Help me research/understand front end swap stuff (XS650)

[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.

 

[jpmobius]

Keeping your bikes dimensions as close as possible to its original specs will likely be your best plan. Find a geometry calculator on the web - there are quite a few good free ones - and plug in all your stock specs. Then when you consider changes you can plug in those specs and compare. Can't imagine you needing to alter your frame. 3 degrees is quite a bit. In general, this is what you are up against: The more modern/racy the bike, the steeper the rake angle likely will be. This is because steeper steering results in quicker handling. it also reduces trail and the self centering or castering tendency. More modern bikes with their tremendously more rigid frames can tolerate a lot steeper rake without being afflicted with problems, so if you wanted to replicate a modern front end on your xs by changing the rake to match all your new front end parts, you would also have to somehow increase the chassis stiffness to match, which is not practical. Fortunately, one of the things engineers do to get some more trail back into the very steep geometry is to reduce the offset in the triple clamps. So if you swap front ends and keep all the dimensions the same - rake, ride height, and tire diameter, the reduced offset will only add extra trail to your bike. And unless your new triples are severely less offset than stock, the slight increase in stability and slightly slower turn in will probably be acceptable. You can try to get a bit of it back by increasing the rear ride height if needed, but keep in mind the resulting steeper rake will load up the chassis more making speed wobble more likely. Be realistic about your expectations. There are substantial benefits to be had with a modern front end. Suspension compliance and damping are a world of improvement, not to mention the huge increase in structural stiffness, but keeping the geometry of your bike very close to original will let you see the benefits of the new parts while keeping its general handling qualities.

 

[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?

 

[jpmobius]

Well, close is of course subjective. I would hazard a "close" assessment, but it would be better if the forks were a bit longer. The forks are shorter, but it is the ride height that counts, as in how high the steering neck is compared to where it was before. Probably a pretty good chance you can achieve the right height fooling around with the springs. I would expect the reduction in offset will be noticeable, but it certainly depends on the rider. Don't forget to consider the mechanical end of such a modification. Often enough bearings can be sized to make things easy, but sometimes not at all, which will take some doing getting a steering stem to work. Length of the stem is also a feature for assembly. Your XS headlight mounts if stock will not work with the larger dia. tubes. Don't mean to be discouraging, just pointing out that there are mechanical bits to sort out as well as the geometry changes.

 

[jetmechmarty]

As an option you can fit you stock forks with custom wound straight springs, cartridge emulators, and a fork brace. Before doing that, you could swap in 1977 or later XS650 forks. That would upgrade you to 35 mm forks and better brakes. Replace the brake rotor with a 2 lb aftermarket unit. The brake caliper would move to the rear of the fork leg and combined with the rotor swap, you lose about 5 lbs of unsprung weight. This would transform the bike from stock without changing geometry. I know this doesn't answer your question, but I think it's worthy of consideration.

 

[sav0r]

I'd go with a FZR 600 fork. They are 38mm, and can be found rather cheaply ($150 to $200 complete with calipers). Use to the FZR calipers and rotors. The rotors will center on the XS650 wheel I believe, however you will have to space the FZR rotor out to meet the caliper. Then you just have to make some spacers to center the wheel on the axle and find bearings that match the XS wheel to the FZR axle. I did this on my RD, but with just a single disk, and it works great.

 

[scott s]

Thanks Sav0r. That's the kind of information I'm looking fork. And single disc is fine by me. How did the triple bolt up? Any mods? I think the RD and XS necks are a lot alike.

 

[scott s]

And you know what? I need to cross reference the part numbers, but that front end looks an AWFUL lot like the Radian front end. And it just so happens I have a set of Progressive springs for the Radian/FJ600.

 

[sav0r]

The FZR stem is a good bit longer than the RD stem, about .9" if I remember correctly. I was going to press the RD stem into the FZR clamps but I couldn't get the stem out of the FZR clamp. In order to save time and get the bike on the raod I just cut a spacer on the lathe that moved the upper bearing down. That allowed me to get away with unmodified FZR triples and stem. I used an AllBalls FZR to RD adapter set as well. Recently however I cut the stem and shortened it. Both setups work, I just grew tired of looking at the spacer. The bars also appeared kind of high, though I run them over the top clamp as they would be on the FZR.

Here's a photo of the arrangement with the spacer.