Making fork yokes

Vulin, have a look at the Project Showcase bikes at Framecrafters. Some nice yokes, amongst other things.

http://www.framecrafters.net/index.php

Chris, Will this be "your" year, Plenty of races and places?

Chris said:

Hi John

Harris performance website does not show the yokes in detail like they use too. But it should give you the idea.
ps I will be sending up my Rickman 41.3mm forks for you to have a look at. ie a Landsdowne kit for them for my triple.

Thanks Chris

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My race bike has a set of reduced offset yokes but it also has 2 degree steeper rake via eccentric steering head bearing inserts.

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What is the felt effect of this geometry Doug? Ie: less force on pilot to change lean/aim, more stable in sharper turns & or does it allow More Power on while turning sharper?

My SV650 has modern 'sharpened' fork geometry with 120x17 tire- which does 2 things I do not like, sucks off pavement just being careful and tends to skip-slip front on tarmac if staying on power, like most do exiting - while trying to use forks [by less counter steer towards straight steer but not into real straight steering] to help pick back up leaving apexes. I must watch not to low side going into turns or hi side coming out, YIKES, too much work and risk for me, so going for broke with factory 750 Commando geometry. Its all the rage to put on GRSX forks with 93mm trail instead of the stock 100mm. 2000 SV650's come with 25' rake.

One thing to accommodate if making fork tubes closer to the stem [as about universal on race bikes] is forks or bars fouling tank before full swing reached.

Shortening the fork tubes &/or raising the rear can lead to front 'tuck' just as its feeling real good... it turns into a bucking bronco with a whip lash through frame/bars so fast no way pilot can do much but let it self correct or just go down. This is what I work up too, more and more "whip lashing" fight back pressing in harder trying to resist it and then the reverse > how much I or bike can take the whip lashing on fast relieving or even letting go of fork pressing.
Guess how I first leaned or rather discovered in surprise about flying a bike out of a turn just as its about to crash, it sure wasn't no corner flundering isolastic Commando. Thot I'd topped out on corner G's as good as moderns can do - far beyond tolerance of skinny tire flexy vintage, til I got laid by my Ms Peel!

@john robert bould:

These are the triple clamps supplied by Framecrafters.net. The bike shown is a Norton wide line with a bunch of trick stuff.

http://www.framecrafters.net/pimg/big/DSC00031.JPG

The fork tubes are stock Commando and you can see the fork tube taper protruding above the clamp and below the top nuts. Rake angle is adjusted by changing out the aluminum bushings which have incremental offsets. The top and bottom of the fork tubes are held in place by pinch bolts.

Mark
It is not 'rider preference' which determined the setup on my Seeley, it is the type of bike, the circuits and the type of power delivery. If it was a two stroke, it would probably hi-side me very often. Yamaha TZs are fairly neutral steering and they still step out at the back when th ey come on song. On my bike the self steering is pronounced. It is longer than a two stroke, and the power doesn't come on with the same sort of bang. If your bike and circuit dictate 'point and squirt' racing , you need straight line stability. If it is underpowered you need nimbleness in corners and the ability to get the power on well back in the corner when coming out onto the straight. Of couse it all comes to nothing if your front brake drags and the bike becomes much more stable and difficult to turn.
Incidentally, I don't like two strokes or four cylinder bikes, they have destroyed road racing. Thunderbikes (single, twin and triple cylinder aircooled four strokes) provide real racing, not 'point and squirt' rubbish.

I take it that you did see this:
http://www.youtube.com/watch?v=2GwGFQnh ... e=youtu.be

Agostini had the MV3-500 set up so that it self steered around corners and was a much better package than the Honda 4 500, even though the MV had less power.
I suggest that with our type of bike, it is extremely important t o be able to ge t the power on really early as you come through t ight corners on circuits which have at least half twisty bits. Also gea r ing is very important . You need t o decide w hether to gear for the straights or the corners . That is why sold my TZ350 and I bought the six speed CR box for my Seeley Commando - gives more flexibility.

I would have to think the forks produced for the stock commando are made to allow the everyday rider to ride with ease, and without the fear of any negative actions. So I think with a couple tweaks there could be great improvements in handling. I would be interested to see what a seeley framed bike would be set to. Now Im sure with the difference in frames would change what rake/trail/offsets you would run. Here is quick mock up I did, there is alot I would like to change. Since the bike is not a full out racer I would still like them to have some style.

I like the yoke itself, but not the tacho mount. If the tacho is not flexibly mounted, it can be destroyed by vibration. I haven't ridden a standard commando, however I would be very careful when reducing offset with the isolastics being involved. If they can get you into the big weave when poorly set up, you could be creating danger for yourself by changing the yokes . On the Seeley everything is tight, and in alignment all the time, so you can really feel how it is steering, and it is usually predictable after you have ridden it for a while. Even now, if I take it out for a practice session, I don't gas it hard halfway around corners until I sense what it is doing . Even tyre pressures have an effect. All you really have to do is leave yourse lf plenty of room in corners when you first ride it after making changes. I have no anxiety with the Seeley, it is a ll good.

I have a motogagdet gauge which is electronic. I also thought about vibration, I could make the hole larger and add a rubber bushing. I may need to rethink the gauge mounting, but I would like to incorporate it into the yoke.

There are so many different variables in making a bike turn than just the triple trees.

.Axle lug position on the lowers
.The place where the handlebars attach to the fork yolk
.The length of the fork tubes or their position in the clamps
.A longer set of rear shocks will make a bike turn quicker (raising the rear of the bike)
.I've even seen guys remove a link from their chains to essentially move the rear wheel closer to the front of the adjustment slot on a stock swingarm, essentially shortening the wheelbase.

Vulin:
I like what you are doing, and would probably be attempting the same thing if I had access to a CNC machine. My only concern would be to not remove too much material
on alloy yolks and weakening them.

A friend of mine has made many sets of fork yokes, I believe they were all made out of 30mm aluminium. I'd use at least 6061, not a 1000 series material. The set that I saw which failed was cast in a mould taken from a steel set, the section between the spindle and the staunchions on the bottom yoke was about 8mm thick. The bike fell a part while negotiating a couple of bends, during a race . Looking at your drawing, you shoudn't have a problem.

3014 hi-copper is 5 times stronger than H80 and machines freely, but will need anidizing,

acotrel said:

A friend of mine has made many sets of fork yokes, I believe they were all made out of 30mm aluminium. I'd use at least 6061, not a 1000 series material. The set that I saw which failed was cast in a mould taken from a steel set, the section between the spindle and the staunchions on the bottom yoke was about 8mm thick. The bike fell a part while negotiating a couple of bends, during a race . Looking at your drawing, you shoudn't have a problem.

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Trying to anodise copper bearing aluminium is not recommended. It usually ends up looking horrible. You might be better using magnesium bearing marine type alloy if it has the desired strength. 7000 series aircraft alloy is good, but expensive , unless you can scrounge some from an aircraft repairer. 6061 is not too bad, we make most of our bits for our racing bikes out of it. I recently bought some Alodine 1200 for treating aluminium to stop corrosion. It is a standard treatment for aircraft fuselage parts, but it has OHS and environmental promblems - full of chrome and hydrofluoric acid. And it doesn't look too flash, but it's cheap and effective, you just dip the parts in a solution of it.
One thing I've learnt about racing motorcycles - minor airports where there are repairers are good places to visit. Most of the big fasteners on my Seeley are now titanium. I've still got to remake my exhaust out of Hastelloy, ( I found a guy who makes aircraft exhausts ) .

RGM yokes 6061 T6 which are milled out under leaving hollow between the mass left around shaft holes. The rear edge lip is ~7/16" thick, the leading edge lip ~3/8".

I may have one of the engineers at work run a stress analysis on it when I am finished. We have plenty of 6061 at work, but I was also thinking of picking up some 7075. Still have lots of thinking, and aesthetic changes to make.

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

7075 is hi strength alloy I used in a few places of hi stress like sprocket adapters, crash cage tangs and caliper mount but its a bit heavier than 6061 so a yoke might weigh as much as cast steel yoke or more. It welds nicely with same color tint as fork sliders or engine cases its being melted into.

here's some specs to compare
http://www.makeitfrom.com/compare-mater ... u-Aluminum

Cast vs milled Al items in 6061 vs 7075
http://www.gunsumerreports.com/review_f ... es_001.php

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

I once rode a 1961 manx which was still fitted with the 19 inch wheels. I think it must have oversteered slight ly under power whe n laid over, because I clearly remember the confidence it gave me, it made me feel like giving it much more stick when it got a bit off-line. My Triton was faster in a straight line, however my lap times were lower when riding the manx. The guys here buy replica featherbed frames with 26 deg. head angles and use 18 inch wheels, so the bikes steer like a suzuki two stroke. I think they've lost the plot.

Thanks for the info Ken.

Glen

On my 24' rake SV650 I've had 3 events of what acetrel and Ken describe happen to me going straight ahead calmly on THE Gravel lumps to end up face planted or elbow driven into rib fractures with bike mass landing on top at 30-40 mph events. Seemed to be a self magnifying jerk around of front too sharp for the lean of the rear just then. Eventually learned a strangely more aggressive yet easier steering method so only go down like 2x's a yr last 6 yrs d/t mud or close call dodge.

At speed on pavement the 24'-25' rake bikes on tarmac also would tuck in too sharp suddenly which unloads rear to skip out sideways enough it then hi sides off the scrubbing front tire angle. Didn't crash at speed in those explorations only because I was going fast enough pure ballistics gave some grace time to recover and sense for fecal ooze then carry on to do it again and again until sunk in what's going wrong to avoid it or use it in my crazy fashion. I had to scare the shit out of me but if I didn't get through this I would of had to quit d/t bike and me broken up most the time too scared-stupid to really ride like the wind.

I like the sense of sharper rake to start a lean then the squatting rear on power planting increasing the rake up to apex max out relief.

Wish others were so fond and full faith on their life saving rubbery skily smooth Commandos as me, as its a high only death will extinguish.