of the bike and the weight distribution. That is probably why a Triton is always inferior to a Manx. Most Tritons seem to feel vague in corners where the Manx is positive.
 
acotrel said:
When I reduced the offset on my Seeley, it moved towards self-steering. Perhaps when I did that I actually REDUCED the trail ?

Just draw yourself a picture, Alan. It is really obvious that reducing the yoke offset increases trail. It's very simple geometry. Reducing offset increases trail, and increasing offset reduces trail, assuming nothing else is changed. You're moving the tire rearwards, and the intersection of the steering axis with the ground isn't changed. How could that possibly be seen to reduce trail?

Ken
 
acotrel said:
Have you actually increased the trail on a bike and found it to become more stable ?

I suppose it might depend on what one means by stable. I tend to use it to mean the ability to hold a steady line through high speed sweepers. It may mean something different to you.

Yes, I have increased and decreased the trail on a race bike and observed the differences. I've had two race bikes with adjustable offset yokes, my Commando PR with Spondon yokes with spacer inserts, and my Ron Wood Rotax with Ohlins front end with interchangeable inserts in the yokes to change offset, and I experimented quite a bit with both on the track. Less trail does make the bike feel easier to turn into a corner, but if you go too far, you will feel the difference on high speed sweepers, particularly bumpy ones. In both cases I settled on an offset that gave me stability in high speed corners, and a neutral feeling in most corners, i.e. not pushing to the outside or diving to the inside under throttle changes. That setup doesn't necessarily let you flick the bike into tight corners quite as quickly, but I never found that to be a big issue. But then I wasn't exactly a Motogp level rider. I understand the top guys like the bikes to be right on the edge of instability to get them turned as quickly as possible, and they just deal with the occasional wobble.

Once you've got the basic geometry (trail, weight balance, etc.) set up to suit your riding style, you can make minor adjustments to suit different tracks by raising and lowering front and rear ride heights slightly, or changing pre-load and damping settings, if you have that option. I never really did much of that. When I got the bike set up the way I liked it, I didn't often change anything to suit different tracks except gearing, and sometimes jetting. After I retired from racing, and had some really fast guys ride my bikes, I noticed that they were much more sensitive to small changes, and we regularly made small suspension adjustments at the track. They pushed the bikes a lot harder than I had, and could feel the differences from small changes that I would never have noticed.

Ken

Ken
 
I take your point about doing a drawing, however I think it would have to be exactly to scale. My problem is that I did not measure the trail before I changed to the reduced offset fork yokes. By stable, what I mean is the tendency to stay upright when braking into corners and run wide when coming out. By self-steering, I mean the very quick turn into corners and tightening the bikes line under power when coming out. I haven't noticed the bike becoming loose or feeling unstable on the one high-speed sweeper on Winton Raceway. However on a bumpy circuit, it might be a problem. The main thing I've found is that on the one very tight hairpin bend, I can brake to about one-third of the way into the corner, then get straight back onto the gas. The corner is a right-hander and if I don't consciously steer, the bike ends up on the right side of the track of it's own volition. What it means is I can use much more power coming through open bends and coming out of the tight ones. I'm still waiting for the bike to mishandle and grab me by the throat. But one thing I've also noticed is that I seem to use less lean in corners when the bike is being powered through them. The bike is vastly different to my old Triton, where the fight was always to keep it on the bitumen when coming out of corners.
I get confused about this effect because there seems to be two major configurations used on production bikes with no happy medium. Some races have very steep rake with very large offset. Others use a much shallower rake with very short offset. If you try to draw conclusions about what will happen by changing the offset, you can be SO wrong. I think the Garden-gate Manx was an example of that. What puzzles me is that Cromie McCandless got it so right when he made the Featherbed frame. It is obviously the combination between rake and trail and wheelbase which affects the handling, however there seems to be no continuous algorithm which expresses the relationship.
I probably have a strange viewpoint about all of this because when I was racing regularly years ago my 500cc Triton was a real bastard, it had minimal torque (all top-end HP) and was very much under capacity, so when I raced it, I had to outride the opposition and handling was at a premium. I'm very conscious about what the bike is actually doing.
 
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