Rearset and drum brake leverage - confirmation my theory will work!

[peteGSX]

Hey all, I'm currently preparing to adapt some ZX12 rearsets to my 82 GS450 and I'm trying to ensure I still get sufficient leverage to operate the drum brake properly.

My plan at the moment is to retain the stock splined drum brake actuator that the stock brake lever operates, and to add a linkage to the rearset brake lever. I have a spare brake lever with the pedal end bent and twisted I'll cut down to use, the spline end is good.

I don't know a great way to explain this but I'll try my best and hopefully the pics will help!

I figure as long as the effective radius of the centre of the splines on the brake lever to that end of linkage is the same as from the centre of the rearset brake lever to its end of the linkage it would replicate the force of the stock brake lever with the same action, resulting in still having an effective rear drum brake.

The radius from the centre of that to where the linkage mounts on it is around 30mm.

The radius of the drum brake splined end is about 18mm, so I'd need to make sure the linkage is only about 12mm out from the splines to get a similar effective 30mm radius.

I've included pic's of my measurements below, can anyone confirm my theory here? Does how I've explained it even make sense? I'll mock it up soon but it requires removing the swingarm axle to do so and I haven't had a chance to do that just yet (this is my daily rider).

https://flic.kr/p/2hPWV2F]Untitled[/url] by https://www.flickr.com/photos/89703844@N08/]starpoint73[/url], on Flickr

https://flic.kr/p/2hPUj2B]Untitled[/url] by https://www.flickr.com/photos/89703844@N08/]starpoint73[/url], on Flickr

 

[Tim]

I put rearsets on my XS650 and never gave a second thought to the mechanical advantage I was losing in the process Still worked just fine.

But I guess the length of the lever being the same on the pedal as it is on the splined shaft the existing pedal connects to makes sense - the travel will be the same, but I'd assume your new rearset pedal is quite a bit shorter than the old pedal. So it would have to be pressed further down to result in the same amount of rotation....

I think

 

[teazer]

Not exactly. The diameter of the splined shaft has no bearing on the leverage. Look at teh length of the stock brake lever from center of shaft to the middle of the toe pad and divide that by the length of the actuating lever - that's the lever on that stock brake arm that pulls the cable or actuating rod. The ratio of those is the leverage so let's say the stock foot lever is say 200mm long and the part that pulls on the brake cable/rod is say 50mm, then the ratio is 200:50 or 4:1 In other words, if the lever moves down say 40mm, then it will pull the cable 10mm.

Now look at the Replacements and see if the ration is anywhere near similar. The new lever will probably be about half the length of OEM part so for the same rotation angle it will only pull the cable half as far as stock.

Since the rear brake is only used to steady the bike or in really slippy conditions, it will probably be OK.

If the new set up won't rotate far enough to operate the rear brake, try to lengthen the operating lever. That will make for a greater effort, but will pull the cable further.

 

[peteGSX]

Cheers guys, input much appreciated! I have a tendency to over think and over complicate things so it's best to get these ideas out and critiqued before I get myself into a world of pain

I took some more pics and measurements tonight. The drum is actuated via an arm rather than a cable FYI. Excuse the disgusting state it's in, like I say it's my daily rider so doesn't get a lot of cosmetic TLC at present.

I get the stock ratio as ~3.5:1 (225mm:65mm) and the rearset ratio is 4:1 (120mm:30mm) as per my measurements, so I think my plan will work. The stock pedal travels ~45mm for the brake to be applied. So if I keep the effective radius the same then with slightly better leverage with the rearset, that should require a little less travel... correct?

I really need to mock this up soon so I can check it all out, hopefully next weekend.

Oh, the plan for brackets is to use some 10 or 12mm thick 6061 T6 aluminium and do away with the stock hangers. That does mean I need to sort out a centre stand bump stop but that's a whole different story.

https://flic.kr/p/2hQ7oMU]Untitled[/url] by https://www.flickr.com/photos/89703844@N08/]starpoint73[/url], on Flickr

https://flic.kr/p/2hQ7oQp]Untitled[/url] by https://www.flickr.com/photos/89703844@N08/]starpoint73[/url], on Flickr

https://flic.kr/p/2hQ3Nv9]Untitled[/url] by https://www.flickr.com/photos/89703844@N08/]starpoint73[/url], on Flickr

https://flic.kr/p/2hQ7oPh]Untitled[/url] by https://www.flickr.com/photos/89703844@N08/]starpoint73[/url], on Flickr

 

[peteGSX]

FYI for thread completeness, here's what I've ended up with which seems to work just fine with the one test ride I've done so far.

Stock brake lever cut down using two rod ends to link to the rearset. The return spring is relocated to the rearset lever and an eyelet on the rearset bracket I made.

https://flic.kr/p/2i1sPyi]Untitled[/url] by https://www.flickr.com/photos/89703844@N08/]starpoint73[/url], on Flickr

 

[bbqb4racin]

Here was my answer to running rearsets with a mechanical rear brake.

 

[pidjones]

Thanks for closing out the thread, Pete! Too many leave them open-ended with no conclusion.

 

[peteGSX]

Interesting solution! How effective is it? I just didn't want to mess with the stock linkage etc.

And no probs on closing the thread out, I 100% agree. It gets so frustrating when you see someone posting exactly the same question or problem but then...