Gilson/MW Band Brake Assembly--Update

#1
I documented this extensively in the Trail Bike build, but I thought I'd bring it back with an update. You can find the actual fab work here starting at post #40: http://www.oldminibikes.com/forum/p...ail-bike-build-resto-kinda-sorta-maybe-4.html

The prototype needed some changes to work more effectively. One of the problems with the prototype was the inability of the lever to fully apply mechanical advantage. Secondly, the hand brake lever needed to exert full force at about halfway in (which is the ideal). To correct this, one part especially needed work: The rod connecting the lever arm to the brake band was too long (1.75") and accordingly too much travel. It was reduced to 1.25". The next two photos show it as originally assembled. Note the band pivot point. This may have to be adjusted again shifting to the right about 1/4".





Here is the version with the shortened arm. The lever is now against the band in the "rest" position.



The other issue was the cable stop. The cable sheath is supposed to stop at the tab on the left. However, this location interfered with the TAV Driven. The other problem is that any sort of bracket off that tab had too much flex and reduced the mechanical advantage of the lever (this is a big problem on the Trail Bike). So, the stop is moved toward the rear and slopes up (for TAV clearance) and is bolted in two places, and no flex. This is slightly thicker stock also. From the factory, there was no spring. This design uses a spring to make sure the lever returns to rest.



Another look. I used a 5/16" bolt with a stop nut and lock nut to secure the cable. Alert eyes will see two nuts on the axle: I needed a stop because the axle is threaded on both sides and the outer will come off.



The jackshaft and TAV assembly got sorted and Copperhead is almost back together. Was way too cold today to even consider starting the bike and doing a test ride.
 
#2
That looks really good. Lucky Mine has a pretty good brake on it still or this would be another idea I'd have to steal from you. I however might be hollaring at ya about getting some measurements off your TAV plates pretty soon.
 
#4
Still More on Band Brake Pivot Point

After all described above plus removing any play in the lever, the brakes still did not grab properly. I know there is a limit as to how well a band brake will bring a bike down from speed, but it just wasn't right. So I went back to school and looked up band brakes and pivot points for maximum efficiency. After doing a lot of reading, I realized I had the pivot off. Engineers are probably saying right now, "Of course, you dumb @$$." My last physics class was in 1979. :laugh:

The image below shows how the pivot point should be located. You can see I was off by roughly 1/2". Two keys: intersecting lines (YELLOW) must go through the pivot of the lever and tangent (a line that touches the perimeter of the circle at one point only) to the drum. And, a tangental line (RED) must pass through both band brake ends. The correct pivot point is identified by the lavender arrow.


To locate the pivot points correctly, you need to clamp the band brake to the drum. Then it's simply using straight edges to find the points. Offset a tad for the bolt(s).

Reinstalled the new assembly and now works as intended...stops better than expected and the mechanical advantage of the lever makes all the difference.

A side note, same principles apply if you're making your own clutch brake.
 
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