Making fork yokes

While the bike sits, and I have the time (currently deployed overseas) I plan to design my own triple trees that I would like to make when I return. I am a machinist on the civilian side, and use CAM everyday. I was able to obtain a temporary solidworks license while Im here. I have limited solidworks experience, but enough I should be ok.

Now, whilst looking among another thread I noticed offsets, spacing, and rake angles of 750/850/featherbeds. The bike is a 73 850 and I am looking to produce two sets, one for clip ons, and one for handlebar mounts. Anything ideas I can get on ways I can improve the yokes to have a more agile bike would be appreciated. I did notice Doug MacRae had a different set on his race bike, so I though I would start in that direction.

Things I will be needing since I dont have access to my bike are offset, spacing, and rake angle. Any of which you think could be improved, the more the better. I also plan on keeping the norton forks, no wild sport bike conversion or anything.

currently how she sits, missing more parts I think though

The Alloy yokes from RGM only weight like .5 less than the factory steel ones so may take a cpmputer to create a CNC possible billet yoke that up to the task. Here's another way to go or savor. 750's 26' rake angle are considered more handy than 850's 27' and that's how its struck me too.

fork-sheet-metal-t13859.html?hilit=sheet%20metal%20yoke

Food for thought...

not normally in to billet stuff on Commando's but that set of yokes looks very nice,
careful though, make a nice set and everyone will want a set,

Check out Tony Foale's website. He has some free apps including one for calculating offtset-trail-rake-wheel diameter. His books are great reads. Good articles too on the site, especially a few on frame design.
Michael Moore's site has a lot of articles regarding design and fabrication.
MC-Chassis-Design is a mailing list on motorcycle design and fabrication. Check it out.

January '74 test in CycleWorld has 28deg rake and 4.36" trail on 4.10 tires listed for the 850. Spacing is 7.375".


http://www.tonyfoale.com/

http://www.tonyfoale.com/progs/castor1.exe

http://www.eurospares.com/

http://micapeak.com/mailman/listinfo/mc-chassis-design

Adjustable rake maybe?
http://www.arttbikepartts.com/rakedtrip ... tural.html

I suggest you should be very careful when making your own fork yokes. Very small changes to the steering geometry have a large effect. It is easy to make something which might chuck you over the handle bars. I suggest the first thing to do is measure the head angle, then note the wheel diameter, and set the offset to something sensible. Tony Foal's book should help. If you get the offset wrong the bike can stand up under braking and turn and throw you off-balance. Or it might oversteer under power when laid over. The latter is preferable, even desirable.
A good benchmark is the 70s Yamaha two strokes, which have 18 inch wheels, 26 degree head angle and 1.5 inch offset - the handling is neutral. If you increase the head angle or the trail, the steering becomes more stable, however other things also happen.
To measure the head angle, it is possible to buy magnetic base protractor from a tool shop, and simply stick it to the fork tube, and read off the angle when the bike is off the stand.
My own bike has one more degree head angle than the 70s Yamaha with all other things the same, and it oversteers under power when laid over, the difference in head angle is critical. Tyre diameters can also affect the result.
If you do this exercise you should be very careful when riding the bike for the first time after the mod, and take note how the bike feels under brakes and when accelerating out of corners.

Hi, I have in front of me a Drawing for the norton commando frame. The steering head angle is 62 degrees for the 850, and 63 for the 750. The yokes are different for each frame. The book says do not put 750 yokes on the 850 frame. I had considered some alloy ones myself. Bit too much agro at the moment. Best to buy a set from someone like RGM, or Norvil. regards, Paul

I have toyed with making some alternative yokes for my 1968 750 to try to cure the instability at higher speeds. The problem is the more I look into it, the more I realise just how complex a subject it is. All the 750 models have a 27 degree head angle or 63 degrees if you prefer. The yokes up to 1971 have the same offset as 650ss & Atlas models at 2.250" parallel. This gives 3.95" of trail with a 90/90/19" tyre. The 850 frame is as previously stated 28 degrees but the offset measured on the top yoke is 3". The yokes are however raked something in the region of -1 degree . Some one else may know the exact figure. These bikes are very stable due to the extra trail but not as lively and nimble as the early 750. Nobody has ever been able to tell me why yokes are raked.
Another example is an oil in frame Triumph twin. 28 deg. frame, 19" wheel, but only about 1.625" yoke offset. These yokes are also raked at -0.48 deg.
Any experts out there?

Would it not be easier to alter the trail by simply changing the yoke offset?

I suggest that introducing more trail by angling the staunchions in the yokes or shifting the axle forward at the bottoms of the sliders, is not the way to go. If the stanchions are parallel with the steering head you reduce the number of variables to the offset, and the steering head angle and wheel and tyre size. There is a lot of information available about what works to give neutral steering, and you can improve from there . (All you need to do is find a bike of about the same specs as you own, which handles well, and measure it.) I would be very careful changing the yokes on a bike which has its handling affected by isolastics , and the head steady. Even half degree change of head angle has an effect. You will note that with the wheel size on the RGV250 Suzuki reduced from 18 inch to 17 inch, the head angle has been reduced from 26 to 25.5 degrees . Perhaps you should stock up on bandages ?

Hi, again folks, I did a lot of drawings for these permutations some years ago when I was making Leading Link Forks. On Commandos,the frame is not a fixed entity, it is a compromise because of the Isolastics. Assuming the frame is straight, and the wheels in line, then reducing the Trail ( steeper head angle ), will turn in faster, ie. fall into the corner. This is the principle of modern large Japanese bikes, which leads to counter-steer, ie. force the bars in the opposite direction to force the tyre to distort to increase the contact area. Nortons only fitted narrow rounded profile compared to todays standard tyres, therefore around 3.75 to 4" trail gives stable handling, especially when the frame has little torsional rigidity. Make the Isolastics solid, then one can fine tune, but the other problem is the frame breaks due to vibration. Its all a compromise. Nortons manufacturing tolerances were pretty awful too, at the end, just like the BSA group were. The frame dimensions are in the later Workshop Manual. All food for thought. Paul

More in this post rake-and-trail-specs-t12097.html

Matchless said:

Would it not be easier to alter the trail by simply changing the yoke offset?

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Could be something as simple as economics. They probably would have had to make up a new casting to handle the necessary offset change but they got away with the old casting by angling the stanchions.

In that link to the other topic on this subject a contributor mentioned that the trail on his 750 commando was correct to the drawing at 90mm. I don't know where the figure of 3 plus inches that you guys mention comes from for trail measurements. My understanding is that the trail is measured by extending a line through the steering head axis to the ground (parallel to the staunchions), and another vertically through the axle to the ground. Are you guys forgetting to subtract the 'forward offset' when you measure the trail ? The trail on my Seeley is 72mm, the head angle is 27 degrees, and with 18 inch wheels it oversteers under power when laid over. The forward offset is 1.4 inch. That is good for racing with skinny tyres, but too quick for road use, and it would be dangerous with isolastics. 90mm of trail is a common measurement on modern racing machines with fat rear tyres, and the head angles are steeper, so the bikes are also less stable due to that to compensate. I believe the standard commando is fairly slow steering, and safe for the average road user. At one stage Peter Williams built some with racing geometry and a few guys in the UK got chucked up the bitumen when they rode over the 'cats eyes' in the middle of their roads . Apparently an experienced rider would have simply ridden through the problem, however beginners hang on too tight . Please be careful when you are playing with this stuff.

he forward offset is 1.4 inch. That is good for racing with skinny tyres, but too quick for road use, and it would be dangerous with isolastics. 90mm of trail is a common measurement on modern racing machines with fat rear tyres, and the head angles are steeper, so the bikes are also less stable due to that to compensate.

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Alan, this is insighful for me to know, amoung other things like your power steering Seeley. My whole cycle hobby revolves around tire breaking free and I swear the hardest I've ever had to press on them to get free was 120x 18". Can't wait to feel that size in race compound. Cycles like musical instruments where everything affects everything else so mostly trial and error. I have great distain now for the sharp over steering moderns walking an inch around their ~1.5" wide contact patch on their ballon tires when the going get tough.

Correct geometry yokes already exist in steel so 2 reasons to fab up your own, just because ya can with an art flair to it or to save some mass in which case its risky to do it in alloy not beefy enough in the right places to avoid fatigue fracture failure.

I was disappointed by the RGM yokes as only .5 lb less than factory, which IIRC is about 7 lb or almost a hp worth of acceleration mass. RGM yoke mass savings cost like $500 a lb compared to other more efficient cost/lb efforts and spending.

Vulin - Sorry to get off topic a bit, but do you have a sound clip of that exhaust arrangement? I'd love to hear it!

Ben

Hobot, we have some really smart fellas doing tricky things to historic bikes here in Australia. I saw a rider crash in an historic race when ther yokes on his bike simply broke. What they had done was use the original casting as a pattern, and taken alumininium items off it. The originals were steel so when the aluminium ones were made the section was not big enough. After his crash, they simply cut the cables and took the bike away in two halves. Years ago a friend of mine was killed when the discs exploded off his RG500 Suzuki under brakes at Bathurst. The factory originals were chrome plated aluminium, and the chrome used to spall off them. So his father arranged to have the discs made out of cast iron. He used the brakes off a fork lift as the basis, they were grey cast iron. The guys are not good on materials. The discs they made might have been successful if meehanite had ben used, some grades of cast iron also have nickel in them and are very strong . The guys who made those discs now live with the guilt.

I suspect that modern bikes don't handle very well with those fat back tyres. They seem to be using 'point and squirt' methods of racing. With our vintage race bikes which have much lower horsepower, it is essential to outride the opposition in the corners . I believe that modern bikes are used on the principle of get it upright then use the power in a straight line. This stuff about moving body weight to get the bike to steer - you do that on the older bikes if they don't handle. I found the steering setup for my seeley by sheer arse. I knew what worked on the older Yamaha two strokes, and figured the one degree difference in head angle would be bearable. It turned out to be excellent.

I captured this bit of video off our local TV this afternoon. It is relevant to what I've said about the handling of modern bikes compared with lower powered older ones :

http://youtu.be/2GwGFQnhs-I