Swapping modern-ish forks onto a Tonti Guzzi

OK, firstly, I know I could accomplish this easier by simply buying some Gixxer forks off Ebay.

Stock Guzzi stem and lower clamp.

Interesting project Fred.

The steering stem is likely on splines into the lower tripple clamp. At a cursory look your wheelbase will be shorter by the difference between the two sets of clamps. Looks like turning radius will be decreased as well with the fork tubes further back.


Mike

the stem either has a shoulder or a split ring on the bottom end. that keeps it from being pulled through the lower triple by the adjuster nut on top... it will only press out "top" to "bottom". you need to do some research on rake & trail.... that much change may not be too good... guessing it would be twitchy

Ok, are fork lengths roughly same ? can the blank wp triple be drilled to match original offset ? need to measure center of both fork legs O-O to center of stem. -O-

I would expect something like 25mm to 50mm . Maybe a Guzzi guru will reveal lost secrets of Italy here ?
Ed

Go here:
http://www.guzzitech.com/

or here:
http://www.v11lemans.com/forums/index.php?act=home

The steering stem on the Guzzi triples I have messed with are just an interference fit with a step to stop it being pulled through.

I cut and steepened the head stock in mine to increase nimbleness. No unforeseen consequences after 25 years.
The Toni frame is pretty conservative, and very forgiving.
The only thing I may have a slight concern about is if the much better forks allow/encourage you to over stretch the rear suspension, which wouldn't be difficult.

All else being equal, length of fork legs, etc, the less offset between the fork and stem centerline will result in more trail. More trail adds stability as the tendency to self correct is greater. Downside of that is that it will require more effort to turn into a corner at speed. Potholes and speed bumps will be a thing of the past

More offset = less trail will speed up the steering, with the downside being that if you go too far it will result in a twitchy front end.

More offset is usually coupled with more rake to balance out the equation. You'll likely want about 4" of trail.


Set of bm' trees and forks that I slapped on a sporty. Both stems wouldn't work for the application, so I turned up a new one.

That's a seriously nice rim, and those forks are pretty sweet, too

If the rotors clear the inside of the upper legs on compression I can't see why it shouldn't work. Machine a stepped axle and a collar/sleeve for the other side where it passes through the the fork, if needed.

You also have the option of playing with various length rear shocks, sliding the tubes through the trees, tire profiles, or a combination of all of the above to get you to where you want your trail number to be.


Something that may be of assistance to you. Don't freak out when you see when you see the pic's of choppers The trail calculator is at the bottom of the page. Have fun

https://www.rbracing-rsr.com/rakeandtrail.html




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Maybe I'm thinking about this wrong, but more triple clamp offset (more of a V shape to the clamp like the originals) would give MORE trail, not less?

Move the tube centerline forward. What happens to the trail ?

Nice project! You're starting with a Cal III? What else are you doing to the Guzzi? I'm looking at improving the stock 35 mm's
i have on my modded Cal II. I want to keep the stock exteriors and swap internals for adjustable cartridge assemblies.

What does the wheel clamps on the stock fork look like? It's not unusual that they are mounted off the fork leg centerline, which in turn also affects trail.

So far I have used Suzuki, Kawasaki and Honda front ends on my Tonti frames. I also have 2 loop projects with modern front ends but have not ridden them yet so no update on handling. Never gave much thought to the offset or frame geometry. I do use a fork that is close to the same length as stock. All of my projects handle better than stock. Some of the better handling can be attributed to the shorter wheel base and much better suspension components. I also use the stock wheels to maintain the older classic look even though the wheels weigh in at crazy heavy numbers. I draw the line on classic look with better brake components. Bottom line, don't over think it, just bolt the front end on and ride it.

Since you have, literally, a blank canvas to work from, begin by determining how much trail you want.

I'll throw out that 110 to 115 mm has been a popular, proven number for lots of years.

Now, determine what your rake and rolling diameter of the front tire are, then figure out what offset you need in the triples.

It's easiest to do this with a full-scale drawing. The drawing has to have: ground plane, tire diameter, the center-line of the fork tubes, fork length, triple clamps and a known steering tube rake angle.

If your forks are significantly longer or shorter than stock tubes, take that into account when determining the rake of the steering head tube. Also, don't forget to include sag when making this measurement.

I consider that it's a given that the fork tubes and steering stem are parallel, although that is not always the case with choppers.

Draw a line at your rake angle which intersects the ground plane XX mm AHEAD of the contact patch of your tire, extending all the way up to the triple clamps. XX being the amount of trail you have selected.

Now you can directly measure how much triple clamp offset you need to achieve the amount of trail you want.

In order to make my guzzi steer faster, I chopped off the steering head and moved it a couple of degrees steeper. It worked OK in so far it didn't become a head shaking mess. I also dropped the forks through the triple clamps about 1/2 inch. I think your geometry changes are reasonably modest in comparison.
What didn't work was fitting wider rims and tyres. Made it all sloppy and vague. I was trying to make the tip in a bit smoother, but big tyres only made that worse. They were a lot narrower than what you are proposing. I think they are pretty sensitive to tyre profile too.
Best change was fitting lighter brake rotors. Biggest single improvement.

There are several different lengths of swing arm available. The late Cali ones being the longest I think. Are you playing with that too?

After owning several Tonti Guzzis since 1979, I came to the conclusion that the factory had optimised the components the best they were going to be. I tried and altered everything, there was not one original untouched component on my bike.
Decent forks (Guzzi ones are pretty crap) may just highlight the inadequacies of the rear suspension and short fat contact patches may affect stability in ways the frame is not prepared for.

Have you tried a dry fit to test out steering lock yet?
Even with small risers on mine, the bars comes close to the tank.

While I was concerned about you denting your nice tank or trapping your thumbs, having even less steering lock than standard I found made the bike surprisingly clumsy.

With guzzi forks I could just pull the bar back to the stop and the bike would twizzle on a sixpence, at least a normal (UK) city street.
After I fitted Marzocchi 38mm ones I had to reduce the lock quite a bit - made U turns a bit wobbly.

You will have even less clearance.

Have fun.

Me likey. This should be interesting and intelligent reading. Thanks for the pictures.

RE: swingarm

Years ago I tried a later, longer/wider swingarm on my T-3 based bike to fit some wider wheels I bought 2nd hand. I think it was off an SPIII. I hated the way it turned. I still have it around and would only use it again if I shortened it.

I went back to the short stock T-3 swingarm and the stock skinny rear tire. It handled way better for me, especially in tighter corners.

I did like the change from the stock 18" rim wheel to a 17" rim in the front.

Could go this route .....

http://www.guzzipower.com/store/SAbrace.html



I copy ,but stop short on the driveline as I wanted to keep the stock tire look.....