160 Fork Upgrade?

[gregwils]

Another newbie question. I bought Tom Bailey's book which is really a great read, and it's helped gaining a little knowledge. Anyway, I now know that the 160 and Bronco shared the same forks, and they are universally considered inferior to the other forks - Tomaselli, I think. He referred to them as "twitchy" at higher speeds. Would that not be a function of the fork rake as opposed to the fork themselves? (Don't flame me, just learning... ) I also got the impression that the better forks can't be swapped onto a 160 frame. Is my understanding correct?

Regardless of the answers above. Is there anything that can be done to improve the performance of the forks? Higher spring rates? Higher viscosity oil? Thanks.

[Rick]

Have you ridden the bike and determined there's a problem with the forks? If so, you may be able to bring them back to a useable state, things to check:
1) if the triple clamps are holding the tubes rigidly
2) The fork slider bushing(s)- I'm not familiar with that fork, but the fork leg should contact the fork sliders at the top of the slider and somehow at the bottom of the tube, but often there isn't a replaceable bushing at the bottom of the leg, just the tube contacting the slider. If you picture a tube sliding inside of another tube but only touching at the top of the outside tube, you can the imagine the potential for the inside tube wallowing around in the leg and getting 'twitchy'. Check the state of the slider bushing(s), not just the seals. If you remove the entire fork leg and spring, you should be able to slide the tube into the slider and not be able to wiggle it around at any point in it's travel. If the only bushing is the fit between the fork tube and the slider, and the slider is worn, there's no easy answer.

And, you can certainly mount a different set of forks to your frame, but finding a set that will bolt right on isn't too likely, so you may need custom machined bearing cups and a new stem- quite a bit of work.
Those forks probably worked OK for that bike at one time, I'd try to bring them back into spec. If you're set on a fork swap, the Honda CB160 fork is very well made and the brake is popular with vintage racers- the CB160 forks uses loose ball bearings for the headstock, but a tapered roller bearing upgrade is available.
Rick

[JimF]

Maybe it's the small fork diameter that makes them seem "twitchy" to some people. All my 60's singles have (or had) 31.5mm forks. As the 70's began for Ducati the engines were getting bigger and the frames in turn got beefier so too did the fork tubes get fatter.

Those 31.5mm fork tubes look strong, but I've a much harder time finding a straight for tube for all the tens of bent for tubes I've run across. And it's not that they are crash damaged, I think it's just road hazards for the most part that did the harm. So if a pot hole can bend the fork tube I imagine the tube can flex a bit in hard cornering giving it what some might think is a twitchy feel.

At least that's my guess.

I do thing you need to check your tubes for straightness (they can be straightened if they are bent) and then try them out. I put beefier forks on a 450 simply to house a 4LS hub on the front wheel but I like the light and easy to steer 31.5mm fork on my 250 and I don't feel that they twitch.

Jim

[gregwils]

Rick and Jim - thanks for the responses. No, I have not ridden the bike. It hasn't been started in a long time, and while it's really low miles, I found some crud in the bottom of the crankcase when changing the oil, I am going to go the conservative route and disassemble the motor to clean and inspect before I start it.

I got the impression of inferior forks from reading a few posts and from Tom Bailey's book. It's my understanding that these forks are different from the 250. According to Tom Bailey, Ducati used the forks from a Bronco, rather than the 125, for the Junior. He indicated they were trying to hit a price point and the 250 forks are more expensive.

The bike isn't a basket case, but I am going to do a complete restoration - my first. I am trying to get educated and applying the "while your in there" mentality to the process. I am not going for complete originality, but would probably prefer Ducati parts. I was hoping there was a straight forward swap even though I haven't come across it in my reading/lurking on the forums. I will keep an open mind to the CB 160 forks. I will confirm they are in spec before I make any decisions.

Jim - I agree. If a fork can get bent during normal street use, you know they are flexing.

Thanks again.

[JimF]

Since you are not limited to the pursuit of originality but you want Ducati components, you could opt for the 31.5mm forks that came on most of the narrowcase 250s including the scramblers and street bikes.

That would keep the components in the Ducati family and give you several options for Ducati handlebars, clip-ons, headlight mounts, etc.

I would just make sure the frame's steering head and the bearing races can accomodate the other forks, and while you are shopping keep in mind that the fender stays attach to the fork so you might have issues with mounting your original fender depending on where the attachment points are on the forks.

The 250 forks also have a boss for the front wheel brake. Here too you might want to be careful that you don't have to source a different brake, though some people on this list that have broader knowledge of all the models can tell you if the brake will be issue between your stock forks and 31.5mm forks.

[Rick]

I've attached a picture from a book you should buy if you want to understand motorcycle suspension systems, 'Race Tech's Motorcycle Suspension Bible' by Paul Thede and Lee Parks- if I plug their book they may not sue me for stealing the image.

fork leg.jpg

From looking through the parts books I'm not sure how the Ducati forks are built- can't see any bushings, but the picture shows the bushings I'm talking about, one on the slider and one on the tube. The bushings keep the slider and the tube in axial alignment- if the tube can flop around inside the slider there will be lots of looseness in the forks. You can also see that when the leg is fully extended the bushings are closest together and least stable, as the slider moves up the leg the entire assembly will become better supported.
I once added bushings to a set of forks- machined the slider to accept a UHMW bushing beneath the seal, and turned a shallow groove into the tube to accept another UHMW bushing- had the slider honed before fitting- lots of work, but the front end felt much better.
Buy the book, it will answer lots of your questions.
If you disassemble the forks, take some pictures so those of us who haven't had a set apart can see what's in there.
Rick

[gregwils]

Again, thanks to both. I'll do some additional research to see if anyone has gone before me and adapted 250 forks to a Junior. Actually, I'd like to try to find 125 forks, because from what I read they are better and both bikes use the same frame. Part of my challenge is that I live in Pittsburgh which is hardly the mecca of bevel. I hate to have to buy 250 forks only to discover they can't be made to work. I have read people put a 160 motor in a 250 frame, but at that point I would feel like I should just buy a 250.

Rick - I see the bushings you are referencing and your comments make sense. It will be a few months - summer house painting project - before I tear into the bike. I'd attack the forks first and do promise to post photos. thanks.