Making fork yokes

The guys who made those discs now live with the guilt.

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that bring tears to me plus my pangs if I've killed someone depending on my skill.

The over powered wheelie prone balloon tire modern bikes can only be considered corner cripples by a very few riders on very few vintage type bikes. When I speak of racer bikes and race pilots as corner crippled its a relative statement compared to what I now know is possible on G forces in turns, not that sports bikes can't corner very fast, its that they have such variable harmonics its hard to predict what they will do when riding them a bit past the stable tire contact planting. I see the elites doing turns all kinds of way, point and shoot and even some phase 3 sideways hooked up wheelies while leaning - but then I see their wiggle worms hit and I know the pilot has hit the limits of that style of handling so bike reliably self corrects its jitters. Only a tad harsher and all hell can break loose if pilot don't let it slide wide crossed up on tires out of best traction effect. You saw that wet race ride crossing up leaning and getting the rear loose by a wonderful pilot, sometimes doing it stable but most the time with little trip outs - that's the weakness I focus on as instants of lost acceleration and control a really Neutral bike can use to zoom around or just widen the gap lost on the last real turn. I think this jitter-jiggle is innate in rigid frames so have some pensiveness about Seeely's pressing into frame flinging flight zones of handling. i don't have much sense of fork geometry expect that my SV650 is too sharp for off road security and 850's are too sluggish to whip around on pasture grass yet soon as as sit on my or my buddies 750's I can sense the handy nees to set the rear spinning and blast around on pasture and Gravel. I didn't like the sluggish handling of 850's on hwy way either though they are just fine for spirited riding just not as easy to steer which at some point becomes enough effort to upset handling limits.

That other big huge reason I use the corner cripple slur for other elite motorcycles, if they require force to steer or lean or pick back up then at some threshold pilot or bike can't take that much force to keep pressing around faster, they go nutzo.
If they require steering dampening then at some threshold the forks can't react fast enough to the road following or pilot corrections and down they go if they don't back off first by the horrific warning signs a new energy threshold has been reached so a new way of handling it required.

Ms Peel also power steered like Alan's Seeley, ie: it didn't fly wider on more power she just turned sharper w/o any fork effort or attention. Peel never lost traction by surprise, on pavement, ever! The coming out of turns would be so fast Peel would hi side back up and land sideways aimed in new direction, the fork geometry reaction was critical at only two points, the launch, as hi sides start at the front, and the landings not to rebound front out of line or jerk out of my grip.

What happens in practice is where the fat tires are slipping or walking around so their frame jiggles and threatens to lift tires and they have to go wide, Peel just shoots straight deep into turn then over power to trip down the lean and skip rear round enough to pop back upright for a bit for better acceleration then trip skip till new power hook up jerk forward - as many times as needed till last facet aims down the next straight away. Anywho its more fun than sex to me so working up to show what ya missing out on but can have just as easy as me, once the chassis stiff ness is worked around.

I don't usually ride dirt bikes, however I tend to go faster in the rain than many others. I think the main thing is to always be relaxed, never stiff or hanging on too tight. If you think about what you are doing , you usually know what is about to happen. The thing to remember is that if it is bad for you, it is bad for everyone else.

Wet pavement still has mysteries to me, though I've gotten in postures like that fantastic racer video > unintended and at much slower speeds > to be very scared to practice limits in wet public places so will have to wait till Peel taken to rainy track day - hopefully with the track all to my lonesome. I already know how to use lean, fork and power to break free on wet but I've not learnt like you have - how freaking long and far various speed angles carry the slip slide before hooking up again...

Fork action angles and such is a mystery to me too, so had to kind of simplify-boil down my use of them, counter steer is for slow going low side help down while fast going straight steer is for inducing hi side saves by prying down on rear when rear is trying to low side and swap ends. I am dependent on Norton factory 750 wisdom on stem and trail and rake until unload or lifted out of traction.

Very small changes! Dont think so!!! My RL FFast back had a front end shunt, the top frame tube is bent over 30mm, yet it handled very well, so the statement "Small changes" didnt make any differance to mine..i bet the head angle is 10 degress out :!: Chopper men just hacked out a lump of tube .bent it back and welded it up.

r

acotrel said:

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.

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John, it is all OK until the bike grabs you by the throat. When we were kids we used t o build Tribsas using Triunph motors and BSA frames. My mate's bike had standard gold flash frame and fork yokes and was scary, my own bike had the same frame with yokes from a 1963 unit Triumph, and was infinitely superior. Some of the choppers have been known to crash their riders through bad handling. I know of one guy who was killed when his chopper took him off a country road through the trees. The difference in handling does not have to appear to be much, but it can still kill you when push comes t o shove. The worst thing I ever did to my old 500cc Triton racer was fit 18 inch wheels to replace 19 inch, to get decent rubber. It never handled after that, it always tried to run wide in corners . It was heavy and exhaustingly dangerous to ride.
There are two factors which combine when you change steering geometry. More rake gives greater stability, more trail gives greater sta bility, but when you increase the rake and reduce the trail........ ? ? ?

acotrel said:

... but when you increase the rake and reduce the trail........ ? ? ?

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You simply change the offset to regain the trail or use adjustable items (inserts) like these 'Attack triples,TL1000 in this case to bring it back to around the 100 mm mark.
That measurement has been used for decades.

I like the look of those fork yokes. Some of the bevel Ducatis had head angles near thirty degrees, it would be intersting to know what offsets were used. I once rode a 900SS, it felt like it was designed to be ridden around bumpy sweeping bends at 150 mph. I think that like a lot of things in motorcycling, when you change steering geometry you need to sneak up on it. If the bike feels like it is standing up as you brake into a corner - watch out .
My Seeley was fitted with Arces yokes off a Ducati 450 when I bought it. Under brakes it stood up and turned the wrong way and threw me off balance. I turned it on again to crash it on the grass beside the circuit, and it came out of its crash dive , 'Arse beats class' ! If it is going to go wrong, it can happen when you are going too fast to step off. I think you might get away with a lot on public roads, but if the defect appears when you are racing, it can be a big crash. My friend who died recently was an excellent rider, and the Seeley caught him. He told me about it, and I took what he said with a grain of salt - almost cost me.

Regardless of what I've said about the hazards of altering steering geometry, I wouldn't deter you from doing it. The benefits can be really great. I believe that most of the historic racers in Australia use geometry suited to public roads, the bikes could be better if played with in the steering department. If the bike self-steers around corners, you can get the power on so much earlier..

I don't believe that steering geometry is in any way "magic".
It's all in the math.
I would bet that any wheelbase, rake and trail you could come up with has already been done and there are hard numbers on it's effectiveness.

Mark, I'm no slouch at maths, I studied it at third year university level. I suggest you should buy the book titled 'Motorcycle Dynamics ' by Vincent Cossalter' and read it. I can follow and understand the logic and even apply the differential equations which are involved, however using the maths to predict an outcome from changing steering geometry is almost impossible. I've read a paper from our Society of Automotive Engineers on motorcycle safety, and the handling aspects were way beyond them. If you got really smart you could set up a computer programme using pattern recognition, and the maths in that book, and you might be doing us all a favour.

acotrel,
In no way do I claim to be a guru in all things steering,
nor do I have the ability to create the computer program that you desire.
With the seemingly endless combinations of rake, trail, wheelbase, rim size, tire profile, rider size, and the ever so arbitrary "rider preference",
setting up a bike to please everyone in every situation would be next to impossible (except for Ms. Peel of course).

I take it from your previous posts indicating your preference for those 2 strokes, that you would obviously lean towards the same setup in every bike that you ride.
Personal preference is really what it all comes down to. Well........ that and ability.

Vulin, you really need to think about joining the MC-Chassis list. Like you, a lot of them are pro's and they all know their stuff regarding motorcycle theory and fabrication. Some of them have Commandos and a lot of them are into vintage racing. This link tells you how to join.

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

There is a whole half a world of off road and stunter motorcyclers using a fork steering method that the motorcycle dynamics gurus never ever even mention let alone tried to mathematically describe it, so I wouldn't thrust their math predictions but as a starting place to begin trial and error refinement from.

[video]http://www.youtube.com/watch?v=xbLovGyW21c[/video]

Well....... that video settles it.
I stand corrected, steering geometry is indeed "magic".

Well....... that video settles it.
I stand corrected, steering geometry is indeed "magic".

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Yep pretty much sums it up for me, too many ways to change a cycle's direction to state any one way is the only best way. The crazy making part is what works great in one condition suddenly don't the next instant. Just don't make a yoke that will break while seeking its magical effects.

Don't forget, if you want to make a set of reduced offset yokes to make the bike steer more like a modern bike, you also need to steepen the rake- otherwise it will not work correctly. My race bike has a set of reduced offset yokes but it also has 2 degree steeper rake via eccentric steering head bearing inserts.

Brly said:

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

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Sorry I dont, I hope to have her running sometime this summer (said that last summer too) Ive had some spotty signal so I have not been able to check the post, so sorry to everyone for the lack of input. I am still going through different sites to see about what changes I will make. It seems like most enjoy the handling of the 750 more then the 850, so I may shoot for that. I thought of hitting up the track guys and see what their geometry looks like, even though it may be a bit aggressive for something street ridden.

I need some help here, this post mentions "adjustable yolk's" i cannot imagin how these work. I assume the top yolk as tapers that match the norton stanchion? so how can the alignments be maintained,surly if the angle is adjusted the holes go out of parallel, unles the top taper's are canted as well? I would like the see this maintenance of alignment in detail.

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

Doug, it was actually your setup I was going to try and mimic. Instead of making eccentric inserts, I was just going to modify the hole angle to achieve the same result. It looks as if your trees were inline with the head?