free float the rear brake?

MaxPower
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MaxPower 1/31/2015 7:53pm
1/31/2015 7:53pm
I say no. We got disc on the rear and no one noticed it went away in 87.. More moving parts, bearings to get worn and sloppy,added weight.
That is a really nice bike to build. I didn't know you could buy a new C+J frame. I'm looking forward to seeing it come together
What suspension are you putting on it?
Lonestar_399 1/31/2015 8:28pm
1/31/2015 8:28pm
Is this a drum brake or disc brake question? I assumed drum...if so, I would float it.
swatdoc
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swatdoc 1/31/2015 10:22pm
1/31/2015 10:22pm
true - definitely more complex and more moving parts, but there has to be some benefit since most bikes went that way just prior to disc brakes driving on the scene.

Does anyone know exactly what the benefit is supposed to be?

And Lonestar, it's definitely drum brake.

Max - will be running Fox Factory Forx up front and Fox Airs out back.

Debating whether to stick with the chromoly swingarm Jeff provides, or to have Karl make me an aluminum one.

another debate i'm having is the rear hub. the stock XR hub weighs 7lbs, 3oz..I'm sure a CR hub is much less. The only issue would be that if i use a CR hub, the brake will be on the opposite side of the brake pedal. I have seen pics of this working - the cable just has to snake around to the other side. Not the cleanest setup, but it might save several pounds

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450exc115
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450exc115 2/1/2015 6:09am
2/1/2015 6:09am
Float a drum brake. The floating arm keeps the distance from the brake arm to drum the same so your brake force doesn't change with suspension movement. If you lock the drum to the swingarm arm that distance changes and it actually shortens and the arm goes up So you would apply more braking force with a brake rod. Cable actuated rear brakes don't have this problem. Smile

Disk brakes don't matter due to using hydhydraulics to apply braking force
MaxPower
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MaxPower 2/1/2015 6:44am
2/1/2015 6:44am
450exc115 wrote:
Float a drum brake. The floating arm keeps the distance from the brake arm to drum the same so your brake force doesn't change with suspension...
Float a drum brake. The floating arm keeps the distance from the brake arm to drum the same so your brake force doesn't change with suspension movement. If you lock the drum to the swingarm arm that distance changes and it actually shortens and the arm goes up So you would apply more braking force with a brake rod. Cable actuated rear brakes don't have this problem. Smile

Disk brakes don't matter due to using hydhydraulics to apply braking force
That is the first time I've heard that reasoning. Thanks 450exc. For years the story has always been the braking torque being directed to the frame and it sounded good but somehow no one noticed it on as bikes progressed.

Swatdoc, go with a hub from a 84 Honda hub. They had left side chains that year. Or RM hubs front and rear to match, they had strait pull spokes.
If you are having an aluminim swingarm built, Is it possible to copy the Works Honda look?
I suppose the only draw back to having an aluminum swingarm made is the cost. I don't see steel being a problem as long as its designed with some though

78rm125
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78rm125 2/1/2015 12:29pm
2/1/2015 12:29pm


I was always wondering how that all works. This made it a bit more clear. Im converting my 75 Rupp RMX to full floating
swatdoc
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swatdoc 2/1/2015 2:24pm
2/1/2015 2:24pm
wow so it looks like there's a benefit whether using a cable or rod actuated brake. Thanks for that diagram. Looks like it would be worth the effort to weld a tab onto the frame.

Max - thanks for the heads up on the 84 hub. After a little research it looks like Honda used that same CR250 hub from 84 thru 86. From the pictures it definitely looks lighter than the XR hub. I'll see what I can pick up on Ebay.

Thanks everyone for your input
yzweldon
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yzweldon 2/1/2015 2:39pm
2/1/2015 2:39pm
I converted my 77 to the 80 arm and had to add the full floater I made myself

MaxPower
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MaxPower 2/1/2015 6:16pm
2/1/2015 6:16pm
78rm125 wrote:
[img]https://p.vitalmx.com/photos/forums/2015/02/01/78866/s1200_dip5000300030002.jpg[/img] I was always wondering how that all works. This made it a bit more clear. Im converting my 75 Rupp RMX to full floating


I was always wondering how that all works. This made it a bit more clear. Im converting my 75 Rupp RMX to full floating
This diagram is what I always understood snd now I like 450 EXCs explanation better

If that diagrams theory made that bike of a difference then why arent braking forces a problem for the new bikes?
swatdoc
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swatdoc 2/1/2015 7:17pm
2/1/2015 7:17pm
yea thats true Max - all modern bikes have the caliper attached to the swingarm..

so maybe since I'll be using a cable, it won't really matter after all

I can definitely see the difference if using a rod activated drum brake - as the swingarm goes up and down the effective length of the rod would change, thereby changing the amount of braking force. With a cable it shouldn't matter.
450exc115
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450exc115 2/1/2015 9:03pm
2/1/2015 9:03pm
That old picture is crap. The braking forces on the shock versus frame don't attribute to much in the way of suspension influence. The interia and mass of the rear wheel far outweigh the force from the brake in suspension movement.

Yamaha went from the floating to non floating then back to floating because of complaints of poor rear brake modulation.

Personally I like the old PE and MR cable actuated brakes. Simple to setup and made for easier wheel changes.
wolfy0067
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wolfy0067 2/9/2015 12:06pm
2/9/2015 12:06pm
450exc115 wrote:
That old picture is crap. The braking forces on the shock versus frame don't attribute to much in the way of suspension influence. The interia and...
That old picture is crap. The braking forces on the shock versus frame don't attribute to much in the way of suspension influence. The interia and mass of the rear wheel far outweigh the force from the brake in suspension movement.

Yamaha went from the floating to non floating then back to floating because of complaints of poor rear brake modulation.

Personally I like the old PE and MR cable actuated brakes. Simple to setup and made for easier wheel changes.
If you race the two setups bact to back, you will find that the floating brake doesn't skip across the ground or follows the terrain allow you to go into braking bumps alot smoother, non floating bounces and wants to skip into the corner, makes a big difference on down hill braking bumps and lap times.
swatdoc
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swatdoc 2/10/2015 7:11pm
2/10/2015 7:11pm
Max - I picked up an 84-86 rear hub from eBay. Looks like it's 2 1/2 lbs lighter than the XR500 hub. Thanks for the tip - it would cost about $1500 to lose that much weight using titanium!!!

Only minor issue is that it uses a 20mm axle, compared to 17mm for the XR. I should be able to have the slots in the swingarm enlarged by 3mm no problem. Will have to get custom axle and spacers made, but that shouldn't be too difficult. If I have Karl make me a whole new swingarm I'll just send him everything and get it made to match up. Gotta make sure I get the spacers just right so the rear sprocket is not misaligned with the countershaft sprocket.

I figured I might as well float the rear brake. Looks like I will only have to weld one tab to the frame just behind the footpeg mount.

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