Help me research/understand front end swap stuff

[scott s]

I've been doing a lot of reading and searching the forums, but I have a rudimentary understanding of this geometry stuff. My eyes start to cross after a while.....information overload.

I have a '75 XS650 and I'd like to upgrade the front end. I have access to a good machinist, welder and a friend that's an engineer. But I need to narrow down the choices a bit, first.
I've seen several GSXR swaps, but they're shorter and alter the geometry a good bit. Ideally, I'd like to find something that DOESN'T require re-raking the neck.

Here are some stock XS650 specs:

Tire size: 26.1"
Rake: 27 degrees
Offset: 1.938"
Fork Length: 29.5"
Tree rake: 0.0 degrees
Trail: ~ 3.9 - 4.47 (I'm seeing two numbers....maybe different between early and later models?)

I've found some CBR 600RR forks. They're traditional (not USD). They're 45 mm, vs. 34mm for the '75 XS.
Online, I've found the CBR has a rake of 24 degrees and trail of 3.7".

I've asked the seller for fork length, steering stem length and a rough measurement of the offset of the triples (stem to triple distance).

The rake and trail of the two bikes seem close, at least with the measurements I have. How big a difference is three degrees when it comes to suspension geometry? Is that, like "Really close and ought to work great!", or is three degrees suicidal? Will it be a twitchy tank slapper or ill handling pig?

I'm not set on USD forks. Traditional but modern and beefy are fine. Can someone help me sort through the myriad charts and info online and suggest a set of forks/triples that are a good starting point? I don't mind mixing and matching. Bonus of the upper triple has handlebar mounts.

 

[scott s]

Just received this about the CBR forks:

Length: 28.5"
Steering stem length: 7" from top of bearing seat to bottom of lower bearing seat.
Offset: 1" offset of steering stem

Those measurements "seem" very close to the XS650, but are they? In terms of steering geometry? Is this one a close match to start with?

 

[Terrible1]

This is good info. I'm not the guy who knows but wanted to say good job for thinking outside of the norm.

 

[Signal]

I am not qualified to comment on the CBR front end, however when doing this sort of thing I find it helps to draw it in a CAD programme then you can easily work out what the changes in rake and trail will be.

Have you considered overhauling your existing forks?

Replacing the springs with a set that suits your weight, fitting a set of Racetech cartridge emulators (650 Central ) & fitting a fork brace it is a lot less work than swapping a front end. The steering geometry will not be altered and your bike will appear largely stock.

Another option is fitting larger diameter forks of a larger capacity XS the 750, 850 or 1100.

I have fitted an SV 650 front end to my XS tracker and have a road XS with most of the upgrades listed above. For my style of riding I would not go to the effort of getting the SV front end to work on an XS again.

 

[hhcustoms]

http://www.chopperhandbook.com/forks.htm

it goes through all the fork types at the top but about halfway down it goes into trail and how to calculate it. use the #'s youve got there and figure out what your trail will be. that will tell you how it will handel darty or sluggish

 

[hhcustoms]

http://bikearama.com/theory/motorcycle-rake-trail-explained/

this may actually be a better link

 

[member3480]

I have done them and here is what you have to consider.
If stock frame and you are going for the CAFE look get the length of front end the closest to stock so you don't change ride height way off. The lower the front end the closer the tire is coming back towards the frame. Now here's the other problems Tire size/axle/brakes. Going with a GSXR or CBR or any of the inverted fronts you will need to run the wheel/axle/brakes that came with that front or you will be machining your ass off to make one off parts. I did a XS650 chopper with a GSXR front and work good but it was a hard tail frame so it was lower already. Size didn't matter that much and I used everything from that front end. I did a KZ chopper with both GSXR front and rear wheels and had to do a little more machine work. So again its the style that may make more or less machine work. Also got to match up the bearing for the neck too.
So you can throw all the numbers around you want but its the end riding of bike that's going to tell. Raking a front also not good if you don't really need it. The more the rake the more you change the ride at higher speeds and have to add steering stabilizer so you don't get TANK slapping.
Hope I helped a little been doing this for over 35 yrs and built some very cool looking bikes that came out UNRIDEABLE after the rake and stretch .
GOOD LUCK WITH YOUR BUILD and there are guys on here running INVERTED fronts on XS650.

 

[scott s]

Right now, after doing some reading, I'm leaning towards FZR 600 front end. It uses the same bearings (or at least has the same upper and lower stem size) as the XS650.
They're 38mm forks and have lots of options for upgrades if I ever wanted to go that route. They use dual piston Sumitomo calipers and the discs have the same pattern as the XS (though you have to use spacers to align them).

Rake is 25 degrees vs. 27 degrees for the XS650.
Trail is 3.7" vs. 3.9"

Best part is, I can use an XJ600/Seca II top triple and get handle bar clamps. It bolts right up and they're cheap on eBay, as is the entire FZR front end.

 

[chizler62]

1st You can't compare data between the stock listings "3.7 vs 3.9" etc You have to take in account the rear wheel, antisquat, ride height, wheelbase, etc.

2nd it's not that hard to figure out. It took me minutes to come up with this for an early CBR


3rd you have to consider the rear suspension in some respect "hardtail or ?" Then figure out Head Tube angles as the suspension compresses. etc

Heres the math for that.


Oh, and lets not get into lean angles and such...

You need to sketch out the entire bike out on paper. It's the easiest. When I was just starting out. I'd use the back of some birthday wrapping paper and mom's dinning room table. A 4' ruler, a protractor and pencil, a hour later I'd have the entire bike drawn out.

If the axle to crown length changes than the entire bike will change.

AXLE TO CROWN is the most IMPORTANT measurement in this conversion. AND that's why you never see it posted!

Post the measurements and I'd be happy to walk you thru it!

 

[TwoManyXS1Bs]

Found this a couple of years ago, can't remember where, but would like to give credit to the originator.

 

[scott s]

And you're right: I don't have a complete understanding of how this all works. I mean, I know what rake, trail, etc., all mean. I understand that the neck of one bike may have a different rake of another. That triples have different offsets. Fork lengths and assembled height make a difference. I get all that. Rudimentary, but I get it. That's why I started this thread.
But what I don't get are how to take the numbers I find online and extrapolate them between two bikes. I've Googled and searched forums and looked at charts and bearing swaps and so on.
I can't always find the information I need online, such as a particular models fork length or neck rake.

How can I find the numbers I need to compare, say, the FZR 600 to the XS650? If someone can help me by plugging this stuff in I would greatly appreciate it.