fj1200 on xs6500? is it possible.

[jamesgs4]

Geez what a load of crap. If it was a f/end off a R1 or whatever, nobody would be talking weight hp etc, and they are probably heavier. Looks as good a swap as any to me, go for it!

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no kidding

 

[jd750ace]

It's not just about weight. If you are railing around a corner, and hit a big bump or dip, with the stock setup, the fork and the frame both flex. With the FJ fork, it is something like 35% more rigid, because of the larger diameter and thicker walls of the tubes. Given that, the frame, at the steering stem, will be forced to absorb the flex, and it is going to stress things. Why do racing flat trackers have gussets welded into the frame? Handling was not the first reason. It was frame cracking that brought this. Also, if you stretch the frame 3 inches, then the distance to the rear contact patch is additional leverage for the flexing energy. Build it if you want, but I would not. There is no "gain" point in the swap that I see.

 

[yamaman]

more crap, where did you get the 35% from? i think you made that up. Also heavier wall staunchions, are they? By your reckoning (read guesswork) i should remove my fork brace, as now my f/end is too rigid for my flimsy frame.

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[jd750ace]

41MM Showa and KYB fork tubes are heavier walled than 35, 36, and 38MM tubes of the same manufacture. I can send you a link to Purdue University's library, where you can view the MMPDS-01. This is the bible by which joint and member strength is calculated for aircraft structure. There is an entire section on column strength (tubes, round, box, hex, whatever shape, and formulas for calculating relative strength. If you have the aptitude, you can calculate the percentage of increase in rigidity of a given member with an increasing diameter, and wall thickness.

Build your bike the way you want. The original post asked for any thoughts. I have thoughts. If you have ever had a machine that you built eat itself up, and then found out later that somone saw a potential issue and did not share thoughts or concerns, it makes you mad. It wastes time and effort, and it can kill people, especially in my line of work. I will make sure not to comment on YOUR projects, but the original post belongs to another user. Sorry if I think to much for you.
Also, the fork brace actually makes the tubes perform in tandem, preventing tortional twist, which is a good thing, so no, I would not say remove the brace.

 

[yamaman]

yeah sure.... back to the question at hand, the FJ front end would be a fine swap in my opinion. Most all improvements put strain further down the line, and indeed that is something to be wary of. Though in this case, id be more worried about getting the rear done right than any issues with swapping the front end for another. sorry for leading your thread down a murky path. goodluck, cheers

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[burns]

yeah sure.... back to the question at hand, the FJ front end would be a fine swap in my opinion. Most all improvements put strain further down the line, and indeed that is something to be wary of. Though in this case, id be more worried about getting the rear done right than any issues with swapping the front end for another. sorry for leading your thread down a murky path. goodluck, cheers

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Being no expert at stiffness (damn it) I venture this opinion with no pretension of any technical knowledge at all:

with a hard-tail frame the stiffness of the fork tubes is the least of your concerns. The forks have SPRINGS in them to take up the slings and arrows of road irregularities and I doubt that fork-tube flex is much of a shock-load factor. Front-end suspension compliance will suffer from the added unsprung weight, but will still be better than a girder or spinger. And you ain't gonna get ANY rear suspension compliance 'cause there ain't any dampening on that end at all and the only "spring" is the flexing of the frame itself.

Dialing in frame flex at the rear will be a challenge.

Seems to me.

BTW, I VERY MUCH doubt that an R-1 front end is heavier than an XJ 11 or 12.

 

[howry]

what do you mean by "Dialing in frame flex at the rear will be a challenge" ?

 

[Tomterrific]

The XS650 front end looks better. Use the money for an extra front brake. Modern front ends look wrong unless the old bike is built to use them.

Tom

 

[howry]

if i had a differ 19inch tire. same specs. i would do that. but i cant find one.

 

[burns]

what do you mean by "Dialing in frame flex at the rear will be a challenge" ?

well, you need some flex, and that will be a matter of wall thckness and geometry since you can't do anything with springs or dampening. Even with modern suspensions manufacturers spend a lot of computer time gettng those factors in the ball park, and race teams tweak it all the time.

So, just welding something up in the garage, with the focus being on looks, is not likely to result in a motorcycle that you are going to want to put in any challenging situations.

But I guess if you ride a hard-tailed bike that is a given anyway.

 

[howry]

well, you need some flex, and that will be a matter of wall thckness and geometry since you can't do anything with springs or dampening. Even with modern suspensions manufacturers spend a lot of computer time gettng those factors in the ball park, and race teams tweak it all the time.

So, just welding something up in the garage, with the focus being on looks, is not likely to result in a motorcycle that you are going to want to put in any challenging situations.

But I guess if you ride a hard-tailed bike that is a given anyway.

would you mind to explain it bit. like what should i focus on to make it better. or prob should just leave that idea. and try find a differ tire, that would be easier.

 

[burns]

People have different tastes in motorcycles. I lean toward bikes that l can dance with, but are still fairly relaxed. Someplace between cafe racer and "standard" I would never consider a hard-tail anything (other than a bicycle).

But there is no reason in the world for you to build a bike that I like - build one that YOU like. I'm sure you will find folks that share your taste.