Overcoming Modern fork swap issues!

[cklamer]

I have a 1981 CB750K DOHC and I am using the entire front end from a 2003 GSX1300R Hayabusa.
Did a lot of research on these and it was a direct bolt on for my bike.
Had to do no major machining except for adding a bump stop to the stock one on the frame but I also use that for the stock steering damper mount.

So if you do your homework and research it can save you a lot of time and money.
Thanks for all of the great information.

 

[bert775]

Thanks for the replies, I want to go with the spoked front wheel which is why I thought using the wheel from Narrow Glide, here's a link to thee site I was looking at, what do you think?

http://vtwinmania.co.uk/product_info.php?cPath=38&products_id=2233

IAppreciate your comments on the rotors/calipers etc, I'm pretty sure I could look at getting these fabricated, cheers

 

[crazypj]

Using complete front end is easy if you WANT 17" wheels.
Converting to spoked wheels is only a little more difficult
I guess a buckled front Busa wheel would be easy to find, just cut the cast spokes off and add spoke flanges ;D
bert, where are you?
That HD wheel seems a bit spendy to me, stock hub is less than $120.00 here.
Not sure what I've got as I'm not really too well informed on HD

 

[erock85]

Here's the measurements I used for a narrowglide wheel and a GSXR from 07 front forks, I sent this message to a member as I was figuring it all out and what spacers I would need.

Finally got in the garage this weekend, I figured with everything off of the front end I would just measure each individual component and do some "simple" math, haha. The measurement I had in question was the Fork Spacing at Wheel Spindle Clamps = 153.5mm that's the measurement I found online. So I measured the inside of the bottom of the forks where the axle clamps in and voila 153mm is what I got. Not sure if half a millimeter matters but I'm going with 153mm. Then I measured the width of the hub at the outer most part of the bearings, 128mm. Measured a couple more components then took it all to the computer. I am a draftsman by trade so I drew some very crude components and centered everything up about a midpoint within the 153mm fork spacing. So I need a 32mmØ x 12.55mm with a 25mmØ through hole. Here's an exploded view:


Then all put together to get the spacer measurement:


The one side of the axle with the sort of built in spacer with a measurement of 53mm actually works I could actually shave off .35mm but idk if it's worth it. So there's the update, whew I feel good about everything now.

 

[spacetiger]

This thread was linked over on the CX500 site as there are a number of front end swaps to the various CX500/650 model.

I see some good info, and thought I'd pass a few guide tools that I used to inform suspension changes. Attached are a 4 guides on safe places to move your steering geometry. As soon as you gather your oem numbers, you can plan your changes accordingly. I found the app for the smart phone to check Rake comes in handy.

Cheers
Jerry

 

[FunJimmy]

This thread was linked over on the CX500 site as there are a number of front end swaps to the various CX500/650 model.

I see some good info, and thought I'd pass a few guide tools that I used to inform suspension changes. Attached are a 4 guides on safe places to move your steering geometry. As soon as you gather your oem numbers, you can plan your changes accordingly. I found the app for the smart phone to check Rake comes in handy.

Cheers
Jerry

Interesting graphs Jerry but I don't understand how a fork swap changes rake. It changes offset which typically increases trail, but rake should remain the same as this is a factor of the frame.

Am I missing something?

 

[Tremelune]

If the length of the fork changes, the rake changes with it.

 

[bajavw]

Is it also not true that some tripples have rake built into them that will change rake, if a total swap is happening.

 

[crazypj]

In effect you 'pivot' frame around rear axle so get steeper head angle which changes rake and trail. It's always been an issue with telescopic forks.
Some bikes get pretty scary under heavy braking, lots tend to weave a bit.
The only way around it is to use a well designed girder fork (which is why the Brittan had carbon fiber girder forks)

 

[FunJimmy]

If the length of the fork changes, the rake changes with it.

Unless you're going from chopper to tracker (or reverse) this change is negligible.
Shouldn't be more than a degree and certainly not 5.3° as Jerry's graph suggests.

 

[jpmobius]

Unless you're going from chopper to tracker (or reverse) this change is negligible.
Shouldn't be more than a degree and certainly not 5.3° as Jerry's graph suggests.

Certainly the degree change in rake would be small, but the effect on handling can be very considerable with an inch of fork length change in either direction as the trail changes along with the rake. Take a bike with as steep a rake as the frame can comfortably tolerate for sharp handling and slide the triples down an inch. If that is the only change, the bike will likely be pretty twitchy and much more prone to speed wobble. Go an inch the other way, and the front will likely noticeably under-steer when pushed hard. If the bike is just for putting around town to coffee shops, maybe it doesn't matter, but aggressive riders will definitely notice.

 

[crazypj]

I think a truer way to put it would be riders as opposed to poseurs

 

[FunJimmy]

Considering most fork swaps performed on this site are old school bikes with modern forks, the risk of negitivly impacting the geometry is pretty minimal. Old school bikes have relaxed head angles with significantly more offset in the triples than modern bikes. Reducing the triple offset by installing modern forks will increase trail. Shorter than stock forks or longer shocks will steepen the rake helping to correct the excessive amount of trail created by the modern triples.

 

[FunJimmy]

Take a bike with as steep a rake as the frame can comfortably tolerate for sharp handling and slide the triples down an inch. If that is the only change, the bike will likely be pretty twitchy and much more prone to speed wobble. Go an inch the other way, and the front will likely noticeably under-steer when pushed hard.

Let's keep this relevant.

This is a good suggestion if you're doing a thesis on motorcycle geometry but the DTT masses are swapping modern forks to vintage bikes. Those vintage bikes DO NOT have "as steep a rake as the frame can comfortably tolerate". They have relaxed angles.

 

[Drewski]

I've done some very thoughtful, but otherwise non-scientific messing around with modern front ends and wheels on vintage/classic bikes. I have nothing other than real world riding results to leave with you.

First the background.

My kz1000 was bought new in September 1977. The z1 and kz1000 were known for twichy handling when they got up to racetrack speeds. Racers back in the day increased stability by increasing the rake and wheelbase by cutting frame tubes to kick out the front end. They usually went to a 18" front wheel then to quicken the steering back up a little.

My frame had the same modifications and bracing done to it back-in-the-day, but I never got round to swapping out the front wheel before life (kids!) got in the way and the bike was shelved.

Fast forward 20+ years and the kids are grown and the bike is back on the front burner! It's 2005 and there are so many more options available for forks, wheels, rubber... it seemed endless! The original plan was for 18" wheels front and rear. The decision was made now to go with 17" rims as better rubber was available and other than lowering the whole bike a little, shouldn't affect the bike in a negative way. Now for the forks.

A set of '04 zx10r USD forks fell into my lap and we were off to the races! All front end dimensions were quickened, but with the stiffer forks and the raked frame, it all just worked! The package is somewhat lower, but everyting together flatout works!! It's stable in the corners, at speed and under braking. It handled the streets and the track so much better than the stock set-up... it's like night and day!






















There have been a few changes and up-grades since 2007 when the project finally hit the road.

Spent a day on the track in 2012:







Got rid of the chicken strips.











So, I agree there is a lot of room for tightening up the handling on these older 70's vintage machines. Consider all the variables and proceed with caution. Remember, when making several changes at once, handling and performance can be significantly changed... sometimes in a not-so-good way.


I'll get into the Kawasaki h2b 750 Triple swaps later! ;D

 

[doc_rot]

Very nice Drewski. Is that a JMC arm? have you noticed any ground clearance issues?

 

[Drewski]

Very nice Drewski. Is that a JMC arm? have you noticed any ground clearance issues?

Yes "doc", that is a JMC. It was one of the last they turned out and a long wait to get it, but the product was worth it. Fit, performance and finish are perfect.

I haven't had any ground clearance issues at all. It is lower, but I'm a little "inseam challenged" so that works just fine for me. Both set's of pipes that I've used are also very good at hugging the contours of the frame/engine. Speed bumps haven't been an issue.

 

[Drewski]

The Kawasaki h2b was a much simpler swap as far as the front end goes.

I wanted to go only slightly more modern with Phase I of the Mongrel Project. I bought a blown-up '86 fz600 Yammie for the wheels, suspension and brakes on both ends. I didn't know it at the time, but the fork tubes were the same diameter, length and the dimensions of the triple clamps were also identical! The forktubes just slipped into the h2b triple clamps and voila... new front end, brakes and wheel.

So the only difference is the wheel size. It's a 16" wheel, where the original was a 19".

Most will testify that h2's are not exactly known for "nimble steering". If anything, they're slow and relatively steady when it comes to turning. I know all about the "Widowmaker" reputation. I rode one back in the 70's as well. That one was pretty sketchy. This one was a differend animal from the get go.

I had already swapped the original wheel for an 18" rim along with improvements to the fork internals, steering head bearings, swing arm pivot, shocks, etc. with a small improvement in steering response. I was warned that the 16" wheel might cause the h2 to throw me on my face when tossed into a spirited turn, but such was certainly not the case. It is predictble in the curves and holds a steady line with ease. Initiating a turn is somewhat quicker, but not too sudden. Stability over the "ton" is great and I couldn't be more pleased with the results. Bridgestone BT-45's are mounted and give great feedback and grip.

The rear end works great with this set-up, but that's another story for a whole other thread. ;D

I should just comment that it's much more than the front end that contributes to quick steering and good handling. The rear swing arm and suspension is crucial component as well as the frame stiffness, tires, etc... but I'd guess you're already well aware of that. ;D


Original restoration, to now... Mongrel Phase II completed.
























Had a fun track-day on the Mongrel back in September. I managed to wear away some of the chicken strips off the BT-45's too!

 

[neophyteCB450]

this good stuff. posting so i can find it again

 

[rzresurection]

Was just reading thru this post. Drewski did a bangup job on the KZ, but I really love the H2 (I'm very partial to 2T's). I love the look of the finished product.

A front end swap isn't all that hard from a 'process' point of view. The hardest part that I found was figuring what to do about a speedo. I upgraded my RZ with an R1 front end which did not have a hub drive speedo.

Like was mentioned above. You need to have an idea of how your bike will handle afterwards. When I went with the R1 front end, the triples had a much smaller offset than my RZ. I also installed a longer swingarm at the time and went with 17" rims - better rubber selection. My turning radius (non handling related) increased due to smaller triple offset. It handles slower than my original geometry, but it is more stable on the highway - it's a trade off. If I wanted super sport bike handling I would have just bought an R1. The RZ frame is bendy at best