Making fork yokes

lcrken said:

worntorn said:

When building the frame for my modified Egli I copied the steering geometry of the Manx Norton racer, as Did Fritz Egli and Terry Prince when they built the original Eglis. The manx is a bike that is known for it's high speed handling and even today can hold it's own against many modern bikes on handling. In the Manx you have a bike that, with a capable rider, can enter most turns at speeds well above anything road legal.
I did a bit of research on Egli and Egli replica frames and found steering head angles ranging from 22 to 27 degrees. I settled on 27, not very radical, but as mentioned, the same as a Manx.

I believe our Commandos have that same steering angle, although there was a slight change somewhere along the way, perhaps Commando experts can chime in. It might be that the setup is pretty well ideal for road uses as is.

Glen

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750 Commando frames had 27 degree steering head angle (rake), and 850 frames were 28 degrees. The 850 used unequal offset yokes to keep the trail measurement the same (approximately) as for the 750.

FWIW, when I was still racing Commandos, I experimented with rakes as steep as 24.5 degrees by cutting and re-welding in the headstock area. I used proportionately less offset in the yokes to keep the trail in the right range. At 24.5 degrees, the bike turned in very easily, but had a scary tendency to highside when pushed hard. I settled on 25.5 to 26 degrees as feeling the most neutral, with trail set to what felt right to me using adjustable Spondon yokes. That, with slightly longer rear shocks and a slightly longer swingarm seemed to work pretty well. That was back in the '80s with 18" racing slicks. When I started racing a Commando in the early '70s with a stock PR, I was very happy with how it handled with the 19" Dunlops, 3.60 front and 4.10 rear, and later with the KR series triangular race tires. I didn't feel a need to experiment with the stock geometry until the switch to more modern profile 18" race tires. I was really happy with the Commandos' performance on the track through the '70s, '80s, and early '90s, but technology has continued to move on since then. Builders like Herb Becker pursued improvements to Commando chassis performance way past what I did, and if I were building a Commando racer now I'd mostly copy Herb's latest designs.

This is a shot of the Spondon adjustable yokes I used on the PR to experiment with trail. The forks are 40 mm Cerianis.

Maybe a little OT here, and sometimes I get carried away with the good old days, so let's not get into another argument here over whether Commandos are suitable race bikes. I don't claim my setup was better than anyone else's, just that it worked well for me.

Ken

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I actually love the old racing stories. Any past experience and knowledge I cant get out of anyone is great. I ideally would like to make the bike trackable (still street ridden, but a track weekend here or there) I am not looking to be competitive, or impress but to have fun with an old bike Maybe a bit of stretch?

icrken,
'At 24.5 degrees, the bike turned in very easily, but had a scary tendency to highside when pushed hard. I settled on 25.5 to 26 degrees as feeling the most neutral, with trail set to what felt right to me using adjustable Spondon yokes.'

How much offset were you using then you settled on the 26 deg. rake ? I'm using about 1.4 inch, but the photo you've posted looks like the yokes have slightly more offset, so the steering might be slightly more stable. I know that feeling that the bike might step out and high side me. I think if my bike had ten more BHP or a rougher power delivery , it could be a real problem. The worst experience I had was w hen I used t oo much offset, and as I braked out of a fast left hand sweeping bend during a race, the bike stood up and turned right and threw me off-balance. I only survived by turning the gas on again when trying t o crash it in a soft place .

acotrel said:

icrken,
'At 24.5 degrees, the bike turned in very easily, but had a scary tendency to highside when pushed hard. I settled on 25.5 to 26 degrees as feeling the most neutral, with trail set to what felt right to me using adjustable Spondon yokes.'

How much offset were you using then you settled on the 26 deg. rake ? I'm using about 1.4 inch, but the photo you've posted looks like the yokes have slightly more offset, so the steering might be slightly more stable. I know that feeling that the bike might step out and high side me. I think if my bike had ten more BHP or a rougher power delivery , it could be a real problem. The worst experience I had was w hen I used t oo much offset, and as I braked out of a fast left hand sweeping bend during a race, the bike stood up and turned right and threw me off-balance. I only survived by turning the gas on again when trying t o crash it in a soft place .

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Good eye. The offset in the picture I posted is not what I used on the Commando with the steeper rake. Those are the yokes and forks I used, but I had different spacers in them for less offset. As shown in the picture, the forks are set up when I put them away in storage, with both the shims installed so I wouldn't loose them. Unfortunately, I don't recall what the final offset was for the Commando. I tried different shim combinations at the track until I found the one I liked best, but I don't know if I ever measured it. If I can find the info in my old notes, I'll post it, but there might not be any info to find.

Ken

I was quite surprised that using the small offset, the bike self-steered under power out of corners. Did you get that effect ?
With my old Triton, I was always dragging it away fr om the outside of bends and it was exhausting to ride . The Seeley with the TZ yokes is dramatically better. I find I can get the power on extremely early, even before I can see into the next straightaway.

acetrel there in lies my insanity over Ms Peel she invited hi throttling up before begining to lean, so over powered before some entries the little bit of lean or fork turn by pilot breaks rear traction to trip er down faster and really pile on the G's into a sharp turn going towards 'apex' on increasing traction by letting off of steering forces. Do ya know what a rush it is to be on almost wheelie acceleration flashing past those almost washing out hard on nose brake lighted elites. Has anyone fitted a 21" front to see what happens?

Hobot,

On flat trackers and other speedway bikes, isn't the balance between slide and drive important ? I would have thought that if the bike oversteers when laid over and gassed when on dirt, the rear end would come around. Wouln't it better to increase the offset, and make the bike more likely to stand up when backing off for corners ? On my brother's speedway sidecars we use plates to move the front wheel forward, and increase the angle on the fork yokes, by cutting the top one and pulling it back and rewelding it, and bending the bottom yoke.

acotrel, 19 posts in one thread,is that a record?

Coydog,
It is a topic which interests me. I feel really stupid that I raced for many years and simply accepted the standard geometry as being optimised. If you are racing, the bike must do something for you, but I suffered a really hard time for no good reason.

I wasn't trying to be dis-respectful as I have the utmost respect for the experienced and knowledgable minds on this forum.I have learned much and am still learning from you guys.The engine and electrical information on this site is very impressive.
The one area that there seems to be a "slight" lacking is suspension,I don't see a lot of information,experimentation and innovation in this area. I wonder how many members have adjusted their sag to the proper settings. I realize this would have to be done through spring rates and may not be easy.
Has anyone tried a steering damper once they had their suspension sorted?

On flat trackers and other speedway bikes, isn't the balance between slide and drive important ? I would have thought that if the bike oversteers when laid over and gassed when on dirt, the rear end would come around. Wouln't it better to increase the offset, and make the bike more likely to stand up when backing off for corners ? On my brother's speedway sidecars we use plates to move the front wheel forward, and increase the angle on the fork yokes, by cutting the top one and pulling it back and rewelding it, and bending the bottom yoke.

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Thank you Alan for exploring and bringing up exciting mystery stuff i only half fast understand how i get away with. I do have a serious thought provoking issue few others fast riders can relate too that may yet maim-kill me,
I Have Never Crashed Going Hard As I Could Sharp As I Could On Pavement, EVER!!!
I believe I've experienced every other surprise combo of going down in *slow* turns on loose stuff or by front brake taking down by stoppie on SuVee or front snatch downs or wheelie up over sideways under WOT on Combats going straight as I could.

I only have a few years on m/c's, most of it this last decade so I too thought the various fast bike geometry was pretty well optimalized by the time I got on em. All bikes I've over ridden or others I've seen 'over ridden' cause the rear to swing out ahead of pilot or even the forks it ya over do it, ie: the ass mass wants to lead the way in slides - regardless of fork geometry. All bikes over steer when over powering the rear while leaned any, even my wifes big scooter is fun-easy to kick rear out while mostly upright to sharpen slow turns on grass and Gravel. It don't have the power nor is it stable on tiny tires to try that at speed on good surfaces.

The best example I've seen to study balancing drive vs slide is that wet racing video showing the slightly crossed up slides balanced by power lean and steering, gives it more to kick rear out for some then eases up before it gets too far out of line it goes down. Most the time his bike jitters to self recover just in time, but a few times he gets it just right so its a smooth combo of slide and power and lean crossed up like a single ski> steering by rear only till back in line enough the forks become effective again for normal tires in line steering. The Gravel tests my balancing slide and drive, trying to always stay in drive w/o much slide, d/t the narrowness of the path and lack of slide stopping soon as let up of power lean or fork angles. In that second or 3 one can explore how fork geometry s or brakes or lean change bike-pilot impact point on ground or into fence but not the final resting place at end of the straight line tangent. Ugh.

Having power enough to spin rear at will makes up for a lot of faulty geometry but also cuts into the rears accelerating forward thrust, so had to learn how to 'jump-jerk" past the crossed up slides everyone is so fascinated by like flat tracking. What i find on fork geometry to over come differently on each type cycle is their point of countersteering sharpness that forces bike down into low sides, by lowering grip of tire edges while also removes downward force on rear patch. This is the transition point that will Hinge an isolastic right off the deck on too a flexy frame or a modern into its fast twitch-judder reaction of tire conflict vectors through too ridge a frame.

To ride past that I have to do like your side car brother nail it even harder, at instant the normal slide begins, at same instant flipping into straight steer [or just letting go some so it auto flips into S-S] that forces bike up into a hi side action while spining rear on counter balances by forcing a low side, yet at same time rear patch being forced harder into the surface by the amount of enough pilot puts into the straight steering, to point it can hi side -stand up- at end of turn in point.

This is what facinates my on Peel, never any twitching or jerking no more force to lean over than pick back up and predicable spin on CoG. Forks just determine when ya got to change styles to compensate for it yet not slow down doing it.

Hobot, Reading your post I can feel gravel rash coming on. I think you are corre ct that most bikes oversteer. I cannot remember any standard bike that I've ridden which did not feel better under power than when rolling through corners. A friend of mine got on an 80s Katana superbike which is currently at the top of our historic racing. Halfway around the big sweeper on Winton Raceway at very high speed, he couldn't decide whether to back off and crash or turn it on and crash. He chose the latter and the bike did the rest for him.

coydog said:

I wasn't trying to be dis-respectful as I have the utmost respect for the experienced and knowledgable minds on this forum.I have learned much and am still learning from you guys.The engine and electrical information on this site is very impressive.
The one area that there seems to be a "slight" lacking is suspension,I don't see a lot of information,experimentation and innovation in this area. I wonder how many members have adjusted their sag to the proper settings. I realize this would have to be done through spring rates and may not be easy.
Has anyone tried a steering damper once they had their suspension sorted?

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There are a couple of people on here at least who have documented some setups, there is a thread about one of the race bike setups which is pretty interesting while others have played with moving shock and swing arm positions as well as midvalve cartridge type dampers. Like you said though there is very little on here about sag or spring rates etc

Winton Raceway at very high speed, he couldn't decide whether to back off and crash or turn it on and crash. He chose the latter and the bike did the rest for him.

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YIKES!!! That's just like my early Graveling lesions rashing flesh off to the bone. Didn't have much armor back then and it got ruined in months so upgraded.

Essentially all wheeled craft go around corners better on good power, up to the point tires start losing it then all sorts of things suddenly matter, wind gusts to pilot position on bike to innate CoG front to back and up and down.

Essentially all Commando type racers said one inch taller rear shocks helps their tract times, which implies a tab less rake is desireable - depending on tire rim size too I see. I extended Peel front 2" base sag and if helped lean and stability. Next time out rear also lifted 2", which made me so pensive to lose Peels prior security I'm making her a low rider to compensate and find out what works for me.

There's a few sharp turns the approach is down hill then the bend that's up hill, its so dicy to try to brake I just enter a bit too hot for brakes and just lean bike for the new direction and let it bleed off speed by the drift up hill which stops with Peel alinged so as rear hooks fully she lifts back up right and carries on forward - for a stunt like drift yet hardly any skill sense to do it. That same method works on pavement just gotta be going a lot faster harsher.

'Essentially all Commando type racers said one inch taller rear shocks helps their tract times, which implies a tab less rake is desireable - depending on tire rim size too I see.'

Hobot, most guys who race commandos are probably on a tight budget. When I had the 'stand up and turn the wrong way under brakes' problem with my Seeley, I was told by a friend who used to work for Colin Seeley, that all the original Seeley frames have 27 degree head angles, and It happened that I had an old Yamaha TZ frame which I knew had 26 degree head angle. I enquired about having a new set of yokes made , however the cost would have been $500, and I didn't have a clue a bout what offset to use. The Yamaha frame with the yokes had cost me $200, so I stole the yokes, and they worked perfectly. I don't know that simply reducing the rake gives the same effect, and once you have your frame you are committed and restricted to minor changes by raising or lowering the rear end, unless you change the yokes. I suggest you are then playing a slightly different game. If you increase the trail, you increase stability in a straight line. If you change the rake or the offset you are changing the trail, but the result is probably different depending on the way you choose to change the trail. A friend of mine was watching a doco on TV about Casey Stoner and Ducati. They opened a draw in the transporter, and there were three sets of fork yokes in there . If you have the option to change the gear cassettes to suit the rider and circuit, as well as the steering geometry, and power mapping .... ? I'm not a believer in MotoGP riders being supermen. It all looks amazing, however I suggest that what they are riding is technically superb, and probably improves their riding dramatically. The feedback loop means thay must always get better.

Well Alan with the listee's help i'm just learning to put numbers on the various bike geometries for what ever horrific to wonderful things that can happen pressing limits. I'd read often and long enough about isolastic racers liking one inch taller shocks so thought if a little bit good then mo is better so put on 2" taller shocks to try on Peel. Realize no one [me] can-should get too frisky on a stupid handling untamed Isolastic with factory funny functioning forks so best wishes if altering its fork geometry to explore its effects on the fly.

I flat gave up on using-trusting forks to steer much when really getting with it in the tighter stuff, like the places widely spaced racers suddenly get all bunched up to start the race all over again neck and neck.

Had me a scare toady w/o intending accelerated to 70's into apparent nice level sweeper to suddenly be bouncing out of inward turning control so had to back off and use body to steady back. Wes said he saw the hinging and that I'd crossed over some humps diagonally. I felt the upset before the humps which just made it more exciting but I know better than race around on a real-factory Commando so recovery reserves were quite sufficient as planned.

coydog said:

Has anyone tried a steering damper once they had their suspension sorted?

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I generally ran steering dampers on all the race bikes. That was pretty general practice at the time. I have raced Nortons without them, and experienced no difficulties. My feeling was that a damper was insurance against the unexpected tank slapper sort of incident. I always adjusted the dampers down to very low damping, because I didn't like the feeling they had on higher settings, and I assumed that they would still work effectively for cases of extreme instability. The preference for settings varies greatly between riders. I've had friends race my bikes who preferred the dampers set up for close to maximum damping. One of the best riders I know used to leave the damping at minimum for riding in the pits, and then cranked it up to max as he got to the pre-grid. There have always been two sides to the steering damper question, with some experts saying they do more harm than good, and others who believe they are essential. I wouldn't know the correct answer to that one. My personal opinion is that you don't need them on bikes set up with conservative steering geometry (i.e. reasonable weight distribution, sufficient trail, etc.), but that they might have real value on race bikes that are set up for quickest turn-in, but at the price of some instability. That might explain why all the motogp bikes have steering dampers. I think most of us would find those bikes to be extremely twitchy by normal street bike standards. FWIW, they are required by the rules for landspeed racing.

Ken

my triton used to have a friction damper, and drum front brake. If you locked the brake, and lost the front it would go into tank slapper, and the steering would crash from lock to lock. If you hung on it would launch you over the front. When it happened, I would steer the bike by grabbing the tank. I never ride without a hydraulic steering damper, even on my Seeley which inspires confidence. My collar bone is still dislocated from the last time it happened.

I usually only use minimum damping settings, however if the steering gets a big kick, the hydraulic pressure in the damper would stop it getting dramatic.

Steering dampers are so popular or rule required because they generally help pilots way more than hurt them. I've a distaste though d/t the times they can hurt me that I encounter more suddenly than times they may help is all. Damper makes just riding around safely a drag to me even set for lowest effect and narrowest range. At some point to me road racing becomes like off road looseness where dampers are very rare. At some point in road racing the bike may tend to fall over on its own d/t serve lean or rear spin or both - at that point the forks will flip into straight steer automatically, if allowed, if not then your ticket into High Side City just got punched.

I think the best type damper are the Scott type which I gave up on but still need to mount on Peel d/t some rules here and there, so wonder how a clever builder would do it on a Commando like set of yokes and be easy to completely disengage most the time.
http://www.scottsonline.com/scotts.php

If the bike goes into a tank slapper, the handle bars can become blur. If you grab them too soon, all the energy of the oscillation comes through them, and you can easily get catapulted over the front of the bike.IF it happens you are in crash mode. The damper stops it from happening. I did one at Calder Raceway yeas ago when I went over the top of another competitor, and grounded a footrest. The bike slid, and I threw it upright. It then went into the lock to lock slapper. I grabbed the tank, and it came out of it. The bars looked steady, so I grabbed them. TOO EARLY - straight over the front I went. The other rider stopped, and said 'I thought you were going to stay on it'. I said 'so did I, until I grabbed the bars'.The point I'm making is that there is no need to ever do that.

Unread postby acotrel » Mon Apr 08, 2013 9:18 am
If the bike goes into a tank slapper, the handle bars can become blur. If you grab them too soon, all the energy of the oscillation comes through them, and you can easily get catapulted over the front of the bike.IF it happens you are in crash mode. The damper stops it from happening. I did one at Calder Raceway yeas ago when I went over the top of another competitor, and grounded a footrest. The bike slid, and I threw it upright. It then went into the lock to lock slapper. I grabbed the tank, and it came out of it. The bars looked steady, so I grabbed them. TOO EARLY - straight over the front I went. The other rider stopped, and said 'I thought you were going to stay on it'. I said 'so did I, until I grabbed the bars'.The point I'm making is that there is no need to ever do that.

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OhMG-UGH Alan, do you ever get PTSD flashbacks images that send involuntary shudders though you now and again? I'm trying to avoid some now after your detailing above, ugh. There's is a lot to learn in your description not all of it has sunk in for me yet but will face reading it slower to see if I can.

I said right off fork dampers are the cat's meow for about any and everyone's various situations, bike set ups and pilot skill, so please wisely install a good one like about everyone else and their sisters do. I did like 20,000 miles with the Scott through thick and thin on my handy dandy spiffed up suspension SV650 so I know to me very bones about all there is to learn on what the best kind of damper can and can not do for me on a 24' rake 17" balloon tire corner cripple I spanked crazy squids just short of slides to point I took Code's course to learn full loss of tarmac traction with bike whiplashing upsets w/o worry of on coming traffic. I didn't like the drag just to cruise, it didn't let the forks flip sides like a sail boom in a tach, then it took me down on THE Gravel by not letting me jerk my own fork tank slaps fast enough to prevent hi side with rear fish tailing. Took if off and now only to sum up confidence, only need to see a damper to strike that one off bothering without them risking too much. IF no damper then I sniff at chicken strips, if almost none I perk up and ask if I can tag along a ways - until...

Hope to have video of how fun Ms Peel is to get crazy on to play tire music by her various ways to take the same turn w/o a dang damper's dangerous dragginess. Can ya even imagine such statements about a rubber baby buggy isolastic Cdo - I sure couldn't believe it could be so good either.