Battery-Carrying Rear Rack Failures. Dampening Needed?

[Kevin Crawford]

Hello all. My 2nd rear rack just failed. It's this one: http://www.ebikes.ca/shop/electric-bicycle-parts/battery-accessories/battrack-dd-ez.html.

It failed at the rear welds, and also one tube snapped (?). This is the 2nd rack I've had fail on me this way in 3 years. They are aluminum. I don't have a particularly heavy setup - just one eZee battery pack, and a medium trunk bag. I am careful and take it slowly over railroad tracks and other impacts, though I do ride about 100 miles per week.

It seems to me that the problem might be that the racks are just too rigid. Or would I be better served by getting a sturdier rack? I like the eZee racks for the battery-locking feature, and I like the double-decker version so I don't have to carry stuff on my back 2 hours a day. But I want my next rack to last. I like the idea of an axle-mounted rack, and I'm thinking steel would be better. But would that really solve my problem? I haven't come up with a good way to dampen vibration on the rack I've been using.

Any suggestions? Thank you very much.

 

[FluxZoom]

I had the same problem with aluminum racks, full suspension frames. The racks I used we're loaded vastly below their ratings, I just looked for the highest load rating and went with that.

https://en.wikipedia.org/wiki/Fatigue_limit

"Other structural metals such as aluminium and copper, do not have a distinct limit and will eventually fail even from small stress amplitudes."

Went with custom steel, no more broken racks.

 

[amberwolf]

I'd recommend looking at the various threads discussing rack breakage; there's a lot of suggestions and some tested solutions to assorted issues.


Unfortunately there's no easy search term set to find them, but here's some stuff you could sort thru for more info:

Some by post
https://endless-sphere.com/forums/search.php?st=0&sk=t&sd=d&sr=posts&keywords=rear+rack+break*+heavy+batt%2A

Some by thread title:

just the racks:
https://endless-sphere.com/forums/search.php?keywords=rack&terms=all&author=&sc=1&sf=titleonly&sk=t&sd=d&sr=topics&st=0&ch=300&t=0&submit=Search

just racks and batteries
https://endless-sphere.com/forums/search.php?st=0&sk=t&sd=d&sr=topics&keywords=rack+batt%2A&sf=titleonly

 

[Chalo]

What size tires and pressures do you use? That makes a big difference in the transient peak forces on your rack, axles, bearings, etc. The max pressure rating of your tires is usually not the right pressure. Using fatter tires and/or lower pressure is probably the single most effective change you can make.

No rack material will have enough damping to make a difference. The problem is direct shocks, not resonant vibration. Steel has less damping and higher stiffness than aluminum anyway.

Batteries are uncompliant dead weight, and the harder they're mounted, the more they beat things up (including themselves). Way back when I used big ol' SLA bricks, I put 3/4" of closed cell EVA foam between them and the rack top. It was still very tough on my rear wheel to carry them, even though they were a small fraction of my weight. The sturdy aluminum rack held up OK.

Another easy change you can make is to carry the battery in a cloth pannier, so the bag serves as a kind of suspension. This works best with a divided and approximately balanced load on both sides.

 

[Kevin Crawford]

Thanks.

The tires are 700C x 32, and I go with max pressure whenever I top off. That's 65 PSI on my current winter studded tires, and something like 100 on 3-season tires.

I figured it was important to have them high, considering the loads on them (though I use a front hub motor). But you're right, I probably could drop that a little without issue. I'll try dropping the rear by 10 PSI and see how that goes.

I hadn't thought of using wider tires. I can see that also helping. Too bad I already have a spare set, and they're Marathon Plusses, also in 32C, which last forever. I'm not kidding about this - I have used these for something like 10,000 miles, and not only have I never had a puncture, they aren't even very worn.