rear drum not grabbing

[Umusername]

I finally go the linkage set up on the rear drum to provide enough mechanical advantage, travel, and pressure after installing rear sets. I thought my issue was poor leverage and not that I have plenty I am finding that the brake still will not grab (it will slow the bike slightly faster than coasting) the shoes are good as I put new ones in same day I did the rear sets. Now it is time to start truble shooting I suppose, what should I attack first, How should I go about it and what should I look for? I will post a picture later tonight I have to run to work.

 

[Umusername]

got a chance to add the picture.

 

[coyote13]

Have you considered putting the linkage under the peg instead of above?

 

[CarbsAndCylinders]

The brake arm and the rearset arm are parallel but the angle of the rod between them looks too steep to me. It looks to me that the force is pulling the brake lever towards the axle and not in the brake arm's proper curve. This might mean that it will take a huge effort to squeeze the pads against the drum. This is just a thought. The other thing I thought of, and this happened to me once, is that if you replace any bolts that screw into the brake, you may have put a longer one in and it could be obstructing the movement inside it.

 

[Bert Jan]

got a chance to add the picture.

WTF...

ive seen some crazy stuff come by here but this mod wins hands down. There is a small hole in the rearset. The one you bolted the strip thru. Connect the brake rod to that and remove that crazy piece of strip. The arm on the foot-lever needs to be A LOT shorter.

 

[Umusername]

Well if it were that simple I am sure it would have worked when I had it set up that way, this is the fourth or fifth revision I have made and this is the most functioning yet. The brake takes more mechanical advantage to match the amount of pull that the brake requires to engage. On top of that I have to clear the frame. I agree that it may be on the long side and I will be working on that but in any case I will need some sort of extension to the rearset. I cannot run it under the peg because the exhaust pipe is in the way.

 

[Bert Jan]

Well if it were that simple I am sure it would have worked when I had it set up that way, this is the fourth or fifth revision I have made and this is the most functioning yet. The brake takes more mechanical advantage to match the amount of pull that the brake requires to engage. On top of that I have to clear the frame. I agree that it may be on the long side and I will be working on that but in any case I will need some sort of extension to the rearset. I cannot run it under the peg because the exhaust pipe is in the way.

Hmm ok... Would it be an option to put the arm on the drum upwards?

 

[rockcitycafe]

Well if it were that simple I am sure it would have worked when I had it set up that way, this is the fourth or fifth revision I have made and this is the most functioning yet. The brake takes more mechanical advantage to match the amount of pull that the brake requires to engage. On top of that I have to clear the frame. I agree that it may be on the long side and I will be working on that but in any case I will need some sort of extension to the rearset. I cannot run it under the peg because the exhaust pipe is in the way.

uhm, okay, you just posted a thread asking for advice why something isn't working and then rejected one of the knowledgeable answers you received. if you're looking for advice, you should be open to it when you get it. thirdly, looking at your setup, you know jack about leverage, and had you looked at how the stock setup WORKED (yes, it worked, those crazy engineers back in the 70's didn't kill their lunch with sharpened rocks you know), you would see just how much of a lever ratio a drum brake requires. I apologize for the curtness, my coffee is still brewing.

 

[Bert Jan]

uhm, okay, you just posted a thread asking for advice why something isn't working and then rejected one of the knowledgeable answers you received. if you're looking for advice, you should be open to it when you get it. thirdly, looking at your setup, you know jack about leverage, and had you looked at how the stock setup WORKED (yes, it worked, those crazy engineers back in the 70's didn't kill their lunch with sharpened rocks you know), you would see just how much of a lever ratio a drum brake requires. I apologize for the curtness, my coffee is still brewing.

It makes me happy if someone who can speak good engish types what i meant to say. thnx.

 

[ApriliaBill]

I'm pretty much a novice at this, so don't take this personal, it's just my take. First, function over form, get the brake set up and worry about the exhaust later. Second, the pictures running in my head, have drum brake levers usually very long and pivoted off the frame and not the peg. Longer arm, more leverage? I agree, the brake arm has to run under, not over. Maybe need an off-set brake arm...

 

[CarbsAndCylinders]

I completely missed what Bert and Rok saw, the fact that you have reduced your leverage with the brake rear set extension.

You have to take that brake apart and see what is going on inside, are the shoes ruined with oil or grease, are they properly installed etc.

No need to insult anyone here

 

[Umusername]

Hmm ok... Would it be an option to put the arm on the drum upwards?

I will try this out next. The brake itself does function either way (like I said I have tried a few different set ups). I would love to do an offset to make it work best. I ment no offense to anyone and after comparing the stock mechanism I can see that I can cut the extension down about 1.5 inches. One of the biggest issue I am having is woking around the huge triangle frame suport (that I actually mounted the rearset on) because it forces an off set of near 5 inches from the top of the rear set to a point near parallel to the brake.

 

[rockcitycafe]

here's the rub, your stock brake lever was about 12" long, it turned on a pivot which has another arm on it which was about 1.5" long at most, right there is an 8:1 lever ratio, now you take that 1.5" and run it back to the rear drum arm, which is about 4" long, so there is another 2.6:1 ratio, now multiply, and you get 21:1 lever ratio roughly from your toe to the rear drum. what your setup shows is that you've got about 1:1. tell me how you think that's going to work. if I put 5lbs of pressure on a stock drum setup, I can lock the wheel, you'd have to put over 100 lbs down to do the same. you need to redesign your setup or just give up on having a functional rear brake

 

[rockcitycafe]

actually, scratch that... your setup shows probably a 1:2 ratio, so you'd literally have to stand on the brake and weigh about 30 lbs more than I do to give the same stopping power as 5lbs on the stock setup

 

[CarbsAndCylinders]

What Roc said makes a lot more sense that what I was thinking.

 

[Umusername]

Ok I know that this is not a permanent solution but this is the set up that I revised to. I still cannot lock it up (I think it is because the bends in the rod give rather than pull the pedal) but it is better. In the future I would like to go up to an 18" rear rim anyway, what is the correct process for moving to a disc brake as that probably makes most sense and by asking now I can be 100% ready when the time comes.

 

[hillsy]

Most disc brake calipers bolt to a carrier that mounts on the axle - and the caliper sits on top of the disc. You will have to move the torque arm to the top of the swingarm to mount this.


Also, you need to mount the brake pedal / master cylinder somehow. If you get sportsbike rear sets (like CBR600) most have a mount for the MC on them.


But you are probably right with the brake rod bending - if you can fix this you'll probably find the drum brake will be adequate.

 

[sprocket2cog]

is the brake connecting rod aluminium ? if so that wont help with the flex.

 

[rockcitycafe]

I still cannot lock it up (I think it is because the bends in the rod give rather than pull the pedal) but it is better.

you're right... physics isn't real.

I hope your front brake works well, and 100% of the time, I hate reading posts about people being in the hospital

 

[Bert Jan]

Ok I know that this is not a permanent solution but this is the set up that I revised to. I still cannot lock it up (I think it is because the bends in the rod give rather than pull the pedal) but it is better. In the future I would like to go up to an 18" rear rim anyway, what is the correct process for moving to a disc brake as that probably makes most sense and by asking now I can be 100% ready when the time comes.

MUCH better already.
Few things, it seems the brake lever for your foot is kinda high up for being a rearset lever. rule of physics is that levers need to be linear to work best. So, make sure the little arm on your lever and the big lever at the drum are linear.
The brake rod will give less if you keep the bends as close to the ends as possible. If thats not possible, i once did this to solve stuff and kill some time:

Obviously, the custom part did get a bearing when the footpeg was installed properly. This was a dryrun photo.

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