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New Guy, Suspension Question

SenorThumpy

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Hi Guys, first post here.
Quick back story, a good buddy of mine put together a CB750 bobber on the cheap and it turned out great. I've been wanting to build a custom bike for a long while now but don't have the ching for a big twin chopper or anything like that. It had never even crossed my mind to chop a honda... obviously its a great idea. I'm looking at an 81 cb750 for cheap ($600), aesthetically it needs work but the motor seems to be fine. I've seen a few pics on google that inspired me and have combed all apparent resources to try and answer this question:

Will a 06 zx10r front end fit and have decent geometry on any year cb750 stock frame? I understand I can have a new stem machined and welded or threaded onto the zx10r triple to accommodate the cb750 neck. I tried searching the question on various forums and tried to hunt down the length of a stock cb750 front end and the length of a stock zx10r front end .... in a rare admission of ignorance, I expected an easy answer of idk 20" lets say. However i was confronted with variables of pre load and length of tubes, length of springs...etc. Can someone please point me in the right direction or break it down stupid proof for me? Sorry for such a long post, just a bit frustrated here :banghead:
Gracias,
-SenorThumpy-
 
Not sure about the zx10 front end. I was going to do a modern fork swap on my bike, I know my CBR954 forks where to short for what I was looking at doing. I think Cylce X put an R6 fork on one of there frames. Just for your information, the 81 fork will be about 1" longer overall than the older 750's. Old 750 is 30" from center of axle to top, 79 is 31" and my CBR is 27" from axle to top. Measurements taken with no weight on the forks by the way.
 
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Thanks for the info dirtdigger, thats what i was looking for.

How do you think the 750 would handle with a 3-4" shorter front end? Is there a range that works well +/- of stock length? I feel like this is a real newb question and I will certainly dig through the forum and do more research later this evening.

EDIT: I'm still curious about how the shorter front end would affect handling & such. However, one of my friends just got back to me about his kawi 636 front end... 30.5" from center of axle to top of triple. I would assume that the 636 front end would be preferable since its apparently bit softer and longer than the zx10r front end.
 
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... I think Cylce X put an R6 fork on one of there frames. ...

Yeah that was Matt from Cycle X. He had me buy the front end for him on ebay! :) I know he had to modify the lower triple tree to make it work. If I remember correctly, he just had to press out the old stem from the R6 triple, grind the weld on the CB750 triple and press that out, then turn it on a lathe and press it into the R6 triple. I can ask him to make sure if anyone is interested.

I think 3-4" shorter on the front end might be too much unless you also lower the back by almost the same amount. I guess it would depend if the trees had any rake in them. I would think it would decrease your rake and trail too much and make it not handle well. I could be wrong though, as I know a lot of sport bikes have less rake and trail than the CB750 and handle great! The people who mentioned spring rates and pre-load and all that stuff are right. It will all factor into how the bike sits and handles, but considering you want a bobber and not a race bike, anything you throw on it that's close to the same length will probably work okay. :twocents:
 
Also the forks on my FZ1 (as it sits in my garage on the side stand, stock springs and pre-load) measures 28-5/8" from the center of the axle to the top of the triple. They are very similar to the forks on an R1.

The forks on my '75 CB750 measure 28-3/4". I wonder why we're getting so many different measurements for the CB750s. Spring sag? Measurements of bikes on their center stands vs side stands?

If I were you I would figure out how much drop you want overall with the bike, and get front forks and rear shocks that are about that much shorter than what the stock bike has.
 
Thanks Travis. I'm thinking the easiest and cheapest way to set up the suspension would be to keep the stock swing arm & geometry and use a sport bike front end thats within an inch of the stock front end length.

1981_cb750-bobber_dave-jansen.jpg

This bike has inspired my plans. I'm planning on building something similar but with a few changes that speak more to my style.
It looks like a stock swing arm presumably welded at the stock frame bolted locations with a triangulated strut welded to the frame to provide the necessary reinforcing for the load usually carried by the rear suspension. The swinger looks to be welded in the stock unweighted position to retain stock geometery... atleast thats what i plan on doing to achieve this look.

Do you guys think thats an accurate statement, or atleast a good way to go about the rear end?

Is they guy who built this bike on this forum or does anyone know who he is and what forum he hangs out on? I'd love to talk to him about the hard tail job & seat set up he did.
 
I think you got it right. You could chop the back end off and do a drop-seat hardtail. That would give you more room for your battery and look less like a welded up swingarm. :twocents:
 
I measure my forks fully extended. We may not be measuring to the exact same spots on the forks either.:shrug:
 
Something ya could do is relocate the motor mounts in the frame to keep all of the juices flowing as, per say, stock dimensions regarding the engines geometrical position in concern to its economical functioning capacity; therefore, eliminating any need to fit suspension to stock existing engine location in concordance with its existing reason for positioning (by factory), essentially moving the bike through a variety of stances & achieving positive engine engagement with any possible sustainable suspension variations.
Meaning, whatever suspended platform ya place the cycle at the engine will perform properly, just as the engineers planned & decided.
Lot of work though.
 
Yeah Im pretty sure Matt used one of Cycle X frames I believe so they could easily have changed the frame dimensions to work with the shorter legs. It would be hard to make modern sport bike forks work on a stock frame but anything is possible with enough money $$ and machine time. I would get some new fork springs (something thats not 30 plus years old and saggy) maybe rebuild the forks with new oil, etc and ride the thing:bike:
 
Yeah it was a Cycle X frame. Just one of their frames "off the shelf" though... not modified for the forks.. I just talked to him today and he said that bike is going to have a 4 page spread in The Horse next month. :thumbsup:
 
I was with Matt at a bachelor party last night and we talked about putting the R6 forks on the Cycle X frame. He said the bike is really low and didn't think those forks would work on a stock frame.. too short. So if you can find the measurement of some R6 forks (2006 if I remember correctly), you'll know you need some forks at least 1-2 inches longer.

I was wrong about how he did the swap. To get the forks to work, he had a custom stem machined that was pressed into the R6 lower. He didn't use a lockring on top of the upper race, just the upper triple and a Harley stem nut.
 
Belle, i like how you think but thats indeed alot of work. If i were to get into that level of fabrication, i'd probably build a big twin chopper.

Travis, great info on the fork swap.

I think i'm going to do the drop seat hard tail with the zx6r front end (custom stem) and if i'm an inch short or so, i figure i can compensate by welding the swinger at a bit of an angle to lower the rear accordingly. Kinda backwards to build around the front end but i'm sure more ass backwards things have been done.

Now when welding the swinger up, i assume its fine to leave the pivot points free and just weld some struts from the swinger to the frame with a bit of additional reinforcing via triangulated tube. Do you think i should weld up the pivot points too and do away with all fasteners that attach swinger to frame?

If everything works out for me financially this week, i should be able to pick up the bike this weekend!
 
I think 3 to 4 in of drop would be a little much, unless your into straight line riding. I dropped my 81 cb 750 2 in front and rear, and the end of the kick stand stayed shinney. if you want to do it on the cheap, get some 2in under tubes for stock legs, Fork by Frank are top of the line, and Tom Summers of Low Riders by Summers has put togeather a 11in HD shock conversion for the CBs if you go that route make sure you tell Tom it's for a 81DOHC, his kits are for the 750SOHC and the thread pitch on the upper mount is differant
 
SenorThumpy, If you want to hardtail it, I would do it right by cutting the back end off and creating a new one. :twocents: The welded up swingarms never look right to me and some people say it makes the frame too stiff and rides rougher than a real hardtail. I don't know though because I never ridden one like that.
 
If your thinking about doing a hardtail conversion, check out CycleOne manufacturing, Red and Kate turn out a real nice hardtail section, for a pretty resonable price.
I'm not sure if Ken at CycleX does hardtails or just full frames, you'd have to check
 
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