Topic: 74 CB750 USD and Mono build

[chowpay]

Hi Guys

I'm extending a stem off a GSXR 1000 2005 to do the fork swap on my 74 CB750

To do this I made a plug out of 6061 alloy :


V-grooved both the plug and the stem pieces, tig'd it up filling the V and rough lathed off the excess




This was all kosher according to an engineer i've been bouncing Ideas off of.

But one thing I didn't consider.. What is the allow type of the stock steering stem? does anyone know?

Thanks for your input

[chowpay]

Hey hey,

No worries .. man I wish it was a 550 sometimes everything just bolts right up to that thing.

The welds are indeed fused. If you go this route be weary because the stem is most likely 2024 or 70xx series aluminum which is not really weldable. Or meant to be rewelded. But got an email back from my RnD buddy looking at my setup and he says it should be OK.

Well OK bothers me so what im doing is, since there is a center bung running through the stem im going to  press in a carbide rod, like a drill bit kinda on both ends , through the center bung. Aint nothing going no where then.

Ah and here's the triple btw, its off a 2005 GSXR1000



*Be sure to support the closest point of where you're actually pressing it out. I got lucky, probably because I was going so slow, that nothing got distorted (bent or cracked). if you're gonna do this support the triple.*





Let me know how your project goes too.

[754]

7075 is considered hard to weld, what did you use for a plug material .. One member on here Elan replaced his with a new one made from 7075.
 What bearings will you use on that stem?

[chowpay]

@747:
hi, I made my plug out of 6061
bearings are from allballsracing just picked it off the list from their fork conversion sections.  The 7000 and 2000 series its not "hard to weld" persay.. I didnt think it was anymore difficult at least not on the tig. The problem is that even if your welds are fantastic the grade alone is prone to cracking. The biggest loads on a steering stems are in shear, not tensile so I think with the added pins it should resolve the issues.

@Cali:
Hi, the pins were there less for the spinning. I was concerned that when the top nut is tightened it would pull hard on the stem and cause it to crack. The pins were there to relief  the stress of the pulling.

It wasn't too much effort to extend the stem, probably about .. oh I say 4hours of actual work. Def much much less that lathing a new stem from scratch. I have gone through and remade a few parts form scratch instead of modifying existing though. The steel in these CB's isnt exactly fantastic haha.

I had to weld some cast iron stuff for my cousin so I havent had many updates but I will post the fork conversion as soon as its complete. I was missing 3 stem nuts off the gsxr which I found on ebay. Once that arrives I'm hoping it'll all just bolt up.

Thanks for your interest, here are the updates on this project that I havent included on the thread:

The swing arm seemed so short on the 74 , for the mono shock setup I wanted to do, I decided to swap in a later model .. i think its a 79
Swap was pretty easy I just had to mill the long end to fit the frame and lathe off the inner race:



On to the mono shock

I noticed not many people post up how to make the top link ... its kinda important. After checking out a few bikes I went with this. Its DOM milled with a 3deg angle to fit into the CB750 frame



I'm using a donor shock from a 96 CBR1000. Which I know has a linkage. Now I can't replicate the linkage with the stock swing arm but talking with one of the RnD guys at honda he suggested as long as I keep the correct ratio it should work fine. To get this ratio down I made a mock strut:

Match the eye-to-eye


Match the travel length


Add a couple of 10mm lines and tada we have mock strut



Figure this will take the guess work from when I get to welding the upper swing arm support

[Tews19]

That is great work... Subscribed

[70CB750]

Parameters of the DOM, please?

[FrankenFrankenstuff]

Dang that looks like a lot of welding and clean up for the top tube hookup on shock. A lot of it. Under your balls. Too bad it can't just sneak under and forward, that way your seat pan isn't up up from frame rails.

I never trusted the stock swing arms for mono alterations. Are you finding pivots and pressure points sufficient? This ain't no 550.

[chowpay]

@70CB750
OD 25.25
IN 18.96

@tew thanks!

@calij
Thanks!

My RnD friend is really cool, he's retired and use to work on all sorts of neat projects. Great fab guy too and lots of great stories.

I was debating on hiding the oil tank but I couldn't find a good spot since I still want to have an electric start. He suggested I use a CRF150 battery. Measured it up and it actually fits in the most forward cavity of the frame under the tank. As long as I have some good cables to the starter there should be just enough amp to turn over the motor.

Converted a 3rd room of the house into a small workshop. It's connected to the kitchen (aka beer room) and the garage.  It's small though so I can only have active projects in the room



@frank
You're right the 750 is a pig compared to the 550
As for the seat pan  I hope not to have too many issues as I'm using the benji seat which has a cavity over the rails. Modifying it a bit as I want to shorten the rear:



The top mount shock tube is at close to the strongest place for a mono shock, at the end of the spine. So the pressure should get transferred right into to the frame. Plan is to make the tabs and weld it to the tube first then weld the piece to the frame. Not too much welding is required..surpringly.

I plan to weld the top loop on top of the stock swing arm at the dropouts so the pressure should be the same as the dual shock. The only other point of pressure is on the pivot end where it connects to the frame. That should be negligible as the weight is really transferred into the shock and the frame. And the pivot end of the swing arm is also really thick.

I realized I need to get the forks mounted first to get the right stance before I work out the angles on the mono shock mount. Soooo in the mean time I started working on the hide away oil tank.

[Terry in Australia]

G'Day Mate, what's the difference in length between the GSXR forks and the CB750 forks? Also, what size wheels are you going to use?

A common mistake newbies make is using short forks and little front wheels, while jacking up the back, which makes for God-Awful handling and stability.

Of course, if you lower the back to make up for the front end drop, you'll be able to remove your sidestand and just use your alternator cover on the left, and your points cover on the right. Have fun! Cheers, Terry.    

[70CB750]

Thank you, Chowpay, neat project!

[chowpay]

@ Terry  ROUGH measurements (I the fork isn't on the bike so im guestimating  the the lowest point of the stem to the axel)
about 5MM

@Calij, interesting.. debating about going with the lion route the size is pretty good.

Couple of updates for the night.

Starting making the oil tank today now that I know I can relocate the electronics under the gas tank. This cut out 6061 aluminum.

After tracing it out of some cardboard and matching it to fit snug under the beji seat I welded a 1/2 inch bar to keep the shape (I know .. I know..I'm half way started the door before I got too damn hot.. i'll finish it in the winter.. thats another project since its gotta be large enough to fit a motorcycle):


Tacked on the back side


Almost started welding this up before making the piping! (almost did a doof)



Here's how it sits (the PVC swing arm I started on to get the angle for the mono shock.. still waiting fork parts before I can set that up):

 

[Terry in Australia]

Excellent work mate, that won't burn your butt will it? If you haven't ridden a CB750 much, it's not hard to burn you leg on your oil tank if your sidecover is missing, with it full of oil nudging 300 degrees F.

Now your forks are only 5mm shorter than stock, so with the 17 inch front wheel you'll have a drop of around 30mm, (maybe more with a lower profile tyre) what is the offset? I imagine it's not much, so with the stock triples, you're gonna have a big stability problem.

When the PO of my Gold K2 built my bike he attempted to use the stock Yamaha FZR1000 triples, but found that even with the 18 inch front wheel the front end "tucked under" in corners, and almost spat him down the road before he machined up some better triples with close to OEM (CB750) offset, and geometry.

Luckily for me he's an old race bike engineer, and he set it up so well, it's the best handling CB750 I've ever ridden. Cheers, Terry.

[chowpay]

Thanks dude, I hope the oil tank wont be a heat issue either. But it does  sits under the cafe rear hump with a good 1" around from touching anything.



I haven't decided on what wheels to go with to help with the offset issue. I've heard good things going over to the 19", the only issue is slight rub under big compression. I think I can get around that issue too.

I like those triples you have on there thats pretty sweet!

[Terry in Australia]

No worries mate, I remember during the "Biker Buildoff" Jesse James built an oil tank into his V Twin Cafe Racer seat and the concern was that he wouldn't be able to ride far before he burned his butt, but I'm wondering if you could insulate your tank from your seathump with some of that Dynamat insulation material that the hotrod guys use, or something similar?

Yeah, those "Slab yokes" are very nicely made and have really cool triangular shaped hollows cut out underneath so they're actually very light. I asked him for a price to make a set for another member here, but said member nearly fainted when I passed on Ken's quote of $1500.00......... Cheers, Terry.  

[Terry in Australia]

Yeah mate, good point, personally I'm more worried about burning my butt than wrinkling my paint, but either way, it needs to be addressed. Cheers, Terry.

[chowpay]

Got some more work done on the oil tank today. Nothing picture worthy just tedious bolt taping and making mounts on the frame. I realized i need to order more 8AN (or 10AN) hose weld-in's for the tank.. I may go the dynomat route for the insulation. Actually what I'll probably use is this roofing material. Its like poorman's dyno mat, from what I can tell it works just as well and even looks the same. (Foil colored on one side and sticky tacky #$%* on the other)

BUT

I did get a chance to knock the forks in (they are backwards because they got in the way when I had it set on the floor):





Interesting thing about the lower bearings, I've always seen people heat the the bearing and whack it with hammer. Instead I tossed the lower triple in the freezer and headed off to get a right size pipe to press in the race. Turns out as soon as I dropped the race on after pulling the lower out of the freeze it fell right to the bottom. A few light taps and it bottomed out. What a nice surprise.

[754]

You do know that aluminum grows or shrinks at a greater rate tha steel..right

[SOHC4 Cafe Racer Fan]

Got some more work done on the oil tank today. Nothing picture worthy just tedious bolt taping and making mounts on the frame. I realized i need to order more 8AN (or 10AN) hose weld-in's for the tank.. I may go the dynomat route for the insulation. Actually what I'll probably use is this roofing material. Its like poorman's dyno mat, from what I can tell it works just as well and even looks the same. (Foil colored on one side and sticky tacky #$%* on the other)

BUT

I did get a chance to knock the forks in:






Interesting thing aYourbout the lower bearings, I've always seen people heat the the bearing and whack it with hammer. Instead I tossed the lower triple in the freezer and headed off to get a right size pipe to press in the race. Turns out as soon as I dropped the race on after pulling the lower out of the freeze it fell right to the bottom. A few light taps and it bottomed out. What a nice surprise.

Your forks are mounted backwards!  You will need to take them out and reverse them left to right, too.

[chowpay]

haha yes I noticed that after, I had it on the floor at first so it got in the way

[SOHC4 Cafe Racer Fan]

haha yes I noticed that after, I had it on the floor at first so it got in the way

Just a bummer that you need to take them out again.  Remember you will need to swap the tubes left to right -- you cannot simply turn them to the rear as they will not match up correctly.

[chowpay]

Ah thats no big deal at all I just put them on to snap a photo. Even when the whole bike is put together I have to pull it all apart again to get the frame painted. Wont know what final tab positions should be, right ride height etc etc till I toss it all together.

[Jerilee]

The pore mans dynomat http://www.lowes.com/pd_154017-81326-PS625_?fromStore=true
I had been 'restoring' (un FUBAR-ing) an 82 El Camino and the folks in those circles have been using this stuff and swear by it. I went with this brand because, at the time, it was rumored to be *the* one.

I had put it on the roof, cab-rear and door panels and inner skins and it *did* help a with some vibration issues and heat... A true heat-shield? I can't say, but it sure was better than bare metal in the summer with a black roof while I had no headliner!

It sometimes has a little trouble with vertical surfaces or moist areas. But, it stuck inverted to my roof like a champ through last year's 100+ degree days -in the sun- last year and a winter, spring and another summer in 'storage'. As well as to the doors and cab-rear after some initial fuss. My 'truck' leaks like sieve in heavy rain so water would get under the adhesive goop, I highly doubt you'd have that problem here.

Hope that helps!
Fun build to watch, good luck!
- Jeri

[SOHC4 Cafe Racer Fan]

Ah thats no big deal at all I just put them on to snap a photo. Even when the whole bike is put together I have to pull it all apart again to get the frame painted. Wont know what final tab positions should be, right ride height etc etc till I toss it all together.

I really like how the seat is taking shape.  Nice job! I'm also impressed by your creativilty in using the PVC pipe to mock up the shock. 

[FrankenFrankenstuff]

I like the loom of your oil tank better than the Benji seat. Tack some cooling fins down the back and call it a day. IMHO.

[Retro Rocket]

what is the offset? I imagine it's not much, so with the stock triples, you're gonna have a big stability problem.

Its actually the opposite Terry, less offset means more trail which adds stability but makes steering slower making the bike feel like its falling into corners...  I used to think it meant less trail therefore less stability but if you look at the way trail is measured, smaller offset clamps bring the forks closer to the steering head so the measurement down through the axle in the vertical plane is the only measurement that changes. The line through the center of the frame neck to the ground sits forward of the axle so if the vertical line that determines trail length is moving backwards towards the front of the bike due to the smaller offset , trail increases, making the front end less responsive to rider input..



Forget what it says at the bottom about "typical values" thats wrong, it is more complicated than simply saying something is typical, rake and wheel size also come into the equation...