Building a race bike
- Thread starter julian
- Start date
phil37
Hero member
- Location
- Lincolnshire
Simonr501
Hero member
- Location
- Lympne, Kent. UK
Great job Julian.
Have you thought about drilling some holes through the centre of the crank ends as well?
The core of of the shaft does not add an awful lot to strength, the trick is figuring out where the core ends though.
Have you thought about drilling some holes through the centre of the crank ends as well?
The core of of the shaft does not add an awful lot to strength, the trick is figuring out where the core ends though.
Hi guys,
My recconing is that 90 degrees is the ideal, have heard the arguments otherwise but my head always says what you deduct from one side you have to add to the other, and ok the firing stroke may be the "slightly" more important balance to factor in but I think not enough to make the effort to complicate the assembly of the crank and cam phasing etc. Am also using very long rods and a very short stroke, so 90 degrees won't be far out anyway. Will check it on a drawing and report back.
Balance factors.
Not fully decided yet, however we have our own crank balancing machine at Formhalls Vintage Engineering (which HCS is now owner of) so will do some real testing to see what we think is ideal. Am going to use "oiliodynamically" (nice new word eh) shaped wedge counterweights with no holes as I feel that having a big hole that fills up with oil and gets flung around etc is not the wisest proposition if you can help it. I always look at the big boys here and take advice from friends in the industry, Look at an F1 crank that they spend gozzillions on designing, If they don't do it, there is probably a reason. (I may add that I am no expert either so I could be wrong)
Rods will be Ti so rather light meaning that whatever counterweight I use, they shouldn't need to be very big. I will of course give myself room to play though.
Trouble is that at 10 to 12k rpm which is where I want max power to be, I think few have been, so I will have to suck it and see really.
Thanks for the interest,
Julian
My recconing is that 90 degrees is the ideal, have heard the arguments otherwise but my head always says what you deduct from one side you have to add to the other, and ok the firing stroke may be the "slightly" more important balance to factor in but I think not enough to make the effort to complicate the assembly of the crank and cam phasing etc. Am also using very long rods and a very short stroke, so 90 degrees won't be far out anyway. Will check it on a drawing and report back.
Balance factors.
Not fully decided yet, however we have our own crank balancing machine at Formhalls Vintage Engineering (which HCS is now owner of) so will do some real testing to see what we think is ideal. Am going to use "oiliodynamically" (nice new word eh) shaped wedge counterweights with no holes as I feel that having a big hole that fills up with oil and gets flung around etc is not the wisest proposition if you can help it. I always look at the big boys here and take advice from friends in the industry, Look at an F1 crank that they spend gozzillions on designing, If they don't do it, there is probably a reason. (I may add that I am no expert either so I could be wrong)
Rods will be Ti so rather light meaning that whatever counterweight I use, they shouldn't need to be very big. I will of course give myself room to play though.
Trouble is that at 10 to 12k rpm which is where I want max power to be, I think few have been, so I will have to suck it and see really.
Thanks for the interest,
Julian
Laverdalothar
Hero member
- Location
- Germany
Hello Julian,
Hopefully the valve drive-train and seats will keep up with what you aim for... :
Sensitive choice of valve springs, potentially a DOHC head needs to be designed for that or Ducatis Desmodromic system needs to be copied - who knows. I am just not sure I have seen an engine doing that kind of revs with the head-design of the Laverda Twins... But you seem to have the right experts around you to make it happen, so I am sure you will achieve it.
cross my fingers!!
cheers
Lothar
Hopefully the valve drive-train and seats will keep up with what you aim for... :
Sensitive choice of valve springs, potentially a DOHC head needs to be designed for that or Ducatis Desmodromic system needs to be copied - who knows. I am just not sure I have seen an engine doing that kind of revs with the head-design of the Laverda Twins... But you seem to have the right experts around you to make it happen, so I am sure you will achieve it.cross my fingers!!
cheers
Lothar
Cheers Lothar,
The idea is to use "original type" stuff, cant put fancy non production head on etc as the rules don't alow. (rightly so)
I will be (am) modifying the head an awful lot though internally, Combustion chambers completely remodelled, ports and valve positions changed etc, Valves and caps will be Ti and possibly rockers also. I'm not worried about the rpm too much as 12k is not such a high number for an ohc engine if you think about it outside of a Laverda, (my pushrod Rocket 3 runs to 9500+ as std and is FAR from fancy) and the engine doesn't know it's a Laverda, only what you tell it to do. It's down to the builder to get the instructions right.
Regards,
Julian
The idea is to use "original type" stuff, cant put fancy non production head on etc as the rules don't alow. (rightly so)
I will be (am) modifying the head an awful lot though internally, Combustion chambers completely remodelled, ports and valve positions changed etc, Valves and caps will be Ti and possibly rockers also. I'm not worried about the rpm too much as 12k is not such a high number for an ohc engine if you think about it outside of a Laverda, (my pushrod Rocket 3 runs to 9500+ as std and is FAR from fancy) and the engine doesn't know it's a Laverda, only what you tell it to do. It's down to the builder to get the instructions right.
Regards,
Julian
KERMIT1200
Hero member
- Location
- Worcester,UK
Hi Julian,
I had my crank on my 578cc Monty(180 degree) balanced to 50%.
Runs rough low down but smooths out lovely above 5500 rpm and I don't run a balance shaft.
Andy
I had my crank on my 578cc Monty(180 degree) balanced to 50%.
Runs rough low down but smooths out lovely above 5500 rpm and I don't run a balance shaft.
Andy
Thanks for the comments guys,
Marnix, I don't think I am doing too much that hasn't been done before, I belive the head in your "new" bike was also treated to quite some work? I am doing it a little diferent to that one but the combustion chambers have a similar look (although mine are in plastic metal at the moment to work out the shape and port bend etc.
Thanks for the thumbs up, please drop by if you are in the area.
Historic Competition Services
Sint Jansveld 5
2160 Wommelgem,
Belgium
That goes for anyone interested.
Best regards,
Julian
Marnix, I don't think I am doing too much that hasn't been done before, I belive the head in your "new" bike was also treated to quite some work? I am doing it a little diferent to that one but the combustion chambers have a similar look (although mine are in plastic metal at the moment to work out the shape and port bend etc.
Thanks for the thumbs up, please drop by if you are in the area.
Historic Competition Services
Sint Jansveld 5
2160 Wommelgem,
Belgium
That goes for anyone interested.
Best regards,
Julian
doing a run of 270 degree ignition boards at the moment Julian if you want one
hall effect trigger set up, can supply magnetic rotor and drive adaptor as well
have around a dozen 750 engines to twist at the moment for customers hence the run of ignitions
we used 60% balance factor on the 270 degree cranks in the past and they run pretty smooth
that was after removing 5kg of weight
hall effect trigger set up, can supply magnetic rotor and drive adaptor as well
have around a dozen 750 engines to twist at the moment for customers hence the run of ignitions
we used 60% balance factor on the 270 degree cranks in the past and they run pretty smooth
that was after removing 5kg of weight
MarnixSFC
Hero member
Well Julian I have seen all kinds of variations, cranks 360, 270 and 180 degree versions and long stroke leading to 1000cc, but short stroke I haven't seen yet and think you're the first one to attempt that. They claimed that the 73 engines reached over 9000 rpm, not keen to try that with my works engine with reduced valve angle, but a 750 twin that does 11000 rpm sounds like good fun eventhough sound-wise I would prefer it to be parallel twin.
Will certainly drop by the next time I'm in the area, look forward to that. Same counts for you if you are in the Wageningen area by the way.
Marnix
Will certainly drop by the next time I'm in the area, look forward to that. Same counts for you if you are in the Wageningen area by the way.
Marnix
piranha-bro2
Hero member
- Location
- Melbourne
Also very interested in the project, Julian. Going the short stroke route is certainly thinking outside the Breganza piazza. Balance factors??? When I twisted my SFC crank I only had pommie twin twisters as a guide and they all seemed to go for 50% - so 50% it was. Unfortunately high revs was where it disappointed me the most.
I theorise that the MUCH wider (4-bearing crank) rocking couple of the SF twin creates a wholly different dynamic to the very narrow two-bearing brit twins. The outcome may very well be quite different with your short stroker, but adding metal once it's gone is one big fat problem compared to removing it if you start high and want to go lower. Just my due lira's worth. (My reincarnated SFC motor is going up to 60%, but with 890cc and a swag of other work - but nothing as rad as what you're doing ... and happy with an 8000rpm limit. As Phil has said his 65% also works a treat - and that was a static balance job)
There's a gun brit crank twister in Canada who seems to know what he's doing, but again, if you look at his creations, the narrowness of the rocking couple is obvious.
http://www.offsetcrank.com/
I theorise that the MUCH wider (4-bearing crank) rocking couple of the SF twin creates a wholly different dynamic to the very narrow two-bearing brit twins. The outcome may very well be quite different with your short stroker, but adding metal once it's gone is one big fat problem compared to removing it if you start high and want to go lower. Just my due lira's worth. (My reincarnated SFC motor is going up to 60%, but with 890cc and a swag of other work - but nothing as rad as what you're doing ... and happy with an 8000rpm limit. As Phil has said his 65% also works a treat - and that was a static balance job)
There's a gun brit crank twister in Canada who seems to know what he's doing, but again, if you look at his creations, the narrowness of the rocking couple is obvious.
http://www.offsetcrank.com/
From what I have read what balance factor is best depends on what frame and engine mounts is used,sending the vibs into the direction of the stiffest frame member. Seems these balance factors have a directional aspect. So different factor works for an Egli frame and not for a featherbed style/
phil37
Hero member
- Location
- Lincolnshire
Vince said:
yes Vince I built a Norbsa nearly 50 years ago and, at the same time my brother had an RGS - BSA - not Laverda! He went to get the crank dynamically balanced at a well known specialist at the time (Hartley) and, he commented that, if I had my crank balanced it would be to a different factor as the out of balance forces on the BSA A10 motor were aimed towards the top front downtubes where the Norton frame is not as strong and to keep an eye out for cracks there. Sure enough after a year or so - the frame started to crack exactly in that area! - Vibes didn't seem any worse than standard in the Norton frame!
So, as the SF has no front downtubes, this could also apply to an even greater degree with the Laverda motor. - regards - Phil
MarnixSFC
Hero member
Had an SF engine in my Egli and recently mounted a 1974 SFC engine. Both work very well and vibrations are no more or less than in the standard frames.
The CND850 with its twisted crank is a different story, vibrations are really bad in footrests and seat. Suspect the crank, but maybe the frame plays a role too. It has a very nice pretty solid looking bracket that holds the rear of the engine, by the looks of it, itis eeehm looks more rigid than what Frits Egli did.
Next step is re-balancing the crank, but not a clue what balance factor it has now..
Marnix
The CND850 with its twisted crank is a different story, vibrations are really bad in footrests and seat. Suspect the crank, but maybe the frame plays a role too. It has a very nice pretty solid looking bracket that holds the rear of the engine, by the looks of it, it
Next step is re-balancing the crank, but not a clue what balance factor it has now..
Marnix
Attachments
piranha-bro2
Hero member
- Location
- Melbourne
Dark art it is (Yoda?). IMO the heavy SF frame is essential in the std bikes, as it helps absorb a lot of the vibes - balance would definitely appear to be fore aft (on the axle plain) or perhaps at 30 deg down at the front up at the rear - which could explain why the rear frame loop ALWAYS ends up broken on stock SFs.
But a different bal factor and all the other variables (like crank out of true, comp ratio) have a considerable effect. Build it for 5000-8500 - track bike? Build it for 3000-5000 - road bike? And the twist on such a wide crank throws the rocking couple in there for good measure ... if it wasn't very effing tricky already!!
But a different bal factor and all the other variables (like crank out of true, comp ratio) have a considerable effect. Build it for 5000-8500 - track bike? Build it for 3000-5000 - road bike? And the twist on such a wide crank throws the rocking couple in there for good measure ... if it wasn't very effing tricky already!!
Hi all,
It's going to be a funny one to balance anyway as the stroke is so short compared to std, the rods and pistons very light, 270 / 450 firing, and a lot of revs.
Will be an interesting road ;o) I will do some maths and see what I come up with.
J
Red,
I think I have an ignition unit from Sachs but will need to find it and see if it's a 90 version, cant remember from the top of my head.
Will be in touch, thanks
J
It's going to be a funny one to balance anyway as the stroke is so short compared to std, the rods and pistons very light, 270 / 450 firing, and a lot of revs.
Will be an interesting road ;o) I will do some maths and see what I come up with.
J
Red,
I think I have an ignition unit from Sachs but will need to find it and see if it's a 90 version, cant remember from the top of my head.
Will be in touch, thanks
J
Piranha Brother 2 said:
Investigating a few standard and SFC cranks showed the factory went different routes for the road and competition versions. It seems the factory put the emphasis on rider comfort for the road, while the racer cranks were balanced for low bearing loads, and fuck-all rider comfort. A friend has a very handy computer program to work out bearing loads, piston forces, etc. Both tested SFC cranks worked out to 58%, road cranks averaged 70%, about +/-3% there. I'm quite comfortable with my results using 64% (Based on a very smooth-running GT I once rebuilt. The crank of this bike had been overhauled and balanced by a Herr Staschel from Hamburg many years before and was simply worn out. He was renowned for his efforts with Ducati and Manx Norton cranks, sadly passed away several years ago.).
"Unfortunately", the crank in the '73 SFC (here we go again!) was perfectly aligned and felt to be OK to be re-used, so I didn't bother pulling it apart, and I've no idea of balance factor, etc. The surprise came when the bike fired for the first time, it is one of the smoothest SFCs I've come across! There is also no heavy frame to absorb the vibes either. Frame, made from thin-walled cro-mo tubing, weighs in at about 9-10kg.
piet
motoddrob
Hero member
- Location
- S/W Australia
sfcpiet said:
That's a light frame Piet. Is it the same basic configuration as a standard SFC frame? Put the Motodd frame on the scales yesterday after receiving it back from repairs, 15.7kg.
The engineer who did the repairs reckon I'd save another kg if I put less paint on it!!! Probably did go overboard with the paint when last repainted.
Dellortoman
Hero member
- Location
- Tasmania, Australia
Piranha Brother 2 said:
Not quite sure what you mean by that Quentin. I presume you're referring to the angle that the 750 cylinders are leaning forward (looks about 30?).
Out of balance forces are either in line with the cylinder or at 90? to the cylinder and crank axis, depending on the balance factor. Assuming for the moment that the cylinder is vertical, 0% balance factor and all forces are in the direction of the cylinder - up and down. 100% balance factor and the forces are horizontal - fore and aft. Tilting the cylinder obviously tilts the out of balance forces.
Note that the maximum vertical and horizontal forces can't simply be added in a vector diagram because they occur at different times in the engine cycle. They need to be plotted against crank angle. What you end up with is a resultant rotating force of varying magnitude for each cylinder. For multiple cylinders, you have multiple rotating forces of varying magnitude. These separate forces create rocking couples which add another dimension to the whole balance issue.
The other complicating issue is trying to work out how the surrounding structure (frame) reacts to these forces and how it will feel to the rider.
You can do a lot of head scratching trying to analyse all this stuff on the drawing board, but achieving a good result usually comes down to trial and error.