Classic Chassis Kits?

[Guest majestyman340]

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

ITSA sounds good Tony.................if we had a "Modern Twinshock" class here, then maybe it would have prevented P65 being ruined by the money men, and twinshock starting to look like its going the same way.

[woody]

Hi Michael, I don't begin to understand the theories around what you describe as regards steering geometry so I'm not going to try and dispute it....

However, I put a pair of leading axle forks on my Ossa MAR once, same length as the Ossa items using the same Ossa yokes, and it completely screwed the steering, the bike was slower to turn and the nimbleness that the MAR has had gone. Obviously, the bike is longer with the front wheel pushed an inch further ahead. The difference was very noticeable when riding. Magical himself said so too when he tried it.

Strangely, a set of leading axle forks on the Majesty doesn't seem to upset it at all

[Guest majestyman340]

Hi Michael, I don't begin to understand the theories around what you describe as regards steering geometry so I'm not going to try and dispute it....

However, I put a pair of leading axle forks on my Ossa MAR once, same length as the Ossa items using the same Ossa yokes, and it completely screwed the steering, the bike was slower to turn and the nimbleness that the MAR has had gone. Obviously, the bike is longer with the front wheel pushed an inch further ahead. The difference was very noticeable when riding. Magical himself said so too when he tried it.

Strangely, a set of leading axle forks on the Majesty doesn't seem to upset it at all

My bike has standard Yamaha twinshock forks, what difference to the head angle does fitting the later forks make, or does the improvement come from the later forks working better?

[Guest majestyman340]

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

If I can find someone to do the work, would it be possible to improve the handling on my stock orginal Godden frame Majesty, in any way other than fitting the later forks? I have heard of people cutting and welding the frame on some bikes, but not sure what effect this has? (if any)

[woody]

If I can find someone to do the work, would it be possible to improve the handling on my stock orginal Godden frame Majesty, in any way other than fitting the later forks? I have heard of people cutting and welding the frame on some bikes, but not sure what effect this has? (if any)

This is getting confusing to keep track of as the same questions are being asked on two different threads, this one and the twinshock thread, so you're missing the answers to your questions. Answers to this question and previous one are a few posts back on this thread, but to reiterate;

I really wouldn't bother altering the head angle on your Godden framed Majesty. It will make an already twitchy bike worse by quickening the steering and shortening the wheelbase (using the existing forks) and the bike is already short as it is. Unless you're top drawer expert class that is not what you need as the bike will require constant steering check and body lean to counter the twitchy steering and keep it pointing in the right direction. Top riders have this, others don't.

There is nothing wrong with the Majesty geometry as it is, an expert may well notice the quicker steering of the new frame but in actual riding in sections 98% of riders would get no benefit from it. The standard bike will easily cope with twinshock sections, the Majesty is a good bike as it is, altering the head angle won't make it suddenly perform like a modern bike. New replica frames have modern geometry purely because that is what is thought correct these days. Doesn't mean it translates into real benefit in sections.

Mono, or any leading axle forks do not alter the head angle if they are the same length as the originals. Forget that line of thinking. As already explained they are fitted to replace the poorly sprung and damped originals. An added benefit is that as they are leading axle they can improve the straight line stability and help preventing tuck in on corners, but this again is personal rather than a hard fact.

Best mod on the bike is to lower the footrests as they are very high (obviously depends on how tall you are as to how much it bothers you) Lowering them to just above or level with the bashplate gives the bike a more comfortable riding position and if you are comfy on the bike that is a major benefit in terms of riding as it is one less thing to worry about. Don't go too far back with them as this will also mess up the steering by weighting the rear to much and making you stretch for the bars, especially on full lock.

You need to read back over this thread and the twinshock thread for the info you're asking about.

[tilertrialler]

I know a guy who cuts and shuts the frame behind the head stock, not sure on exact angles but the modern bikes are around 23 degrees and the older ones were around 26. the trouble with the yams is they are short to start with ( not sure about the godden frames) so if you tip in the headstock you really need to lengthen the swinging arm, and this is done often.

About the forks a lot of guys are putting modern forks on the twinshocks and many have fat bars.

[michaelmoore]

However, I put a pair of leading axle forks on my Ossa MAR once, same length as the Ossa items using the same Ossa yokes, and it completely screwed the steering, the bike was slower to turn and the nimbleness that the MAR has had gone. Obviously, the bike is longer with the front wheel pushed an inch further ahead. The difference was very noticeable when riding. Magical himself said so too when he tried it.

Hi Woody,

If you get the books on motorcycle chassis design and construction by (respectively) Tony Foale and John Bradley you'll find they have both done a very good job of explaining things on those subjects.

I'd expect the handling to be quite different from those changes. I'll guess the Ossa had 27 degrees of rake and 85mm of ground trail (those numbers seem to be ballpark for the mid 1970s. A 21" front wheel's about 26" OD/660mm. That gives some numbers to evaluate the change.

The center axle clamps would have 74mm of offset for that geometry and you'd have a real trail of 75.74mm (real trail is the actual number that does the work, you can read about that in Tony Foale's writings). If your leading axle forks had 35mm of lead (I'm figuring 35mm stanchion with 10mm thick sliders and the 15mm axle right up on the leading edge of the slider - 17.5 + 10 + 7.5 = 35 so they may have had even more than that but that looks close to the way the Bultaco forks were in a photo that is handy) your new offset is now 109mm. That gives ground trail of 45.8mm and real trail of 40.8mm.

I think it should have been pretty easy to have the front end knocked off line if it hit a rock -- you'd have to hold on pretty tight as it wouldn't have as much self-centering available. If you were just riding straight ahead on fairly smooth dirt at a slow speed you might not notice that effect very much. Your small rearward change in the weight distribution might also make it a little more "nervous" in the rocks.

Moving the wheel 35mm or so forward moves a big part of the steered mass forwards too. That's the issue I was discussing above.

It looks to me like much of the nimbleness/quick turning on a trials bike may be more in the wheelbase than in the steering geometry. 48" wheelbase bikes will easily make a turn where a 55" wb bike doesn't stand a chance. For a given radius of bend a larger wheelbase means you've got to turn the forks more to make it. If the forks are already at the stops you are out of luck. I just looked in a copy of Tony's book and he even mentions trials bikes in the section on wheelbase "A practical consideration for trials bikes is that, for a given degree of steering lock, the minimum turning circle is smaller with a shorter wheelbase. For this reason, trials machines have wheelbases as short as 49-50 inches." Gaunt's little Jawa was reported by Morely to be 50.5", and the 123 Cota as 49.6".

Your experience is why I tried to point out that you often can't change just one factor without having other things change too, and that sure muddies the waters when trying to determine what effect a change has. My previous post was addressing strictly the different placement in mass in leading axle vs center axle forks as I've seen comments over the years that the leading axle are supposed to be "better" by having a lighter feel to them and it seems to me that we're talking pretty small changes in mass locations and and normally fairly slow and smooth steering movements so you don't have a lot of momentum being built up and slowed down. To evaluate only that factor by going out and riding you need to keep wheelbase, rake, trail and overall weight distribution of the bike constant.

You could have had the same experience on the Ossa by keeping the center axle forks and putting on a set of clamps with extra offset equal to the offset in the sliders of the forks you tried with the Ossa clamps. What you did was make some very significant changes in the steering geometry and wheelbase (and a little on weight distribution) and was quite a different experiment from what I'd proposed. It was interesting to hear your comments.

As has been noted, you can't just cut and shut and change the steering head angle without changing the wheelbase and possibly also raising the front end from the steeper forks. One change leads to another. If you want to keep wheelbase the same while steepening the rake you need to cut off the steering head and rotate it forward (and away from the frame) around the front axle, unless you are planning on doing a cut and shut and then lengthening the swing arm to get the wheelbase back. And once you've done that you've moved the engine/footpegs closer to the front wheel and changed the weight distribution too. You've also changed the trail when you changed the rake and retained the same forks and clamps.

I made my comments because it is clear that a lot of people don't have a good understanding of this (I won't claim to be an expert, but it is something that has interested me for decades) and they often tend to focus on one thing that they can see and say "there's the difference." Leading axle vs center axle is one of those "oh look, that's different, that must be it" things. But if both sets of forks put the axle through the exact same path from full bump to full rebound you've made zero change in the steering geometry. Swapping leading vs center axle fork tube assemblies around in the same clamps will make a very significant change.

It is sort of like how people will say "my frame is so light because it is made from 531 or 4130" when those steel alloys have the same mass as mild steel. It isn't the type of steel that makes the frame light, it is having less of it used to make the frame. But they see a "531" sticker and figure that obvious difference has to be the reason.

cheers,

Michael

[michaelmoore]

FYI, one of the engineers on my chassis list says that the steered inertia will be a radius squared effect, and not a linear one as I laid it out. Sorry for the error.

cheers,

Michael

[Guest majestyman340]

I know a guy who cuts and shuts the frame behind the head stock, not sure on exact angles but the modern bikes are around 23 degrees and the older ones were around 26. the trouble with the yams is they are short to start with ( not sure about the godden frames) so if you tip in the headstock you really need to lengthen the swinging arm, and this is done often.

About the forks a lot of guys are putting modern forks on the twinshocks and many have fat bars.

Very helpful info.............is this guy able to modify a Godden Majesty chassis, and any idea of what he charges?

[Guest majestyman340]

Hi Woody,

If you get the books on motorcycle chassis design and construction by (respectively) Tony Foale and John Bradley you'll find they have both done a very good job of explaining things on those subjects.

I'd expect the handling to be quite different from those changes. I'll guess the Ossa had 27 degrees of rake and 85mm of ground trail (those numbers seem to be ballpark for the mid 1970s. A 21" front wheel's about 26" OD/660mm. That gives some numbers to evaluate the change.

The center axle clamps would have 74mm of offset for that geometry and you'd have a real trail of 75.74mm (real trail is the actual number that does the work, you can read about that in Tony Foale's writings). If your leading axle forks had 35mm of lead (I'm figuring 35mm stanchion with 10mm thick sliders and the 15mm axle right up on the leading edge of the slider - 17.5 + 10 + 7.5 = 35 so they may have had even more than that but that looks close to the way the Bultaco forks were in a photo that is handy) your new offset is now 109mm. That gives ground trail of 45.8mm and real trail of 40.8mm.

I think it should have been pretty easy to have the front end knocked off line if it hit a rock -- you'd have to hold on pretty tight as it wouldn't have as much self-centering available. If you were just riding straight ahead on fairly smooth dirt at a slow speed you might not notice that effect very much. Your small rearward change in the weight distribution might also make it a little more "nervous" in the rocks.

Moving the wheel 35mm or so forward moves a big part of the steered mass forwards too. That's the issue I was discussing above.

It looks to me like much of the nimbleness/quick turning on a trials bike may be more in the wheelbase than in the steering geometry. 48" wheelbase bikes will easily make a turn where a 55" wb bike doesn't stand a chance. For a given radius of bend a larger wheelbase means you've got to turn the forks more to make it. If the forks are already at the stops you are out of luck. I just looked in a copy of Tony's book and he even mentions trials bikes in the section on wheelbase "A practical consideration for trials bikes is that, for a given degree of steering lock, the minimum turning circle is smaller with a shorter wheelbase. For this reason, trials machines have wheelbases as short as 49-50 inches." Gaunt's little Jawa was reported by Morely to be 50.5", and the 123 Cota as 49.6".

Your experience is why I tried to point out that you often can't change just one factor without having other things change too, and that sure muddies the waters when trying to determine what effect a change has. My previous post was addressing strictly the different placement in mass in leading axle vs center axle forks as I've seen comments over the years that the leading axle are supposed to be "better" by having a lighter feel to them and it seems to me that we're talking pretty small changes in mass locations and and normally fairly slow and smooth steering movements so you don't have a lot of momentum being built up and slowed down. To evaluate only that factor by going out and riding you need to keep wheelbase, rake, trail and overall weight distribution of the bike constant.

You could have had the same experience on the Ossa by keeping the center axle forks and putting on a set of clamps with extra offset equal to the offset in the sliders of the forks you tried with the Ossa clamps. What you did was make some very significant changes in the steering geometry and wheelbase (and a little on weight distribution) and was quite a different experiment from what I'd proposed. It was interesting to hear your comments.

As has been noted, you can't just cut and shut and change the steering head angle without changing the wheelbase and possibly also raising the front end from the steeper forks. One change leads to another. If you want to keep wheelbase the same while steepening the rake you need to cut off the steering head and rotate it forward (and away from the frame) around the front axle, unless you are planning on doing a cut and shut and then lengthening the swing arm to get the wheelbase back. And once you've done that you've moved the engine/footpegs closer to the front wheel and changed the weight distribution too. You've also changed the trail when you changed the rake and retained the same forks and clamps.

I made my comments because it is clear that a lot of people don't have a good understanding of this (I won't claim to be an expert, but it is something that has interested me for decades) and they often tend to focus on one thing that they can see and say "there's the difference." Leading axle vs center axle is one of those "oh look, that's different, that must be it" things. But if both sets of forks put the axle through the exact same path from full bump to full rebound you've made zero change in the steering geometry. Swapping leading vs center axle fork tube assemblies around in the same clamps will make a very significant change.

It is sort of like how people will say "my frame is so light because it is made from 531 or 4130" when those steel alloys have the same mass as mild steel. It isn't the type of steel that makes the frame light, it is having less of it used to make the frame. But they see a "531" sticker and figure that obvious difference has to be the reason.

cheers,

Michael

All sounds very complicated................are you saying modern steering angle is worth doing on a t/s bike, or no not unless you also have modified fork yokes to alter the trail at the same time?

[paioli]

Grinder+brickwall+home made guage+mig= better stearing, works every time for me !

no x=y-x+$ to the power of 6 just a bit of trial and error and picking the brains of a bloke

whose been building twinshocks for the last 40 years !

[michaelmoore]

majestyman340, that's one of those "it depends" situations. Do you need a faux-replica of a modern bike in order to clean your twinshock sections or Pre65 sections? Is the steering geometry on a modern bike the determining factor, or does 40-50lbf less weight play a part?

"Ideal" rake seems to steepen as the years go past. Tony Foale has done some experiments showing that you need some trail, but bikes with zero rake (this was his R75 BMW being used as a test bed) were quite rideable, and it appears that getting rake down into the 10-15 degree range can have some benefits. You can see an article about that at http://www.tonyfoale.com/Articles/RakeEx/RakeEx.htm But with a bike with a regular steering head it gets really awkward to get the steering head attached in a position to allow those steep rakes.and there are photos in the article that show how far the steering head has to move forward. Also note that Tony comments that with steeper rake the bike is less sensitive when dealing with ruts, and that may be a definite factor in steeper rakes for trials bikes.

Are 2-3 degrees less rake going to make you suddenly clean every section you ride? I wouldn't expect that if I were the rider. I think that getting enough practice and skill so that we can ensure the wheels are exactly where we want them in a section, instead of just within +/- 12", is far more likely to improve our scores than changing from steering geometry that has proven to be "good enough" for many people. If you are converting some bike that originally came with 32 degrees and 5" of trail into a trials bike, then I'd think that yes, deraking it down into the 26-27 degree range is probably going to be a good move. If you are starting with something that was pretty well designed to be a trials bike you are probably going to start seeing a case of diminishing returns.

What I am trying to say that if you make a change to one thing a person needs to consider all the possible ramifications and then decide if they also need to be addressed at the same time. If you change the rake you've now got a different trail number if you didn't change clamps. If you moved the front wheel around while changing rake (or changing the clamps) you now have a different wheelbase and weight distribution.

Does changing just the rake and letting all the other factors end up where ever they happen to end up make things better? Beats me. Like Dirty Harry said in the movie, you've got to ask yourself "do you feel lucky?"

When I build a new frame for my KT250 or my TTY400 projects I'll probably just use 26-27 degrees as the different forks I've got were all designed with offsets for that kind of rake, and I won't have to (at least at the start) have to make new triple clamps too. And for a pre 1978 class bike (AHRMA Modern Classic) the front end of the bike will look period with that kind of fork rake, where if it had 21 degrees it wouldn't.

I'd find it interesting to do some experimentation to try and determine what changes do have a significant effect, and whether that effect is good or bad, and how that change interacts with other changes. Footpeg and handlebar positions seem to have noticeable effects without changing the frame around. Engine position (up/down, fore/aft) can have an effect without changing steering geometry. Mix four or five things up and who knows where you'll be, or if you'll be able to say "oh, it was obviously the steering rake change that made the bike better, and the changes to foot pegs and wheelbase and engine location had no effect."

But I'd do that kind of experimentation for the fun of it, not because I expect it will make me an expert trials rider. It may be that the experimentation would lead to some conclusions like "the footpegs are better off at the leading edge of the rear tire and 12" off the ground instead of 3" behind that point and 3" higher" and I could modify my bike accordingly and make some small changes where the original designers don't seem to have gotten things right.

I'd be loathe to make any blanket recommendations like "every trials bike in the world should be modified to have 22.5 degrees of rake, and you should sacrifice any other dimension needed to arrive at that rake angle."

Don't forget too that different people like different things. Some people love to slide a MX bike around a course and go just as fast as someone who loves to square off every corner. Figuring out what YOU are comfortable riding and what works with YOUR riding style is important.

Paioli, trial and error can definitely get you there. But knowing what counts as "error" so you can avoid trying it is likely to get you there quicker.

cheers,

Michael

Edited March 6, 2008 by MichaelMoore

[andy m]

It was all getting a bit technical until Paioli's post then it came back down to earth. Nice one, no nonsense modifications. Grinder + brickwall

No doubt picking the brains of J B.

[michaelmoore]

Cheetah (Gollner?) seems to have offered a disc brake on a trials bike in the late 60s from articles I've seen.

FYI, a copy of the first "Twinshock" book just arrived and there is a photo of the 1967 Cheetah works team (Mick Whitlock, Chris Cullen and Arthur Dovey) with all of their bikes having front disc brakes.

cheers,

Michael

[Guest majestyman340]

Grinder+brickwall+home made guage+mig= better stearing, works every time for me !

no x=y-x+$ to the power of 6 just a bit of trial and error and picking the brains of a bloke

whose been building twinshocks for the last 40 years !

Would your friend who has been building t/s bikes for 40 years be interested in modifying a std Godden Majesty chassis? Not sure I like the brickwall idea that much, and no problem paying to have the frame done in a proper jig.