Chain, Swingarm Clearance

[tbone222]

Well, this is a conundrum. Starting to put this little brat/bobber together and the chain clearance on the swing-arm doesn't work. What I have done so far - lowered the front forks 2"; switched from the original RK chain to a new 530 chain; switched the "raincatcher" 18" rear wheel and tire that came with this '78 to a 16" wheel and tire and replaced the 34 tooth sprocket with a 33 tooth. All I can think of is that by lowering the front forks it has thrown off the geometry of the angle of the chain. Any solution other than changing the forks back to stock?

 

[Guy]

Raising , lowering forks has no effect on rear tire to chain clearance. With OE rear 16'' tire there is very little clearance, not much room for a wider tire. Are you talking about the chain touching the top or bottom of the swing arm pivot?

 

[gggGary]

Well, this is a conundrum. Starting to put this little brat/bobber together and the chain clearance on the swing-arm doesn't work. What I have done so far - lowered the front forks 2"; switched from the original RK chain to a new 530 chain; switched the "raincatcher" 18" rear wheel and tire that came with this '78 to a 16" wheel and tire and replaced the 34 tooth sprocket with a 33 tooth. All I can think of is that by lowering the front forks it has thrown off the geometry of the angle of the chain. Any solution other than changing the forks back to stock?

Pictures!
Hard to tell what your problem is from Baraboo, even with binoculars ....

 

[tbone222]

Raising , lowering forks has no effect on rear tire to chain clearance. With OE rear 16'' tire there is very little clearance, not much room for a wider tire. Are you talking about the chain touching the top or bottom of the swing arm pivot?

Top of the swing arm

 

[tbone222]

Shorter Shocks oughta do I think!

 

[5twins]

Yes, with the Brat kit "laying" the shock down more, and stock length shocks, the swingarm is angled down more. Shorter shocks is the easy "fix". Moving the top shock mount up would also work but is more difficult.

 

[Wingedwheel]

I had the same issue on a brat build. Does it clear with your weight on the bike?

 

[gggGary]

I had the same issue on a brat build. Does it clear with your weight on the bike?

good point. A shock that's laid down has an effectively lower spring rate so it will prolly sag more under rider's weight.

 

[tbone222]

I had the same issue on a brat build. Does it clear with your weight on the bike?

I'll give it a try and let you know

 

[Wagner73]

Yes, with the Brat kit "laying" the shock down more, and stock length shocks, the swingarm is angled down more.

I have to say I'm slightly confused by this; I would think it would be just the opposite. If the upper and lower shock mounting points stay in their same planes i.e. the top shock mount moves further forward but in the same plane, the stock lower mount is retained and the shock length remains stock, the resulting vertical distance is reduced and the swingarm angles up more. Of course, if the top shock mount moves forward but also down, the case may be different but it's just geometry.

What am I missing?

 

[Wingedwheel]

What am I missing?

The 16” wheel in conjunction with stock shocks and a shorter distance between mounts and a smaller diameter sprocket. Everything adds up.

 

[Wagner73]

I'm still not really getting it, but okay. I concede that one tooth less on the sprocket marginally reduces the diameter and moves the chain ~1/4" (if that) toward the top of the swingarm. The shorter rear tire & wheel combo does nothing to affect the chain...yes the bike sits a little lower in the rear but just pivots around the front axle per se with no real chain effect.

And then there's the shock geometry which I apparently have misinterpreted.

 

[bosco659]

Shock length and upper shock mount position are giving you headaches. The upper mounts will be a pita to change. Best bet is to block it under the frame, remove the shocks and raise the swing arm to the desired height for clearance and you’ll see the compressed (amount while sitting on it) shock length you’ll need and or where the mounts will need to be. Maybe you’ll decide to hard tail it (although I wouldn’t).

 

[Mannyroad]

I'm still not really getting it, but okay. I concede that one tooth less on the sprocket marginally reduces the diameter and moves the chain ~1/4" (if that) toward the top of the swingarm. The shorter rear tire & wheel combo does nothing to affect the chain...yes the bike sits a little lower in the rear but just pivots around the front axle per se with no real chain effect.

And then there's the shock geometry which I apparently have misinterpreted.

Looks to me, from looking at a stock frame diagram, that the frame in the photos has been modified to the extent that the top shock mounts are significantly lower than stock frame mount points, so using stick shocks will cause the significant downward angle on the swingarm, which is clear in the photos. Shorter shocks or re-configute the height of the top mounts is zbout all you can do.

 

[Wagner73]

Looks to me, from looking at a stock frame diagram, that the frame in the photos has been modified to the extent that the top shock mounts are significantly lower than stock frame mounts...Shorter shocks or re-configute the height of the top mounts is zbout all you can do.

Yes, true. Based on your point(s) I have compared the photo of TBone's frame & top shock mounting point to my 78E (pictured). The mounts are significantly further forward and 3"-4" lower than stock. So this definitely would create the swingarm issues. As has been pointed out by @bosco659 and @Mannyroad , it would seem the only choice would be to arrive at a shock length that will allow the swingarm to end at the desired height.

And as @gggGary has said, the rated spring rate will have to be stiffer to provide appropriate realized spring rate/action in the laid-down configuration.

 

[tbone222]

Hello, Sat on the bike and got some relief between chain & swing arm but I wouldn't be comfortable riding it that way (about 1" between chain and top of swing arm but chain was tight with little play in it). The shocks on the bike are 12.5" eye to eye so ordered a pair of 11" eye to eye shocks from ebay. Thinking that between these shocks and my weight this should work out (hope so!). Will keep you posted.

 

[gggGary]

Did a bit of looking and a straight line through rear axle, swing arm pivot extended forward will be about at the top bolt of the front engine mount. True on madness and another stock special and also on this bobber frame. These front shock mounts look like they're about 1 1/2"? further forward than yours are. All have 13" shocks.

Madness has a nylon rub strip on top of a thicker than stock swing arm with a 32 tooth rear sprocket, the chain sits on the swing arm on the stand, that piece has been on there for at least 20K miles.

 

[tbone222]

Did a bit of looking and a straight line through rear axle, swing arm pivot extended forward will be about at the top bolt of the front engine mount. True on madness and another stock special and also on this bobber frame. These front shock mounts look like they're about 1 1/2"? further forward than yours are. All have 13" shocks.
View attachment 360282 View attachment 360283
Madness has a nylon rub strip on top of a thicker than stock swing arm with a 32 tooth rear sprocket, the chain sits on the swing arm on the stand, that piece has been on there for at least 20K miles.
View attachment 360276 View attachment 360277 View attachment 360278

Thanks I will take this into serious consideration

 

[tbone222]

Just an update, Put on 11" eye to eye shocks which worked out really well (had 12.5" eye to eye standard shocks on). Was considering a pair of 11 3/8" but these will work. Bike sits just fine from what I can see, even with me sitting on it. With the shocks on I have almost an inch clearance off the frame and it pretty much stays that way with me on the bike. I think this solves this piece of the puzzle.

 

[halfmile]

Did a bit of looking and a straight line through rear axle, swing arm pivot extended forward will be about at the top bolt of the front engine mount. True on madness and another stock special and also on this bobber frame. These front shock mounts look like they're about 1 1/2"? further forward than yours are. All have 13" shocks.
View attachment 360282 View attachment 360283
Madness has a nylon rub strip on top of a thicker than stock swing arm with a 32 tooth rear sprocket, the chain sits on the swing arm on the stand, that piece has been on there for at least 20K miles.
View attachment 360276 View attachment 360277 View attachment 360278

Gary, I see you got the brat kit welded up. Looks great. Maybe I should make some more this winter?