Overcoming Modern fork swap issues!

[JimmyD750]

So I can find this later.....

 

[Durgam_K]

Being able to use CAD has huge advantages. I was able to track down XS650 frame specs and from there, able to design an entirely new setup for my old 650. I was able to determine determine swingarm lengths, and use the frame shock mounts to finalize where to weld the shock mounts on the swingarm. And with that new swingarm, determine the correct Rake and trail for that wheelbase for optimum performance.

A helpful trick is...Try to find a modern bike that will fit your riding and handling needs. Than mimic those dimensions if possible. As another member mentioned, times have drastically changed in bike design, and attempting to get 20005 GSXR handling from a 70's bike is near impossible. BUT.... getting things safely close, makes a huge difference in feel.

Kris

 

[NagChampa]

Anybody have any real world experience with increased offset triple clamps? I am not aware of anyone building a bike with these. Thinking about going this route if I do a USD fork swap with 18 or 19 wheel. Unfortunately they are also costly so any insight on the topic is appreciated.

 

[redwillissuperman]

What is your target trail value? Work back from there.

Good info there Fun.

 

[crazypj]

USD forks are almost always shorter than the originals (GSXR750 and CB360 are same length, XS650, about 2~3 inches longer then GSXR)
The trail has a pretty large increase which makes bike very stable by 'slowing down' steering

 

[NagChampa]

What is your target trail value? Work back from there.

Good info there Fun.

Not sure what my target trail value is at the moment, probably close to stock.

Have to take into account the taller CB900 shocks being used on the rear, also have to decide on wheel and tire size before I will be ready to do the math. Bike has to be safe before anything else comes in to play.

USD forks provide the best available braking upgrade but come with a few problems that need to be corrected, like the geometry change from being shorter, as well as the decreased turning radius, or smashed tank if you do not have steering stops. It seems like the offset triples will reduce some of the problems that come with USD forks.

 

[rzresurection]

USD forks provide the best available braking upgrade

How so? In terms of modern calipers? All you need are modern calipers and some new 'adapter plates'. The plates may be a PITA if you have to relocate the calipers in 3 dimensions - that will require a machinists time.

This may save you the aggravation of worrying about new offsets and how to graft a new front end onto your bike.

You must also consider getting the front end resprung to match the weight of your bike.

If you're not looking for the suspension upgrade, but want better braking, just go with some multi-pot calipers and plates.

 

[NagChampa]

How so? In terms of modern calipers? All you need are modern calipers and some new 'adapter plates'. The plates may be a PITA if you have to relocate the calipers in 3 dimensions - that will require a machinists time.

This may save you the aggravation of worrying about new offsets and how to graft a new front end onto your bike.

You must also consider getting the front end resprung to match the weight of your bike.

If you're not looking for the suspension upgrade, but want better braking, just go with some multi-pot calipers and plates.

Definitely would do it for the suspension upgrade, not just the brakes. I do not have a 'Machine Shop Guy' at the moment so I would like to keep the machine work to a minimum and not spend a ton of money on a front end that I may just end up replacing with what I originally wanted, and it is certainly not aggravation, just gathering info and experienced opinion.

 

[Cody.linhart]

Can you put a 2000 Yamaha R1 front end on a 1978 Honda CB750K? If so what all would it take? If I'm correct just a front wheel and triple tree stem correct?

 

[rzresurection]

I don't know much about a 78 CB750. I did graft a 2000 R1 onto the front of my 87 RZ350. Not too hard to do. I needed to get the correct taper bearings. I didn't go thru any stem swaps. I had to use a few strategically placed spacers.

The hardest parts - welding on new steering stops and figuring out how to get a speedo back. Modern sport bikes don't use a gear driven cable like in the old days. I had to go with a GPS unit

Worked out well

 

[mrvass123]

Hey everyone,
New here and first post in fact so please go easy.
I'm planning a cafe project for my 1984 Kawasaki gt550 and one of the ideas was to do a front end swap. Sound's easy enough but i have a problem. My brother (who is helping me with the project) is saying that the stems are welded in and can't be removed and that we have to find a front end with the exact dimensions for the stem in order to do a straight swap, front end with front end.
My response was simply to do a stem swap... That's the genera Idea but, wait until you read about the parts i'm looking at.
The new front end consisting of some bad ass Brembo calipers and some beautiful inverted forks off a Ducati 696 monster. Can anyone help me with some info on the best way to go about this?
Thanks, great content all round.

 

[Streetfighterkz]

saying that the stems are welded in and can't be removed...
My response was simply to do a stem swap...
Can anyone help me with some info on the best way to go about this?

The GT550 stem can most definitely removed even if it’s welded in. I removed the stem on my Gpz750 and it was welded. I simply ground down the weld until I could see the seam between the lower triple and stem. Then using an appropriately sized socket, hammered the old stem out.

The trick is getting the old stem in the new triple clamp. You may luck out and the stems’ OD’s are the same. If not you may need turn up a bushing to adapt the old stem to the new tree if the stem is too small (most likely in your particular case).

I have pics of a couple stem swaps I’ve done that I can dig up if needed.

Later, Doug

 

[mrvass123]

hey, that sounds awesome! i would love to see those if possible. Although, I've had a little change of heart, instead of putting the complete front end off the ducati on the gt550 i would simply just use the parts i need such as the forks and calipers. i'm planning on getting a similar yoke and tree which fits both the kawasaki and the forks etc. what do you think?

 

[teazer]

....... I've had a little change of heart, instead of putting the complete front end off the ducati on the gt550 i would simply just use the parts i need such as the forks and calipers. i'm planning on getting a similar yoke and tree which fits both the kawasaki and the forks etc. what do you think?

First off I would remove your bike's forks and measure the overall length and the stem dimensions. If the donor (Ducati) fork stem is close enough, it might be possible to use it or have it modified. If not a new one can be machined and pressed in. The probability of pressing out teh GT stem and using it in the donor forks is slim.

But before you go too far down that rabbit hole, check the overall length of both sets of forks to be sure that they are close to the same. If your new forks are much shorter then you will be changing rake and trail and ground clearance and if those changes are not well understood you could make the bike unstable or even unsafe.

Next thing to consider is that you will be changing from a skinny 19" (or is 18") front rim to a fatter 17" rim if you use the Ducati wheel and brakes and it will be much wider than the rear so that will look odd. And it will lower the front end and change rake and trail and clearances.

A simple way to see what will work is to take a clean picture of your bike form the side and then measure forks and wheel diameter and measure the parts from the Ducati and draw them onto your picture and work out how much it changes things. There are also more accurate tools on line to do the same thing.

 

[crazypj]

Just about all USD forks are way shorter than 'old school' 1980 (and earlier) bikes although 2000+ GSXR forks are just about identical length to CB360 / CB400F. The yoke offset is also 20~40mm less so you increase trail dramatically. The trail increase is somewhat compensated by lower front end which restores some of the trail. I have seen clamp on extensions made to increase overall length of fork leg but to be strong enough they look ugly as they are so oversize (basically a piece of 6061 about 2.5"~3.00" diameter, 5"~6" long bored to slip over fork top then clamp in place with top reduced diameter to fit 'original' top yoke)
I recently saw on You Tube a neat way of clamping using a single bolt (Stefan Gotteswinter, chamfer cutter) I think it would be possible to 'hide' the extension adapter but would require a lot of machining. IIRC, Kawasaki GT550 has quite ling forks plus 19" wheel, even 'back in the day' they looked spindly

 

[teazer]

There are quite a few sets of 37,39 or even 41mm conventional forks that would be a better upgrade. For example on a GT750 (similar length 35mm forks) I could upgrade to 37 or 39mm forks from say a GS1100 or even a GL1000 and for 41mm I could go to GSX600 forks or even a set of FZR400 (39 IIRC) forks. Lots of options that can be made to fit with less work and less upsetting to the geometry.

https://www.kiwavmotors.com/en/faq/about-fitment/23-motorcycle-fork-tube-size-chart

or http://www.dotheton.com/forum/index.php?topic=20950.0. Choose the same make as your bike and look for say 37 or 39mm from a similar year and maybe somewhat more engine displacement and then see what's available.

You could probably bore your triple clamps to take a set of suitable 37mm legs and maybe even a set of 39s and then rebuild or replace the wheel to suit.

Forks can also be "stretched by an inch or so with custom gull wing (dropped) top triple clamps.

 

[crazypj]

I picked up several sets of GSX-F (Katana) 41mm.forks 16 or so years ago when no one wanted them so VERY cheap. They were cheap at the time as you could get BSX-R cartridge forks for around, $150. Suzuki had zero finance, etc (way too many crashed with less than 100miles One had about 5 ft, never made it off lot) Katana are conventional spring and damper but stiffer by far than the originals (and lighter) Put two sets on XS 650's and a set on CB360. Dead easy as they have steel bottom yoke (same as Honda) so swapping stems is simple

 

[teazer]

I have a set here and sometime soon I should put them back on ebay. Too many forks, not enough projects to put them on.

 

[Streetfighterkz]

Here are some pics from swapping a Gpz750 stem into a Zx11 lower triple. The Zx11 stem’s ID was the same as the Gpz’s OD so I cut the Zx11 stem flush and used it as a bushing. Then welded the Gpz stem into the “bushing” as both were steel.

I also swapped a ‘74 H1 stem into a ‘00 Zr7 triple clamp. That was a simpler press out the Zr7 stem, press and weld the H1 stem.

Later, Doug

 

[teazer]

I have a slightly modified GSXR alloy stem pressed into an RD350 lower triple to reduce weight on our RD350 drag bike.

On GT750 Suzukis a GSXR lower triple and stem slide straight on with a set of allballs taper rollers.

I have a set of TZ350 triples with a custom alloy stem pressed in.

On another GT750 I plan on machining out the triples from 35mm to 37 -39 to fit later model forks ( amount of metal left and choice of forks dictate the size).

Lots of ways to get thicker fork legs onto an old bike that don't necessarily include USD forks or using the OEM stem. For example, any competent machinist can machine up a new custom stem or modify an existing one.