Viability of a putting a modern sportbike front end on a CL350

I have a set of GSXR750 forks on one of my 76GT750s and there is no way that it's period. It's not supposed to be. It's my bike and I basically know what I'm doing and can make a reasonable assessment of the risks. I can measure steering geometry and make a reasonable guess as to how it will handle. Better front brakes to me means even less reason to have a back brake weighing the back end down. They don't contribute much to braking except in the wet when they are a godsend.

As someone mentioned ealier a front end swap is not cheap. By the time you get forks and new rims and disks and capilers and brackets and spokes etc it can get quite pricey. Using the huge rims from a modern sportbike on a tiny twin is overkill and won't help handling much so new rims are probably going to be on the shopping list.

From my perspective, modern suspension can look good on a custom and after all we are building custom bikes not replicas of the sixties as a rule.

The big issue with fitting modern forks is not stem length or bearings or caliper brackets or any of the other details that still have to be worked out. It's steering geometry. That can bite if you get it wrong so take a lot of measurements before during and after and if it looks unsafe or the numbers are out of bounds, change it.

There are some seriously stupid and unsafe custom bikes around. Probably 75% are less effective as motorcycles than they were before the "builder " started so think twice and cut once. It's a fashion, yes, but that doesn't mean that we have to follow every stupid idea just because it's trendy.

The irony is that people used to ride cafe racers to be different now we build them to be the same sort of different as everyone else. Be original people. And be safe and have fun being creative.
 
Tremelune said:
I thought about the 550 swap, but I'd still wind up with mediocre (disc) breaks, limited tire selection, and (most likely, depending on where they came from), worn forks. It would probably be a small difference in money for the swap vs the excellent breaks, excellent tire selection, and much better forks of many front-ends from the last decade.
Brooklyn/Manhattan? have you been to "Sixth St. Cycles" lower East side.
As far as the idea of 550 forks and brakes, never go half way across a stream, if you want the modern front end just do it. On my current Yam Rt360/250 conversion I'm only using a single 320mm rotor from an '09 Honda CBR1000R so the single rotor gives a nice light airy look. Properly chosen you can find significantly shorter forks (my '03 GSXR600 forks are only 28.5" axle center to top cap) which can help the rake angle will also be stiffer. The much larger tube diameter (45mm) looks heavier but in reality are not because of much thinner wall tubing. Aluminum triples are not only lighter but have much less offset which will give quicker steering and shorter front wheelbase. As far as period correct that's funny, 1959 didn't have too many disc brakes or digital ignitions or three cylinder two strokes with chambers etc. Cafe has become pretty much an open category. I see the current trend of a "cafe" bike with off road tires and "cafe" bikes with the 16" Firestone balloon tires as well. Which is fine with me, I like the individualism of all custom bikes.
Cheers, 50gary
 
ohiocaferacer said:
I did the USD modern fork swap several years ago.......very easy to do.....just swap the stems.
This bike is a 1969 CB350 with 1998 GSXR750 inverted forks.
Tankside2-150.jpg

During mock up:
cb350rr-right150.jpg

How hard was it to use a modern rear wheel and disc break in the original swing arm? I'm assuming some fab work is called for? I took my swing arm (cl350) to a mates the other day to offer it up to a wheel (from and SV650) he wants to sell me and It looks like it might fit but with about a gnats cock on either side. Any input? I've been perving on Ohiocaferacers.com for a wile and I'm amazed by the 350 project but need a bit of help getting my head around the rear end. Any advice of help would be great.
 
Hi ohiocaferacer, nice work... What rearsets are you using and why the clamps vs welding mounts on the frame?

I'm doing a 400F build and sorting all this out now...

Thanks!
 
Hey guys! Change of direction here but how about the viability of using a modern rear end? I've got an SV650 rear wheel I want to use on my CL350 and Cant seem to figure out how to make it work. Any ideas?
 
I would swap swing arms. If you use all this modern suspension, plus the wheels and tires I would extensively brace your frame. Typical old tube frames are pretty soft, mild steel and flexible. With increased traction available due to your upgrades the chassis will wind up like a spring. For a 350 I would think a 4.5" rear wheel and a 160 tire would be a bit much, just two cents. It may well not even fit inside the frame rails?
Cheers, 50gary
 
Dose anyone have any pictures of pressing out stems and any spacers they have made? That would be super cool. I'm sitting here with 2 triple trees scratching my head. If you haven't guessed I'm not the most mechanical inclined.
 
1969Honda said:
It can be done, my '69 has a '98 R1 front on it currently. Drive the bottom wedge out of the cb/cl350 tree with a long punch, then press out the stem from the lower triple. On the R1, remove bottom circ-clip, press out stem. Press the 350 stem into the r1 lower and you're good to go. Just measure your initial stem length before removing and duplicate with the new tree.

IMG_8684.jpg

IMG_8686.jpg

Are you sure about this? I'm looking at my Cl tree and the bottom section seems to be one piece.
 

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The stem is welded to the bottom clamp. Theoretically, if you grind off the weld you can press out the stem.
 
AHHHHHH I GET IT NOW!!! and how do I afix it to the new triple tree? Here is a pic of what I'm dealing with.
 

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and all 3 bits are supposed to magically sit there and hold your front end on because its clamped around the forks?
 
The stem is pressed into the lower triple clamp. A proper "Press" fit will hold a great deal of weight and force. The press fit is used instead of the welding because the aluminum cannot be welded to the old steel stem. Typically the press fit will be with the lower end of the stem being oversized by .001" per inch of diameter I.E. if the hole in the lower triple clamp is 1" the stem must be 1.001" if the hole is 1.25" diameter the stem must be 1.00125" and so on. The stem is pressed into lower clamp with a heavy duty arbor press exerting tones of force. The resulting friction causes the stem to be stuck (to use a non technical term) in the triple clamp. This would be done at a machine shop not usually at home. If the stem is smaller in diameter then you must build it up to a larger diameter (I used a method called "spray welding" then have it precision ground to the proper size and lastly press in to the lower clamp)
Cheers, 50gary
 
50gary said:
The stem is pressed into the lower triple clamp. A proper "Press" fit will hold a great deal of weight and force. The press fit is used instead of the welding because the aluminum cannot be welded to the old steel stem. Typically the press fit will be with the lower end of the stem being oversized by .001" per inch of diameter I.E. if the hole in the lower triple clamp is 1" the stem must be 1.001" if the hole is 1.25" diameter the stem must be 1.00125" and so on. The stem is pressed into lower clamp with a heavy duty arbor press exerting tones of force. The resulting friction causes the stem to be stuck (to use a non technical term) in the triple clamp. This would be done at a machine shop not usually at home. If the stem is smaller in diameter then you must build it up to a larger diameter (I used a method called "spray welding" then have it precision ground to the proper size and lastly press in to the lower clamp)
Cheers, 50gary

yeah, what he said.
 
Interesting topic for me as I just did this conversion to 2003 GSXR. But most years are the same.

I ordered the conversion bearing kit from AllBalls...$44
Removed upper cup and pressed in new AllBalls cup (I use 1/2 of an aluminum mountain bike straight handlebar to beat it out)
Removed lower Honda cup, ground it down to 4mm with a grinding wheel and last step using sandpaper on a flat surface plate.
Pressed in the "spacer" made from the old cup
Pressed in the new AllBalls cup on top of the spacer
Installed new AllBalls race on steerer/lower triple tree
last step is to install triple tree and fiddle with washers and spacers at the top for a perfect fit. I didnt use anything non-GSXR for this.
Done.

If I had it to do over again and wanted the very easiest way I would get a modern Honda front end. The bearings and stems seem to be pretty consistent over many decades, at least the bearing sizes according to AllBalls website. the stem lengths are another issue as the head tubes for various bikes have some variation but in theory as long as the 350 is on the long side as far as head tube goes then you can make others work using spacers.

There are lots of ways to ride and stop better the best being experience. My humble opinion is that riding can be pretty dangerous and there is no such thing as overkill when it comes to braking smoothness, dexterity, and control in bumps and emergencies that a modern front end provides. That is why I put disc brakes on my 66 bug. Control in braking isnt about being able to lock up the front wheel (although that is a good measure of braking power) it is about not locking up your front wheel in all kinds of situations. I do like the aesthetics of the older rigs and that is why I went with a 2003 fork instead of the upside down style. I also opted to use the standard CB350 front wheel with the drum brakes (this takes machining a custom axle conversion). http://www.flickr.com/photos/44613580@N03/8517120597/in/photostream/

This conversion shortens the wheel base by approx 2 3/8" and all of that is off of the "front-center" dimension. Less fork offset means greater trail (offset and trail are opposite forces so to speak). This means that the fork is going to be more "floppy" or "twitchy" at low speeds and more stable at high speeds. It is a complicated relationship but one worth understanding.

Anyway...pressing out the steerer tubes if you can do it yourself is definately the cheapest way to go because you can use your standard bearings.

Another thing to consider are the steering stops on the new fork are wider apart. You will need to fab a steel or aluminum extension for your cb350 head tube steering stop thingy. You can design one that has a grommet for the GSXR steering dampener and have it set up all neat in front.

I look forward to seeing how your project turns out.
 

Halekai, Those are very nice conventional forks the GSX-R (03) is also what I'm using on my Rt dirt bike to cafe/track bike conversion. I like the look and they are shorter than most other forks by about an inch. This helps with a more aggressive rake (caster) angle. They are also very stiff being large tube diameter (45mm) and again shorter than average. Cartridge forks are way better as well. I am using the GSX-R axle and machined the doner wheel (GS450 1.85" x 18") I like the big dia. axle for it's rigidity. I'm only using a single (320mm) rotor and have removed the caliper lugs on the right lower fork leg, looks so much cleaner. Sorry to jack the thread but it's all good information.
Cheers, 50gary
click to enlarge.
 
50gary, Lets ee a picture of that machining you did to your hub.

This rake and trail thing is complicated. 50gary, Yes the shorter by one inch forks act to decrease the overall head tube angle of the bike by lowering the front. This decreases the trail. to keep steering "the same" one would need to also decrease the fork offset...which the gsxr fork does as well. Of course we are not talking specific numbers only generalities here.

There are some good wiki articles on understanding these relationships. I am a bicycle framebuilder by trade and it took me quite a while and quite a number of fork/frames before I really got this concept down. For me it all seemed anti-intuitive.

When discussing the viability of a fork swap, rake and trail definately need to be considered. Racing bikes have exact numbers that are "Magic" and are closely guarded secrets. A lot of think time goes into calculating tire diameter at various speeds, fork dive in braking etc. and it is all factored to arrive at the race settings.

I think one of the common myths is that Honda had it exactly right when they manufactured the bike back in 1970 and anything we do to modify the steering will be dangerous if you dont do it to factory spec. (I always get weird looks and usually a wrong explanation about steering geometry from other motorcycle people) It is my understanding that the triple trees on these bikes and all of the CB's were similar offset yet were put on many different styles of bike with all kinds of head tube angles and wheel sizes (another key factor is wheel and tyre diameter!). Just look at the changes the CB750 went through. I think every year had changes to the stearing geometry to get it to handle better....So I have read.

I doubt very much that Honda or any other manufacturer had it just right and most bikes could improve on the handling by analysing the numbers and making appropriate changes.

For our purposes there is a range that is going to feel good. extreme differences may have an impact on safety ie. shimy in turns while braking etc.

Just my two cents.
 
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