Classic Chassis Kits?

[Guest majestyman340]

So what we saying here is , if you own a MAJESTY you can fit mono wheels/yokes/forks/swing arm

because its PERIOD/IN THE SPIRIT and the purest don't matter because these are the RULES in our opinion!

well guys until any RULES are set in stone its time to wake up and smell the coffee, stop winging about it and

campaign for some better RULES, but don't be disappointed if fitting mono parts gets outlawed, hope you kept

hold of all your old majesty bits !

The COTSWOLD ridden by Chris Koch was manufactured as a twinshock, a one off with a air-cooled yamaha

donor engine

[hrc1]

Is this it?

[paioli]

that's the baby, Jon Bliss rode this bike 10 months ago and this is

the first moans about it , a gasgas motor was suggested but the cost

was too high and he could not be bothered with fitting all the water works,

i wonder if you would still be moaning if he had fitted a old ty twinshock

motor or if Chris Koch had not done so well at the Phil King !

[frenchie]

If the rules or lack of them allow this type of machine to be ridden in a classic trial being classed as a twinshock then the rider has every right to ride then so be it. Chris Koch is a superb rider and would have possibly won the trial had he been on a Bultaco or even an Italjet as if my memory serves me correctly, he was sponsered or part sponsered on both of these if not others in the halcyon days way back when.

Its about opinions, you cannot compare a Majesty with mono forks to this and anybody who does has, in my humble opinion been sniffing too much Bel ray........

Bonsoir

[paioli]

so a 300 fantic with a

[Guest majestyman340]

that's the baby, Jon Bliss rode this bike 10 months ago and this is

the first moans about it , a gasgas motor was suggested but the cost

was too high and he could not be bothered with fitting all the water works,

i wonder if you would still be moaning if he had fitted a old ty twinshock

motor or if Chris Koch had not done so well at the Phil King !

If this bike is ok, then there doesnt seem anything to stop someone buying a new super trick RS Honda chassis kit, with modern steering geometry and handling, and bolting in a 4RT motor and swinging arm, and acing it up at all the twinshock events?

I guess the first of the highly modified "P65" bikes didnt cost anywhere near what people are paying today, and unless someone thinks up some sensible rules to nip this sort of thing in the bud as regards t/s, how long before people are going to be spending

[paioli]

MODERN STEERING GEOMETRY AND HANDLING that

[tony283]

Our newest Organization here in the USA was formed to cover all the "Twinshocks" that currently are not allowed to compete in the AHRMA Championships which currently cover all bikes up to model year 1979. Crazy as it sounds, especially after Bernie Schrieber's wins all the 79-85 bikes (the last of the Twinshocks) have nowhere to ride! ITSA (International Twin Shock Association) has 3 specific classes which encompass all Twinshocks:

MODERN TWINSHOCK Air Cooled Drum Brakes Twin Rear Shocks

HISTORIC 1973-1979

PRE-HISTORIC Pre 1973

So basically here is a class which allows the rider to build "whatever" to whatever cost he or she wishes to invest as long as it conforms to those 3 simple rules of Air Cooled, Drum Brakes, and Twinshocks. An early RTL Honda with Twin shocks will be out there soon! All the TY 350's could be in if someone puts twinshocks on them. I know we do have 2 new Majesty "lookalike frames" which will be competing which were made by the same company that makes Valentino Rossi's roadracing frames in the UK.......I dread to think what they cost???

It's only going one way unless YOU guys get the ACU to lay down some firm rules.

Tony

[majesty320]

Hi majestyman340, I thought you were going to send me some pics of the marzzochi fork mods we talked about. Cheers Mark

[Guest majestyman340]

MODERN STEERING GEOMETRY AND HANDLING that

[michaelmoore]

The steering angle is determined by the axis through the steering bearings in the frame. The forks could only change it by being grossly longer or shorter than the originals and so tipping the frame up or down.

Parallel vs non-parallel triple clamps change the wheel path and how quickly trail is lost or gained as the suspension collapses and the axle moves towards or away from the steering axis.

Generally, non-parallel clamps will have more offset in the lower clamp and they'll lose trail more slowly because the axle now moves towards the steering axis instead of paralleling it. If you kicked the lower clamp way out you could get it to where the offset decreased quicker to increase the trail faster than the change of rake from the steering head dropping causes trail to decrease.

Go to Tony Foale's website and download his free steering geometry calculator to make it easier to see what changes in offset etc do to real trail and ground trail.

IIRC Tony had a KT250 for a while when he was in England.

cheers,

Michael

[woody]

I think both the new Majesty and RS Honda chassis have modern type geometry, and you are quite right thats something thats very important. Not altogether sure how fitting a set of mono forks is going to achieve the same thing though? My bike has original ts forks, and I wonder does fitting the mono forks alter the steering angle to the same as that of the modern chassis kits?

The main reason for fitting mono forks or Marzocchi forks or whatever to the Majesty is because the original forks are too soft in both spring and damping. Depending on how heavy you are this may not be an issue but it is when you're 17+ stone. The forks sag badly and top out, essentially they are crap. It's nothing new, I've seen Majesties fitted with mono forks for years, particulary in Cumbria and I rode a Majesty 340 15 years ago or more fitted with a Fantic front end.

Some Majesties, when they were new, had the fork damper rods revalved by Shirty to give a much more plush action, for supported riders only I would guess, normal punters got the stadard TY fork set up. I've tried a bike with these revalved forks and they are on a par with Marzocchis as fitted to Fantic, Armstrong etc. So fitting Mono or Fantic forks is only putting the bike's front suspension on a par with other bikes from the era or the 'works' bikes with the revalved standard forks.

Other people like the leading axle of the mono forks for added stability and as a cure for tuck in on turns but that is down to personal choice. It can't alter the steering angle as already explained but it makes the steering slower than standard. A new Majesty frame with steeper steering angle fitted with the standard forks (ie; in line spindle) is going to turn quicker still than the original bike which should be interesting as they are a 'nervous' bike to begin with. Fitting leading axle forks may slow the steering up a bit to something like a standard bike but with the steeper head angle I guess it will turn tighter.

[michaelmoore]

Hi Woody,

The axle doesn't know if it is in front, behind or dead below the fork tube. All leading axle forks do if the axle position and path during the suspension movement are held constant is to slightly reduce the polar moment of inertia of the steered mass. And since the wheel is the biggest part of that and doesn't change position, the change should not be a very big one. I guess people the likes of S. Miller may be able to discern that kind of small change (just as V. Rossi seems to be able to detect a 2mm change in fork height) but I suspect that few mere mortals can notice the difference.

I'd be surprised if most people would notice the reduction in effective steered mass when a trials bike (to the best of my knowledge) normally isn't "flicked" down with a quick snap of the steering. Roadracers do those kind of sharp steering inputs, but even there what you are mostly noticing is the the gyro effects of the front wheel and disc rotors. Put on a lighter tire and significantly lighter rim/wheel and 300+mm brake rotors and that change gets noticeable during those sharp steering inputs at speed.

Extra stability would normally come about from reducing the offset of the axle from the steering axis and so increasing trail. Putting center axle forks in a set of leading axle triple clamps would give you that, hopefully without the more rearward wheel position causing it to bottom out on the exhaust pipe at full bump. But it also reduces the wheelbase which would tend to reduce the straight-line stability a little, while shifting a little bit of the weight distribution more forward which should help to increase the stability. Changing just one thing gets very difficult to do as you need to watch multiple variables.

It would be interesting to do some blind tests to see if people could actually notice the difference in the change of polar moment. You wouldn't even have to change forks. Get a couple of 1-2 lbf bars of lead that will fit inside of the fork springs and figure out how to attach them to the bottom of the fork caps. Put a bag over the upper part of the forks between the triple clamps (or something that shields that area from the rider's view). Now have someone move the lead bars from the fork cap to taped to the front of the fork tube to the back of the fork tube (f that is possible without it cutting down on the steering lock) to not even on the bike at all, varying the position randomly and ensuring the rider doesn't know where the lead is and send the rider through the same section over and over. Do it enough times with enough different changes to give a reasonable sample and record the rider's guess about where the weight is.

eta: you could probably avoid fiddling with putting the lead bars inside the forks, just position them front, back and sides of the tubes. That makes it easy to do it all with just duct tape or zip ties.

If you want to play with trail have a spare set of the "other" kind of forks from what you normally have. Run the forks as leading axle, swap in the center axle forks, and then swap those for the leading axle forks running reversed for a trailing axle. I think there is about an inch of lead in the Bultaco sliders so that would get you a pretty noticeable change in trail. You'll have to adjust the amount the forks extend through the clamps to keep the steering head angle from changing. Of course, you also end up with changes in wheelbase and weight distribution muddying up the experiment, along with changes in the polar moment of the steered mass from moving the wheel a couple of inches forwards and back.

Filling the inner tube with water should increase the gyro effects and make the bike really stable. But when it does finally get knocked off line you may find it difficult to overcome the stability of the new direction.

As to damping, a friend has done a fair bit of work on four-stroke Husqvarnas over the years, including fork/rear damper valving. He told me that he's seen rear dampers from the same model and year bikes that were not very many serial numbers apart have very different shim stack specs from the factory. It doesn't matter how trick the damper is, if it is valved for someone who weighs 50 pounds different and who goes 20% slower or faster the external adjusters are unlikely to have enough range to get it anywhere close to being right. And if you don't have that adjustment available (as on our vintage stuff) you are really out of luck.

cheers,

Michael

Edited March 6, 2008 by MichaelMoore

[michaelmoore]

FWIW, I grabbed some numbers and did some simple calculations which hopefully I haven't fouled up.

Based on a modified Yamaha DT2MX front end with the lightest combination of parts (from Barry Watkin's "Lightweight Weapon" article)

center axle triple clamp w/stem assy 5.75 lbf

front wheel assy 18.5 lbf

fork tubes complete (both) 13 lbf

My KT250 clamps look to be roughly 6.875" OC for the fork tube spacing. For an easy comparison I decided to use 3" offset in the clamps for a center axle fork and 1.5" offset for a leading axle fork (with axle still at 3" offset).

For the center axle fork leg the center of the tubes are on a radius of 4.563" (from the steering axis). The leading axle tubes would be 3.751". The wheel radius is 3".

I decided to use 1.25" radius for the center of mass of the clamp assy with an assumption that the changes between flatter clamps/axle boss and deeper clamps/no axle boss would be a wash.

offset 3.0" on forks

center axle

fork leg radius 4.563", mass 13lbf, mass X radius 59.319 lbf/inch

triple crown radius 1.25", mass 5.75lbf, mass X radius 7.1875 lbf/inch

wheel radius 3", mass 18.5lbf, mass X radius 55.5 lbf/inch

sum = 122.01 lbf/inch

offset 1.5" on forks

leading axle

fork leg radius 3.751", mass 13lbf, mass X radius 48.763 lbf/inch

triple crown radius 1.25", mass 5.75lbf, mass X radius 7.1875 lbf/inch

wheel radius 3", mass 18.5lbf, mass X radius 55.5 lbf/inch

sum = 111.45 lbf/inch

The leading axle forks end up about 91% of the total of the center axle fork. An interesting thing about actually doing some numbers is seeing that I forgot that the fork tube assys would end up on a longer radius than the wheel assy which magnifies the effect of moving them around. If you put a 2.75 lbf rod onto one of the leading axle fork tubes so that it is on the same 3.751" radius you'd have pretty much the same steered mass number as the center axle fork assy.

Still, the difference is not a very large percentage. If you are riding in loose dirt/sand/mud I wonder how much that might serve to damp out that small difference enough to make even the aces not be able to reliably notice it? And if your handlebars were an inch or two wider or narrower between the types of forks you might offset the difference and have them feel the same.

Would I notice it? I don't know. I might be able to concentrate on noticing the difference during an experiment (especially on a frictionless surface so I was only noticing the fork mass effects and not the tire squidging about) and detect something, maybe not. In a section it might depend on how much I was having to concentrate on not going out of bounds.

cheers,

Michael

Edited March 6, 2008 by MichaelMoore

[Guest majestyman340]

Hi Woody,

The axle doesn't know if it is in front, behind or dead below the fork tube. All leading axle forks do if the axle position and path during the suspension movement are held constant is to slightly reduce the polar moment of inertia of the steered mass. And since the wheel is the biggest part of that and doesn't change position, the change should not be a very big one. I guess people the likes of S. Miller may be able to discern that kind of small change (just as V. Rossi seems to be able to detect a 2mm change in fork height) but I suspect that few mere mortals can notice the difference.

I'd be surprised if most people would notice the reduction in effective steered mass when a trials bike (to the best of my knowledge) normally isn't "flicked" down with a quick snap of the steering. Roadracers do those kind of sharp steering inputs, but even there what you are mostly noticing is the the gyro effects of the front wheel and disc rotors. Put on a lighter tire and significantly lighter rim/wheel and 300+mm brake rotors and that change gets noticeable during those sharp steering inputs at speed.

Extra stability would normally come about from reducing the offset of the axle from the steering axis and so increasing trail. Putting center axle forks in a set of leading axle triple clamps would give you that, hopefully without the more rearward wheel position causing it to bottom out on the exhaust pipe at full bump. But it also reduces the wheelbase which would tend to reduce the straight-line stability a little, while shifting a little bit of the weight distribution more forward which should help to increase the stability. Changing just one thing gets very difficult to do as you need to watch multiple variables.

It would be interesting to do some blind tests to see if people could actually notice the difference in the change of polar moment. You wouldn't even have to change forks. Get a couple of 1-2 lbf bars of lead that will fit inside of the fork springs and figure out how to attach them to the bottom of the fork caps. Put a bag over the upper part of the forks between the triple clamps (or something that shields that area from the rider's view). Now have someone move the lead bars from the fork cap to taped to the front of the fork tube to the back of the fork tube (f that is possible without it cutting down on the steering lock) to not even on the bike at all, varying the position randomly and ensuring the rider doesn't know where the lead is and send the rider through the same section over and over. Do it enough times with enough different changes to give a reasonable sample and record the rider's guess about where the weight is.

eta: you could probably avoid fiddling with putting the lead bars inside the forks, just position them front, back and sides of the tubes. That makes it easy to do it all with just duct tape or zip ties.

If you want to play with trail have a spare set of the "other" kind of forks from what you normally have. Run the forks as leading axle, swap in the center axle forks, and then swap those for the leading axle forks running reversed for a trailing axle. I think there is about an inch of lead in the Bultaco sliders so that would get you a pretty noticeable change in trail. You'll have to adjust the amount the forks extend through the clamps to keep the steering head angle from changing. Of course, you also end up with changes in wheelbase and weight distribution muddying up the experiment, along with changes in the polar moment of the steered mass from moving the wheel a couple of inches forwards and back.

Filling the inner tube with water should increase the gyro effects and make the bike really stable. But when it does finally get knocked off line you may find it difficult to overcome the stability of the new direction.

As to damping, a friend has done a fair bit of work on four-stroke Husqvarnas over the years, including fork/rear damper valving. He told me that he's seen rear dampers from the same model and year bikes that were not very many serial numbers apart have very different shim stack specs from the factory. It doesn't matter how trick the damper is, if it is valved for someone who weighs 50 pounds different and who goes 20% slower or faster the external adjusters are unlikely to have enough range to get it anywhere close to being right. And if you don't have that adjustment available (as on our vintage stuff) you are really out of luck.

cheers,

Michael

Cant say I understand all of that Mike but very interesting all the same................what sort of steering angle is used on modern machinery, and how does this compare to an old twinshock bike?