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Offline CR750

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'Period' Front end swap on CB750 classic racer ?
« on: January 04, 2017, 08:59:15 am »
I have been doing more and more trackdays last two years with my '94 VFR750 and a Yamaha Tr1 classic racer of a buddy of mine, and now want to move into some classic endurance racing next year with a classic CB750 K7 or F2 ( have both these project bikes).

Besides the specific rules to the class I am thinking of participating in I was wondering about an 'period' upgrade that would be accepted in most classic racing series for a CB750 in Europe.

Besides modding the original cb750 with preload and valve tech emulators and progressive springs, I can think of taking the thicker 37mm GL1000 forks (I am also using in my CR replica for the looks) and doing the same mods.

Anybody else got any experiences with more or less 'period' forks... so no UPSD forks etc ! ;-)
I am interested in getting the best performance package for the alternative front end (and maybe using the brakes as well) while still being able to participate in classic races.
 
« Last Edit: January 04, 2017, 09:05:48 am by cr750 »
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Offline simon#42

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #1 on: January 04, 2017, 11:05:01 am »
if you want something period that will be eligible in europe the gl1000 forks would be a good bet , if you are a serious racer though i would forget emulators and have some proper cartridges fitted . these will bring the forks bang up to date and give you plenty of adjustment .
 
 
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Offline livefast_dieold

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #2 on: January 06, 2017, 04:34:21 am »
I'm running with a front end taken from a Guzzi T5 made by Bitubo, 40 mm diameter, with Brembo calipers, and I couldn't be happier with the set up! You need to get some good cartridges inside and you are good to go!
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Offline SOHC4 Cafe Racer Fan

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #3 on: January 10, 2017, 10:23:55 am »
I didn't race mine, but I had a CB750K7 with a GL1000 front (shortened fork tubes, Racetech gold valve emulators and springs) and preload adjusters.  It definitely felt more planted in corners (and the GL brakes were a big improvements paired with twin EBC rotors). 

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1975 CB550K1 "Blue" Stockish Restomod (http://forums.sohc4.net/index.php?topic=135005.0)
1975 CB550F1 frame/CB650 engine hybrid "The Hot Mess" (http://forums.sohc4.net/index.php/topic,150220.0.html)
2008 Triumph Thruxton (http://forums.sohc4.net/index.php/topic,190956.0.html)
2014 MV Agusta Brutale Dragster 800
2015 Yamaha FZ-09 (http://forums.sohc4.net/index.php/topic,186861.0.html)

"There are some things nobody needs in this world, and a bright-red, hunch-back, warp-speed 900cc cafe racer is one of them — but I want one anyway, and on some days I actually believe I need one.... Being shot out of a cannon will always be better than being squeezed out of a tube. That is why God made fast motorcycles, Bubba." Hunter S. Thompson, Song of the Sausage Creature, Cycle World, March 1995.  (http://www.latexnet.org/~csmith/sausage.html and https://magazine.cycleworld.com/article/1995/3/1/song-of-the-sausage-creature)

Sold/Emeritus
1973 CB750K2 "Bionic Mongrel" (http://forums.sohc4.net/index.php?topic=132734.0) - Sold
1977 CB750K7 "Nine Lives" Restomod (http://forums.sohc4.net/index.php?topic=50490.0) - Sold
2005 RVT1000RR RC51-SP2 "El Diablo" - Sold
2016+ Triumph Thruxton 1200 R (http://forums.sohc4.net/index.php/topic,170198.0.html) - Sold

Offline CR750

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #4 on: January 10, 2017, 12:31:51 pm »
Do you remember how much you shortned them ?
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Offline CR750

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #5 on: January 10, 2017, 12:34:38 pm »
Livefast; are you racing ? Those are 35 mm like the originals right  ?
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Offline SOHC4 Cafe Racer Fan

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #6 on: January 10, 2017, 12:46:11 pm »
Quote from: cr750 on January 10, 2017, 12:31:51 pm
Do you remember how much you shortned them ?
I believe it was 1.5" shorter. 
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1975 CB550K1 "Blue" Stockish Restomod (http://forums.sohc4.net/index.php?topic=135005.0)
1975 CB550F1 frame/CB650 engine hybrid "The Hot Mess" (http://forums.sohc4.net/index.php/topic,150220.0.html)
2008 Triumph Thruxton (http://forums.sohc4.net/index.php/topic,190956.0.html)
2014 MV Agusta Brutale Dragster 800
2015 Yamaha FZ-09 (http://forums.sohc4.net/index.php/topic,186861.0.html)

"There are some things nobody needs in this world, and a bright-red, hunch-back, warp-speed 900cc cafe racer is one of them — but I want one anyway, and on some days I actually believe I need one.... Being shot out of a cannon will always be better than being squeezed out of a tube. That is why God made fast motorcycles, Bubba." Hunter S. Thompson, Song of the Sausage Creature, Cycle World, March 1995.  (http://www.latexnet.org/~csmith/sausage.html and https://magazine.cycleworld.com/article/1995/3/1/song-of-the-sausage-creature)

Sold/Emeritus
1973 CB750K2 "Bionic Mongrel" (http://forums.sohc4.net/index.php?topic=132734.0) - Sold
1977 CB750K7 "Nine Lives" Restomod (http://forums.sohc4.net/index.php?topic=50490.0) - Sold
2005 RVT1000RR RC51-SP2 "El Diablo" - Sold
2016+ Triumph Thruxton 1200 R (http://forums.sohc4.net/index.php/topic,170198.0.html) - Sold

Offline 754

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #7 on: January 10, 2017, 03:46:41 pm »
Why did you shorten them ?
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Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline SOHC4 Cafe Racer Fan

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #8 on: January 11, 2017, 03:28:19 pm »
Quote from: 754 on January 10, 2017, 03:46:41 pm
Why did you shorten them ?

The GL1000 forks are about 2" longer than the 750 forks.  I didn't want to change the stock 750 geometry or have excessive portions of fork tube sticking out the top of my triple tree.  Instead, I chose shorter tubes and had RaceTech provide a spring based on the new fork tube length and weight of my 750.
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1975 CB550K1 "Blue" Stockish Restomod (http://forums.sohc4.net/index.php?topic=135005.0)
1975 CB550F1 frame/CB650 engine hybrid "The Hot Mess" (http://forums.sohc4.net/index.php/topic,150220.0.html)
2008 Triumph Thruxton (http://forums.sohc4.net/index.php/topic,190956.0.html)
2014 MV Agusta Brutale Dragster 800
2015 Yamaha FZ-09 (http://forums.sohc4.net/index.php/topic,186861.0.html)

"There are some things nobody needs in this world, and a bright-red, hunch-back, warp-speed 900cc cafe racer is one of them — but I want one anyway, and on some days I actually believe I need one.... Being shot out of a cannon will always be better than being squeezed out of a tube. That is why God made fast motorcycles, Bubba." Hunter S. Thompson, Song of the Sausage Creature, Cycle World, March 1995.  (http://www.latexnet.org/~csmith/sausage.html and https://magazine.cycleworld.com/article/1995/3/1/song-of-the-sausage-creature)

Sold/Emeritus
1973 CB750K2 "Bionic Mongrel" (http://forums.sohc4.net/index.php?topic=132734.0) - Sold
1977 CB750K7 "Nine Lives" Restomod (http://forums.sohc4.net/index.php?topic=50490.0) - Sold
2005 RVT1000RR RC51-SP2 "El Diablo" - Sold
2016+ Triumph Thruxton 1200 R (http://forums.sohc4.net/index.php/topic,170198.0.html) - Sold

Offline livefast_dieold

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #9 on: January 12, 2017, 04:13:18 am »
Yes I am racing, those are 40 mm diameter
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Offline simon#42

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #10 on: January 12, 2017, 02:15:41 pm »
i would not worry to much about the diameter of the forks if i were you ,  while the gl1000 forks are thin by modern standards they have a thick wall and are pretty stiff .
i raced plenty of tz 750's with 35mm forks and believe me it wasnt the fork diameter that was holding me back .
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Offline voxonda

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #11 on: January 12, 2017, 02:41:33 pm »


We raced both the GL front and the 1979 DOHC. Now use the DOHC, is lighter and the fork has a longer overlap. Biggest improvement is using of 20 mm axle and stabilizer. Have changed the damping and the package is really working.
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Offline 754

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #12 on: January 12, 2017, 02:55:44 pm »
I think the 20 mm axle is a big plus.. Plus the 4 bolt bottom clamps help even on the small axle GL fork..
 Steps in the right direction..
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Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline slikwilli420

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #13 on: January 12, 2017, 03:51:07 pm »
CBR rear axles for the front. Cheap, more modern materials, and hollow. I'm using one on each end of my race bike.
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All you gotta do is do what you gotta do.

Vintage Speed Parts Mashup: http://forums.sohc4.net/index.php?topic=133638.0
Rickman CR Parts Kit Refresh: http://forums.sohc4.net/index.php/topic,154837.0.html
AHRMA CB750 Racer: http://forums.sohc4.net/index.php/topic,158461.0.html
AHRMA Superbike Heavyweight Racer: http://forums.sohc4.net/index.php/topic,173120.0.html
'76F CB750 Patina Redemption: http://forums.sohc4.net/index.php/topic,174871.0.html

Offline SOHC4 Cafe Racer Fan

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #14 on: January 12, 2017, 04:31:42 pm »
Quote from: voxonda on January 12, 2017, 02:41:33 pm


We raced both the GL front and the 1979 DOHC. Now use the DOHC, is lighter and the fork has a longer overlap. Biggest improvement is using of 20 mm axle and stabilizer. Have changed the damping and the package is really working.

Killer bike, Rob.  Now you've done it.  The prices for the DOHC fronts just went up! ;)

Does that '79 use a 20mm axle or are you using a custom one?
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1975 CB550K1 "Blue" Stockish Restomod (http://forums.sohc4.net/index.php?topic=135005.0)
1975 CB550F1 frame/CB650 engine hybrid "The Hot Mess" (http://forums.sohc4.net/index.php/topic,150220.0.html)
2008 Triumph Thruxton (http://forums.sohc4.net/index.php/topic,190956.0.html)
2014 MV Agusta Brutale Dragster 800
2015 Yamaha FZ-09 (http://forums.sohc4.net/index.php/topic,186861.0.html)

"There are some things nobody needs in this world, and a bright-red, hunch-back, warp-speed 900cc cafe racer is one of them — but I want one anyway, and on some days I actually believe I need one.... Being shot out of a cannon will always be better than being squeezed out of a tube. That is why God made fast motorcycles, Bubba." Hunter S. Thompson, Song of the Sausage Creature, Cycle World, March 1995.  (http://www.latexnet.org/~csmith/sausage.html and https://magazine.cycleworld.com/article/1995/3/1/song-of-the-sausage-creature)

Sold/Emeritus
1973 CB750K2 "Bionic Mongrel" (http://forums.sohc4.net/index.php?topic=132734.0) - Sold
1977 CB750K7 "Nine Lives" Restomod (http://forums.sohc4.net/index.php?topic=50490.0) - Sold
2005 RVT1000RR RC51-SP2 "El Diablo" - Sold
2016+ Triumph Thruxton 1200 R (http://forums.sohc4.net/index.php/topic,170198.0.html) - Sold

Offline voxonda

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #15 on: January 13, 2017, 06:52:52 am »
I use 20 mm axle's from either CB900 or CBR600, both are already hollow and then shorten them to correct length and insert some M14x1.5mm thread and finally make up a very light aluminium secure bolt. Makes the front very rigid, especially under braking.
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Offline turboguzzi

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #16 on: January 13, 2017, 11:03:41 am »
hi rob, beautiful bike indeed,
about the 20mm front axle, only seen it done with a custom axle which had a machined bigger step to pull the wheel against one of the fork legs by a nut in the other end.

could be nice to see a detail pic of your solution as it sound you are using a flat axle, i dont really understand how you secure the wheel sideways
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Offline slikwilli420

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #17 on: January 13, 2017, 11:27:56 am »
I will leave Rob to explain his solution but I will offer mine as well. Once I got the CBR axle cut to length, I made spacers for either side that would center the wheel. The width of spacers and hub obviously matches the distance between the inside of the forks. I am using early GL forks with the 4-bolt caps. To assemble, I put everything together with the front nuts on the caps tightened some, but not all they way, then I can torque the axle nut to load the bearings, then finish torquing the front and rear nuts on the caps. The hollow axle is nice stiffness upgrade with likely no weight penalty from a solid one.
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All you gotta do is do what you gotta do.

Vintage Speed Parts Mashup: http://forums.sohc4.net/index.php?topic=133638.0
Rickman CR Parts Kit Refresh: http://forums.sohc4.net/index.php/topic,154837.0.html
AHRMA CB750 Racer: http://forums.sohc4.net/index.php/topic,158461.0.html
AHRMA Superbike Heavyweight Racer: http://forums.sohc4.net/index.php/topic,173120.0.html
'76F CB750 Patina Redemption: http://forums.sohc4.net/index.php/topic,174871.0.html

Offline 754

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #18 on: January 13, 2017, 12:00:44 pm »
Are you using a boring head in the mill to enlarge the axle holes .
The stock twobolt cap is not even a machinrd surface...flimsy..
 The way the betors and cerianis are made, I believe is stiffer, it wraps completely around the axle, then the slit closes a bit to clamp it up.
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Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline 754

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #19 on: January 13, 2017, 12:08:11 pm »
I believe the way Vox is clamping the axle is this way.
 Pretend a fork tube is the axle..shortened and tapped inside....then picture the fork tube cap with a much wider flange under the hex portion. When you turn the nut and flange (against side of fork leg) it pulls the axle toward the nut... Then you tighten the axle clamps, and its done . The end of axle has to be slightly down in the hole of the fork leg, if it bottoms it cant preload the bearing assembly.

 If you dont have a mill and lathe, try this cut the axle (keep it inside the fork leg) and get it threaded, should be done on lathe, unless its at tap size. Then buy a bolt that fits and use a large washer... done..
« Last Edit: January 13, 2017, 02:22:16 pm by 754 »
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Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline turboguzzi

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #20 on: January 13, 2017, 02:10:49 pm »
interesting ideas guys, but still, will be interesting to see rob's....
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Offline voxonda

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #21 on: January 13, 2017, 02:33:41 pm »
Quote from: 754 on January 13, 2017, 12:00:44 pm
Are you using a boring head in the mill to enlarge the axle holes .
The stock twobolt cap is not even a machinrd surface...flimsy..
 The way the betors and cerianis are made, I believe is stiffer, it wraps completely around the axle, then the slit closes a bit to clamp it up.

No need to drill the axle holes, they are 20 m/m. I prefer the GL clamps but that fork is much heavier.
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Offline voxonda

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #22 on: January 13, 2017, 02:46:05 pm »
Quote from: turboguzzi on January 13, 2017, 02:10:49 pm
interesting ideas guys, but still, will be interesting to see rob's....

No real good pic's at the moment but maybe this one will do. You can see that there is a big aluminium flanged bushing between leg and bearing, same on other side. These have the wheel centered. Made it a nice fit not to tight.
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Offline turboguzzi

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #23 on: January 14, 2017, 06:53:36 am »
ok, got you... the only thing is that i always like to leave one clamp free, pump the fork a few times to let it self align and then close it. with two spacers you are essentially not leaving this freedom. not a biggie, but i prefer a setup where you can snug it against one of the legs. maybe am a bit fixed over this kind of detail :)
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Offline bwaller

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Re: 'Period' Front end swap on CB750 classic racer ?
« Reply #24 on: January 14, 2017, 07:24:31 am »
I'm with TG on this. Leave any fork brace loose during his description, tighten axle clamps, then adjust brace, etc. Helps eliminate any binding/stiction. Understand Simon too, but always wise to start with as good a set up as possible. I just sent a set of legs for internal anodizing, anyone tried this?
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