Topic: Aftermarket frame - group purchase

[turboguzzi]

i think the main difference between a CB on 18" 110 front, 130-150 rear to a SBK on 17" 120 F, 190 R will be in overall ride and CG height as well as cornering clearance.

Surely Simon can add here...

As soon as you add some height on the back, there is a big chance of the top run of the chain to touch the swingarm, on an ideal frame an adjustable height swingarm pivot can remedy that. it also lets you change "squat" under power, but thats an advanced subject...

on all my stock framed bikes, i always reached a point where that put a limit on rake reduction

 

[simon#42]

this is a difficult subject !  but i will make a few points . unfortunately   the internet is not a great source of answers its mostly half truths and guesses .  the first thing you have to consider is the tyres , wheel size and grip levels will be major factors in geometry .  the greater the grip the more forward weight bias you are going to want . wheel size not only alters how far forward you can move the engine it also has a significant gyroscopic effect to take into consideration . next  you need to think about the swinging arm pivot point , modern engines have stacked gearboxes and are very short they also use relatively long swinging arms  this is the main reason for me that you cannot just copy modern geometry . you can get the rake and trail and the wheelbase but you cannot get the pivot point in the right place . consider why you need modern geometry anyway . a classic bike is not ridden like a supersport 600 so why copy it ?  just one more thing do not under any circumstances use the rear ride height at adjust your rake !!!   to get one thing better you mess up everything else , do not do it !!!

here is the picture i use as my avatar , this was a frame i made for a rotax engine if you look at the front tyre you can see how far forward i have placed the engine !  it still did not work very well the engine was just to long .

[livefast_dieold]

this is a difficult subject !  but i will make a few points . unfortunately   the internet is not a great source of answers its mostly half truths and guesses .  the first thing you have to consider is the tyres , wheel size and grip levels will be major factors in geometry .  the greater the grip the more forward weight bias you are going to want . wheel size not only alters how far forward you can move the engine it also has a significant gyroscopic effect to take into consideration . next  you need to think about the swinging arm pivot point , modern engines have stacked gearboxes and are very short they also use relatively long swinging arms  this is the main reason for me that you cannot just copy modern geometry . you can get the rake and trail and the wheelbase but you cannot get the pivot point in the right place . consider why you need modern geometry anyway . a classic bike is not ridden like a supersport 600 so why copy it ?  just one more thing do not under any circumstances use the rear ride height at adjust your rake !!!   to get one thing better you mess up everything else , do not do it !!!

here is the picture i use as my avatar , this was a frame i made for a rotax engine if you look at the front tyre you can see how far forward i have placed the engine !  it still did not work very well the engine was just to long .

Thanks Simon for your insights.
So assuming that modern tyres offer obviously a vastly improved grip, rake can be reduced and the engine can be moved forward which is what going to happen already using a 24° rake and 100mm trail.
As for pivot position: this is quite an issue as the engine layout make it really difficult, so I don't think there's any room for improvement here. The 750 has already a short swingarm because of this, I was planning to advance like the engine.
As for the need to ride it like a modern bike, well actually thanks to modern tyres I think the riding position it's not that different, apart from being slightly more "seated".

I don't understand your last point though, are you suggesting that under any circumstances one should NOT - for example - increase the length of the rear shock to reduce rake? Seems to me it's quite a common modification... I did it too on my oem frame.

[Tintop]

I'm taking simon#42's comment to mean - changing rake from the back also introduces changes to many other variables, so if you want to change rake do it at the front (triples?).  It's the old adage, 'only make 1 change at a time".

[livefast_dieold]

I'm taking simon#42's comment to mean - changing rake from the back also introduces changes to many other variables, so if you want to change rake do it at the front (triples?).  It's the old adage, 'only make 1 change at a time".

In that sense I agree, any change in rake needs to be balanced with a change in trail as well, if this was the sense of his comment

[simon#42]

tintop has understood me perfectly .  if you raise the rear of the bike to alter the rake it will have other effects . first it will raise the centre of gravity which will make the bike harder to roll ( lean from side to side )  it will also make the bike pitch more  ( dive under braking and squat under acceleration )  second it will alter the wheelbase ( very slightly shorter ) thirdly you will raise the swinging arm pivot which will cause the bike to squat more under acceleration ( which will alter the rake and soak up power as the engine works against the suspension ) . some riders will notice this and some wont but it will be happening regardless .

[westfieldandy]

At www.good-bits.co.uk we have been quietly working on new chassis for the smith honda F750 race bike. The new chassis is awaiting final approval for UK championship status etc, and we intend to have complete bike ready for some races next year.

The frame is available to order now. if anybody would like one. we have jigs already manufactured

For F750 rules the frame has to look the similar to original road frame, so it is twin shock etc, but the tubing used is lighter and more rigid and slightly larger in dia. we also took the opportunity to change the geometry of the chassis, and use a period looking box section swing arm in conjunction with new magnesium front and rear hubs to get rid of as much weight as possible.  This will be the best way forward to create a competitive machine, the std chassis/running gear is just not good enough to cut it with the rob north framed triples fully developed in todays championships

The frames are made here in uk by an established race frame manufacturer and are a real thing of beauty.

Frame and swing arm kit is £3600

I would love to get some out there running in f750 classic racing. and would consider some kind of support package if a good rider commits to a full championship.

andy

Andy,

Nice to hear about your frame project.  I’d love to see pictures of your work at some point.

George

[livefast_dieold]

Hi everyone!

It has been a while since my last post here, but all is good in Italy!
After COVID the Italian motorcycle federation dismantled the vintage class, unfortunately, and I lost quite a bit of interest in racing my SOHC, which has been left untouched since 2020. A few months ago, I was visiting the Honda museum in Japan, and I got bitten by the bug again. I can clearly remember thinking: "well, life is too short not to have my dream racing SOHC".

Long story short, I've ordered quite a few parts to upgrade the bike as I always wanted (970cc pistons, carrillo rods, etc.) and most notably an EGLI style frame from JL Mekkanika, some of you may have already saw some pictures Janne posted on Facebook, but here are some more!

I have to say it's really a beautiful thing, welds are nicely done, all in CrMo, and it weighs only 8.8 kg!

[grcamna2]

Beautiful work.

[Don R]

 Nice welds and fitting, I notice it's all straight tube and triangles.

[livefast_dieold]

Nice welds and fitting, I notice it's all straight tube and triangles.

exactly! There are just 2 bends in the whole frame

[scottly]

From the large heat affected zone, I'm guessing oxy-acetylene was used rather than TIG welding?

[simon#42]

That looks good .

[livefast_dieold]

From the large heat affected zone, I'm guessing oxy-acetylene was used rather than TIG welding?

It's TIG welding only

[Leino]

I'm pretty sure that's why Janne wanted to borrow one of my engines...

[livefast_dieold]

I'm pretty sure that's why Janne wanted to borrow one of my engines...

Thank you Leino!!! :-)

[Jerry Rxman Griffin aka MuthaF'er]

This will be interesting!! 

[Leino]

I'm pretty sure that's why Janne wanted to borrow one of my engines...

Thank you Leino!!! :-)

No worries, it wasn't the first time  I have a few bits and pieces so he borrows them from time to time.
Janne also made me a Bimota replica swingarm a few years back.

[Leino]

As this was originally supposed to be a group purchase type of thing, I don't mind borrowing parts if anyone is still up to it 

[turboguzzi]

Ricardo told me about this during a very nice dinner, almost chocked on my food! didnt see that coming!

Guess that much of the charm of those frames is their simplicity.

I know of a similar Segoni frame in a barn, i believe with a tuned motor next to it, maybe i should go and have a look at it again so we can have some outings together