Frame modifications

[mcarrel44]

Hello

This is my first post on my build. I have a bunch of bits and pieces of an xs that I am turning into a modernized cafe racer. I was wondering if there is any structural reason to leave the cross members that hold the bottom of the side covers in place, or if they could be removed.

 

[gggGary]

Looks great so far!
Just M2C; there needs to be some triangulation between the backbone and the swing arm mounts, but maybe not where Yamaha had it.

 

[atomic22]

I don't think there would be enough support without them, unless, like gggGary said, you put them somewhere else. I could be wrong...it wouldn't be the first time.
I like where you are going with the mono-shock. I got the same one, and I am hoping to get it on in the next couple of months, after riding season is over.

 

[5twins]

Adding a brace to the swingarm pivot area is supposed to help. Just a simple tube brace run across between the pivot plates and attached to the main backbone down tube .....

For best support, swingarm braces usually extend farther back and mount to the axle plates.

 

[barkcollar]

Like the mono shock

 

[mcarrel44]

Thanks for the responses guys.

A couple of questions..

5twins, what O.D. did you make that brace from?

Barkcollar, Is that the pivot arm from the R6 that you used for the top mounting bracket, and how does it ride?

 

[gggGary]

If you add that cross tube brace have the swing arm in and torqued before welding. I have seen some major frame deformation needed to get the slop out of the system. I ended up making shims so I did not mis-align the swing arm pivots. Yamaha also has shims for that purpose.

 

[barkcollar]

It is the R6 bottom bracket on the top ans used the top for the bottom.
It's a little weak and have a bump stop to hold my 165# ass up.
It need a stiffer spring. I even made a 3/4" spacer under the spring. Helped some.
I'll give more pics if u want

 

[barkcollar]

here u go

 

[newell]

Bark, I really dig that set-up, that is what I was thinking, I had thought about using a radian swing arm but it looks like I can use the stock swing arm... any details or more pics would be great... love the look of all the mono pics...

 

[5twins]

I don't know the diameter of the brace tube. Those are just pics I saved from the internet. Shouldn't be too hard to figure out though.

 

[mcarrel44]

Barkcollar, how much does your spring compress with the weight of the bike on it. I got mine all mounted up and took it off the stand and it dropped like a rock and it seems like there is very little travel left and its real soft. I can pick the back tire up with one hand and compress the shock a couple of inches without really straining myself. Does this sound like I've got a bad shock? or should I redo my mounts lower. When I did the math on the shock with the whole weight of the bike on it, the compression should only be like 7/8 of an inch, and dropped a full two and any additional weight makes it botttom out completely.

 

[Travis]

I'd be interested in hearing about the math that goes into that. Do you measure the weight of the bike less the swingarm (and everything bolted to it), then figure out how far the lower shock pivot is from both the swingarm pivot bolt and axle, then figure out the angle of the shock compressed and uncompressed, then do some trig maybe to figure the force applied at an angle, and bam! you have your numbers for what spring rate and shock to use?? I supposed you might work backwards to see if the shock you bought will work and where to mount it and what angle to mount it.

Obviously, I just kinda made that up... but would interested in hearing how it's actually done if you care to explain.

 

[Travis]

So I've been thinking, the best way to measure the weight would be from the new upper shock mount location with you on the bike. (maybe that's not correct either???) It's not as simple as the weight of the bike less the swingarm because you have a front wheel that carries weight too

So once you get that, you figure out the leverage gained from the swingarm (based on the length and lower shock mount position, and the angle of the shock relative to the movement of the swingarm to figure the force applied at the lower shock mount. Take all of that knowing what sag measurements you want with you on the bike, and then you'll have your spring rate???

I feel like I'm missing something..

 

[kent_in_kc]

I cut out the stock braces and welded in curved pieces (from a 7/8" handle bar). I changed the V of the stock braces into a U to gain enough space to put a battery.