Author Topic: USCRA Supervintage (900cc limit) cb750 based road racer build, ideas?  (Read 10614 times)

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Offline gschuld

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I am far from committed on things like the seat, bars, rear set make(if not custom) and position.  Luckily Haybus is 6'6" tall and a little heavier than I am so I am very interested in seeing what works for him and for bigger guys in general on cb750s.  Road racing a bike is very new to me.  I have developed no good or bad habits on modern bikes since I have made it a point not to ride anything newer than the mid 70s.  I have ridden CB sohc4 for 20 years, but not on a road race course...  I am looking forward to it.

George
« Last Edit: May 16, 2015, 04:44:47 pm by gschuld »

Offline Haybus

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George, get ready to be hooked. My biggest regret is not doing this earlier in my life. But also be prepare to undo all that you do. My wife asked me last week if I was going to tear my bike apart after every race weekend. I hope not, but tinkering with the bike is currently an addiction I can't shake!!

You're not on the bike for long periods at a time, so comfort isn't all the important. However, you wouldn't want to be so cramped you couldn't move around. I made attachments to the frame that allow me to change the location of my pegs. I built a dummy plate with 16 holes so I could move the pegs around and see what would work. I sat on the bike in my leathers for quite some time. But that still doesn't really tell you how comfortable it will be when racing. My point is, try to set it up so you could change it if needed. I, like many others I'm sure, used Brent's thread for inspiration. This is not a knock on Brent in any way, and I liked his foot peg idea, but I was worried about how I would change it if I couldn't tolerate the position, even if only for 5 or so laps.

I thought once I had it all together I would just race. Ha, was I wrong. At least so far.

Alan
75 CB750
76 KZ900

Offline Old Scrambler

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Regarding setting up the chassis................After playing with my first salt-flats racer set-up, I copied the comfortable fit to start my second build on the CB750 chassis.............I started with the same back-of-seat to stem-nut at the neck, and the seat-to-footpeg measurements. The result was a very comfortable tuck with the 'chin' of my helmet about 1-inch from the tank.  At your height, I would think the seat-back would be several inches behind the upper shock-mounts. I have noticed that some riders like a relatively flat seat, while others prefer a rounded seat-top.
Dennis in Wisconsin
'64 Triumph Cub & '74 Honda CB750 Bonneville Salt Flats AMA Record Holder (6)
CB750 Classic Bonneville Racer thread - http://forums.sohc4.net/index.php/topic,135473.0.html
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Offline gschuld

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George, get ready to be hooked.

I thought once I had it all together I would just race. Ha, was I wrong. At least so far.

Alan

Yeah I know.  I have put this desire off for a while, we'll see where it goes.  If I really get hooked and want to stay in the USCRA heavyweight supervintage class and want to be competitive in it, I will likely need to build a very custom, hard core, high caliber race bike.  I'm not certain if my desire will go that far.  Otherwise, I'd have to travel more ...much more.

As far as making constant changes, I was hoping that you would solve all my problems and set a cb750 up perfectly for us big guys so all I had to do was pilfer your setup and I'd be good to go ::).     ;D

I have been thinking along the lines of making things like the peg positions and rear shock height easily at the track adjustable.  Perhaps I can get/make a fiberglass race seat and make several shapes and sizes of cushions or foam padding to be able to easily and quickly trade them out at will to experiment with on track days.  Set up a range of seat positions based on the ranges I find on successful cb750 racers.

George

Offline Haybus

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Having your seat back behind the shock mounts might look kind of weird, like you're sitting in the back seat  ;). But everybody is built different. I have really long legs and arms, so I need my seat back a ways. The back of my seat is about even, or slightly ahead of the shock mounts. About where the seat should be if I had a race tank I think. It's really comfortable for what it is, except the foam is hard as a high school bleacher seat. I also spent quite a time pondering how far back and how high to mount the seat. Being tall, I would suggest you spend some time figuring that out because it's a little harder to change later on. There's a lot of design elements you have to consider all at once, so take your time and try to mock them up however you can. Your peg height affects your seat height, but it also affects your exhaust and linkages. You want a low seat height, but you want your pegs high enough they don't drag the corners. It's kind of a challenge for us tall guys. For reference, my pegs are 12" and my seat 31" off the ground. My pegs are about even with the front of my back tire. But this is just what works for me so far.

If you really get hooked? Ha ha, that's a good one George.

Alan
75 CB750
76 KZ900

Offline Retro Rocket

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Successful Aussie 750 racer... ;)

750 K2 1000cc
750 F1 970cc
750 Bitsa 900cc
If You can't fix it with a hammer, You've got an electrical problem.

Offline Haybus

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Notice the upper shock mounts have been moved forward on the T-Rex.
75 CB750
76 KZ900

Offline Retro Rocket

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Notice the upper shock mounts have been moved forward on the T-Rex.

Yes, thats something Pops Yoshimura did on a lot of his race bikes, the Yoshimura GS1000's are a good example..., I would love to know what the preferred angle is and its effect on the rear suspension...?
750 K2 1000cc
750 F1 970cc
750 Bitsa 900cc
If You can't fix it with a hammer, You've got an electrical problem.

Offline bwaller

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It allows more wheel travel, but less shock travel so damping/spring rates can change. The angle grows with travel, I've read 38 degrees is a good starting point.

Offline Tintop

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Weren't the KR Kawasaki's the 1st to use the 'lay down' shock config?
1977 CB550/4 Cafe - Speed Warrior / BOTM 03/11
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Whittaker GBF Vintage Racing Sidecar (XS750 power) - ITG / 151's / CMR Racing Products (SOLD)
1976 CB400 SS - stock / BOTM 04/11 (SOLD)
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Offline Retro Rocket

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It allows more wheel travel, but less shock travel so damping/spring rates can change. The angle grows with travel, I've read 38 degrees is a good starting point.

Thanks Brent...
750 K2 1000cc
750 F1 970cc
750 Bitsa 900cc
If You can't fix it with a hammer, You've got an electrical problem.

Offline Old Scrambler

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G...........since your considering frames.............take a look at the CB750A.............same as K units except for the raised tubes above the motor.  Valve cover and head removal is possible with motor in frame ;) ;) ;)
Dennis in Wisconsin
'64 Triumph Cub & '74 Honda CB750 Bonneville Salt Flats AMA Record Holder (6)
CB750 Classic Bonneville Racer thread - http://forums.sohc4.net/index.php/topic,135473.0.html
'63 CL72 Project(s)
'66 CL77 Red
'67 Triumph T100C
'73 750K3 Owned since New
'77 750F2 Cafe Project
2020 ROYAL ENFIELD Himalayan

Offline JohnN

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T-Rex looks nicely done! Appears to have a boxed swingarm but I can't tell if the length has been altered. Front end has been dropped and the offset looks very short. Any further details on these items available?
CR750 replica
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Offline bwaller

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I was going to try to make contact with Rex as soon as it slows down a tad here. If anyone else has an "in" go for it.

Offline bear

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As far as I'm concerned it looks bloody dangerous,  it's a wheel short as far as I'm concerned. ;D
We have a meeting in Victoria next weekend.
Rex may be there, if not some of the blokes riding those bikes will be.

I'm happy to do a little digging.

I do know Rex has moved on from the P4 bikes and concentrates his development on the later P5 & P6 bikes these days.

What am I chasing, chassis mods?

Cheers,
Brian

The older I get the faster I was.

Offline Retro Rocket

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As far as I'm concerned it looks bloody dangerous,  it's a wheel short as far as I'm concerned. ;D
We have a meeting in Victoria next weekend.
Rex may be there, if not some of the blokes riding those bikes will be.

I'm happy to do a little digging.

I do know Rex has moved on from the P4 bikes and concentrates his development on the later P5 & P6 bikes these days.

What am I chasing, chassis mods?

Cheers,
Brian

Gday Brian, yeah, chassis mods, rake and trail, triple clamp offset, swingarm length, Frame bracing, anything related to handling.... ;)
750 K2 1000cc
750 F1 970cc
750 Bitsa 900cc
If You can't fix it with a hammer, You've got an electrical problem.

Offline bear

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Ok, I'll take my note book and pencil. ;D

Cheers,
Brian
The older I get the faster I was.

Offline Retro Rocket

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Ok, I'll take my note book and pencil. ;D

Cheers,
Brian

I was thinking that when i wrote that post.... ;D   Thanks mate... ;)
750 K2 1000cc
750 F1 970cc
750 Bitsa 900cc
If You can't fix it with a hammer, You've got an electrical problem.

Offline bwaller

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Thanks Brian. Five minutes face to face is way better than twenty emails in several directions!

Offline gschuld

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As far as I'm concerned it looks bloody dangerous,  it's a wheel short as far as I'm concerned. ;D
We have a meeting in Victoria next weekend.
Rex may be there, if not some of the blokes riding those bikes will be.

I'm happy to do a little digging.

I do know Rex has moved on from the P4 bikes and concentrates his development on the later P5 & P6 bikes these days.

What am I chasing, chassis mods?

Cheers,
Brian

Gday Brian, yeah, chassis mods, rake and trail, triple clamp offset, swingarm length, Frame bracing, anything related to handling.... ;)

Bear,

   What Retro Rocket said...   And any photos would be greatly appreciated.

George