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Coaster brake becoming unpredictable

Old 11-13-19, 03:33 PM
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wesgreen
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Coaster brake becoming unpredictable

Hi,
I have a SRAM i-Motion 3 Speed coaster brake hub, supposedly maintenance - free, bought with a Dahon Vitesse folding bike in 2013. It still shifts perfectly, after 6 1/2 years and less than 4000 miles, but for the last few months sometimes the brake does not engage until I reverse - pedal for about half a rotation. Very unpredictable. Sram CS says it could be hardened oil residue; maybe replacing the oil would fix it. Is that the most likely problem?
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Old 11-13-19, 04:03 PM
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"Maintenance free" just means disposable

Any mechanical device can and does wear. With most all the CB hubs I've dealt with there's no real adjustability of how much cog rotation is required before the brake engages. The cog/ring ratio does effect this but that's a constant. So as surfaces wear it takes more cog rotation to bring together the internal parts. Additionally if the surfaces that rub/roll against each other are not lubed then both greater wear and less easy function will follow.

Does SRAM offer servicing instructions? Seems to me that a tear down, cleaning and relubing on reassembly would be a good thing to do. Andy
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Old 11-13-19, 04:23 PM
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Originally Posted by wesgreen View Post
...
Sram CS says it could be hardened oil residue; maybe replacing the oil would fix it. Is that the most likely problem?
What did SRAM say is most likely when you asked them? Why not give it a try? [No need to answer that rhetorical question; I already know why.]
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Old 11-13-19, 05:05 PM
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Originally Posted by wesgreen View Post
Hi,
I have a SRAM i-Motion 3 Speed coaster brake hub, supposedly maintenance - free, bought with a Dahon Vitesse folding bike in 2013.
"maintenance free" only means that it will work, until it doesn't. Once that happens, it is time to look into rebuild options that the manufacturer never thought necessary. It may also be possible that the bike manufacturer misrepresented the component manufacturer's specs thinking that nobody would ride that bike as far as you have.
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Old 11-13-19, 07:06 PM
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wesgreen
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Originally Posted by Andrew R Stewart View Post
"Maintenance free" just means disposable

Any mechanical device can and does wear. With most all the CB hubs I've dealt with there's no real adjustability of how much cog rotation is required before the brake engages. The cog/ring ratio does effect this but that's a constant. So as surfaces wear it takes more cog rotation to bring together the internal parts. Additionally if the surfaces that rub/roll against each other are not lubed then both greater wear and less easy function will follow.

Does SRAM offer servicing instructions? Seems to me that a tear down, cleaning and relubing on reassembly would be a good thing to do. Andy
Thanks for your responses, alcjphil and AndrewRStewart. Although Sram claims the hub is permanently lubed and maintenance - free, they did publish disassembly and lubing instructions. Maybe I should take that as a hint. I think Shimano recommends relubing every 5000 miles for the Nexus 3. The Sram rep I spoke to today was not familiar with the hub, but aware of the recall issued for it this year (brake malfunction). Sram stopped making hubs, so no more parts or feedback from them. This hub doesn't seem to have the greatest rep for longevity, but is very efficient, quiet, lightweight, shifts great and allows for super easy wheel changes. I guess you can't have it all.
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Old 11-13-19, 08:16 PM
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Originally Posted by wesgreen View Post
Although Sram claims the hub is permanently lubed and maintenance - free, they did publish disassembly and lubing instructions. Maybe I should take that as a hint. This hub doesn't seem to have the greatest rep for longevity, but is very efficient, quiet, lightweight, shifts great and allows for super easy wheel changes. I guess you can't have it all.
Maybe you can have it all if you follow directions for disassembly and lubrication. Longevity can be improved by just doing that
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Old 11-13-19, 08:37 PM
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I never used coaster brakes and don't know anything about it. I was curious and watched this RJ bike guy video for single speed coaster brake hub:

I would imagine you're getting problems with the clutch engaging when backpedaling. Some stuff is either blocking the teeth from contacting, or the driver is loose. Or the brake pad is worn and loose. But the i-motion will have different mechanisms and you'll have to look at the manual to figure it out as you disassemble it. I also would imagine that the braking creates a lot of metal dust if the i-motion brakes work similarly to the hub that's in the video. But I could be wrong. You'll only find out if you disassemble it.

You can find some photos of the i-motion 3 disassembled here:
https://www.rideyourbike.com/sramIGH.shtml

I've disassembled my old SRAM Dual Drive II several times in the past, and it looks surprisingly similar to the i-motion 3 except that the Dual Drive has a freehub and no brakes, but I can't figure out how the brakes on the i-motion 3 works just by looking at the photo. The Dual Drive gets very dirty from wear and old grease, so I used to clean it out and regrease/oil once every year or 2. Following the instructions the first time was confusing but easier on the 2nd time. One of the main things, assuming the construction is similar to the i-motion 3, is that you release the left nut first and most of the internals then freely pull out from the right side. In reverse, you assemble all the internals onto the axle, then slip it into the hub shell from the right side.

Work on a very large table so you can lay everything out flat. And I didn't take photos but it would have helped during my first time if I had an image of every step of disassembly to review when I forget how to put it back together.

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Old 11-13-19, 10:17 PM
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ttt1232- The only problem with the possible problem you describe is that the clutch doesn't slip WRT the brake shoes like it does with the drive function. The part of the clutch that the pointer is pointing at is that drive side and, yes, it's shallowly splined surface can wear and/or get glazed. But the other side of the clutch (not pointed at and being the brake shoe engaging portion) usually has two lugs or protrusions that hold the shoes from slipping WRT the clutch. Andy (who was nervous when he first overhauled a Bendix CB hub way back when and after having done a number of SA AWs and S3Cs).
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Old 11-14-19, 11:01 AM
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A recall for this hub has been issued by SRAM. Google "SRAM recall" and you'll see the hub in question.
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Old 11-15-19, 03:33 PM
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Beautiful video, and hub explosion pics. Thanks, tomtomtom123. I'll be taking a look over the holidays, and favoring the front brake until then.
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Old 11-16-19, 12:26 PM
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Originally Posted by Andrew R Stewart View Post
ttt1232- The only problem with the possible problem you describe is that the clutch doesn't slip WRT the brake shoes like it does with the drive function. The part of the clutch that the pointer is pointing at is that drive side and, yes, it's shallowly splined surface can wear and/or get glazed. But the other side of the clutch (not pointed at and being the brake shoe engaging portion) usually has two lugs or protrusions that hold the shoes from slipping WRT the clutch. Andy (who was nervous when he first overhauled a Bendix CB hub way back when and after having done a number of SA AWs and S3Cs).
Did you ever get into a Bendix Automatic? I had those as a kid in the late '50s. Pretty amazing thing and a terrific brake with its three large shoes but I never found a grease exactly like the factory grease. It looked like white soap but was soft and not 'stringy' at all. I used one again years later in the '80s and learned that you don't use a Teflon grease in a CB hub! That really should have been obvious.
BTW, I think on the OP brake hub the problem is wear resulting from lubrication failure. The wear will be manifest in the drum/hub braking surface and the shoes. If new shoes don't make it good enough the brake sector of the hub might be a write-off.

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Old 12-29-19, 05:34 PM
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I had asked the SRAM cs rep if over - tightening the axle nuts could cause this, and he said he didn't know. But a few weeks ago I loosened them a little, and that seems to have solved the problem.
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