Weight Balance Questions

[bwthor]

I converted a 1980 Special to a Cafe racer. Sitting on the bike, I'm in a forward leaning position, rather than the more relaxed sitting upright position. The Special also had a two up seat, so I'm guessing the rear springs are way too stiff for my 170lbs when in the leaning forward position. I've already softened up the rear springs to their lowest setting, and I think there was a way to stiffen up the front, but I haven't changed the front yet.

Does anyone have any recommendations on better rear springs for my setup? How about the front, should I run them in a stiffer setting if they are set soft?

 

[ThatXS650Guy]

What handlebars are you using? My '79 café has an upright seating position but I am running superbike bars, Bonneville shocks, 18" rear and 35mm forks with a 2" drop.

 

[bwthor]

Clubman type bars like these...

 

[5twins]

I don't think you'll have issues with the suspension being too stiff, it will probably be the opposite, especially if the shocks and forks are stock originals. The rear shocks may be OK for a while if they're not blown out and leaking oil, but you will want better aftermarket replacements eventually. The usual routine is to go about 1" longer. Stock shocks are 12.5" or 12.75" so low to mid 13" replacements work well. This raises the rear end a little which is a good thing. That more heavily loads the front end and that gives a little better steering input and feel.

The stock fork springs are a nice dual rate but are too soft, especially after all these years. You'll find this out the first time you grab the front brake. The forks will practically bottom out, lol. Even with the preload adjusters set at the max, the forks are still too soft. There are a couple inexpensive things you can do to improve the forks. First, use slightly more than the recommended amount of oil (just under 6 ounces). Use 6.5 to 7 ounces. This will greatly reduce the brake dive. Also, add 1" pipe preload spacers to the springs. That's pretty much the most you can add without the springs possibly getting coil bound. Anything less is pointless really because you can accomplish that with just the adjustable preload caps. The caps have 2 stiffer settings. Each increases the preload by 10mm. An inch is about 25mm so a spacer that long will put you about a half step stiffer than your max adjuster setting to start with. Then you'll have 2 stiffer steps after that if need be.

I also like to "Minton Mod" the fork damper rods. Much has been written about this so if you do a search, it'll come up. It's another low cost (practically free) mod and I think it really helps.

 

[grizld1]

Whatever you do, make sure that tapered roller bearings have been installed in the steering head, nylon swingarm bushings have been replaced with bronze, and a fork brace has been installed correctly.

There are two things going on with shocks and forks that impact comfort and compliance (of tire to road). You're only thinking about one of them--spring rate. The other is damping--how quickly the suspension is valved to respond. OE shocks were pretty bad out of the crate, and like most damper rod forks even today, the forks on the XS650 are sprung too loose and damped too hard for optimal performance.

What to do depends on how you ride. Once upon a time I rode pretty aggressively, and it took a lot to satisfy me. But age has slowed me down, and a stock fork with preload set right and a pair of middle-of-the-line shocks from Hagon or Progressive Suspension (as opposed to the custom made Works Performance shocks on my bike) would probably satisfy me now.

So--if you're not in the habit of pitching the bike over till your boot rubs the pavement but want quicker, more linear steering, get a good pair of shocks--meaning something in the range of $200 to $300 a pair, not Chiwanese crap--in a length between 13.5" and 14" eye to eye (I recommend a range 1/4" less for Standard models with shocks mounted closer to vertical). Give the vendor your weight and the bike's weight (a ballpark number is OK for this) and let him pick the springs for you, then set your preloads where they feel right, and call it done.

I don't think much of the Minton treatment on the forks (in fact there isn't much of his writing that I do think well of, he was a slob and made some errors that can and will cost time and money if his procedures are followed uncritically). If you do want really good performance, the next step is to install cartridge emulators and straight rate springs in the forks; a competent vendor will be able to suggest spring rate, but at your weight 1.85 kg./mm. springs should work for you unless your bike has been lightened by more than 50 lbs., in which case you'd be looking at 1.80 kg. springs. To make a long story shorter, the valves enable damping action to respond to the road surface. They do well what dual rate springs do badly, and enable you to run comfortably with tighter spring rate. You'd set up the suspension by first setting laden sag (amount of suspension travel that's taken up by the weight of machine and rider) and then fine tuning it by trial and error. Instead of repeating the recipe here, I'll refer you to a great cookbook: google Suspension 101 ebook. I can recommend two trusted vendors for emulator valves and fork springs: Race Tech and Traxxion Dynamics. Enjoy!

 

[5twins]

That's the thing about the Minton fork mods, if you want to switch to emulators later, there's nothing stopping you. Minton did incorrectly list drill bit and hole sizes in his article but we've figured them out. I think they really help, especially in making the forks more responsive to the little stuff.