DIY rear rack battery mount (or not?)

[saucerful]

I electrified a 20" bicycle (Bike Friday) with an em3ev shark battery. However, I don't like where I put the battery (vertical on the seat mast) and want to move to rear rack. I see two options:

1) Premade -> switching to a different battery? Like this, but it just barely fits the 76 mm thick (3") battery it is designed for. The only quality hard case battery I can find in those dimensions is Grin's ezee. Getting a new battery is possible but expensive. Also I'm not sure how much I trust this rack's strength loaded with 30-50 lbs of groceries etc? I trust racks from Axiom, Topeak, etc. more

2) DIY -> using em3ev battery and a high quality rear rack but will it last? Conveniently, a 26" wheel rack on a 20" wheel bike leaves about 5" of space between the wheel and the rack. Some rear racks even have lower rails but these are ~2" from the top-- not low enough. I would modify the rack to add a metal shelf underneath.

The shelf would be attached either hanging off the horizonal rails (top or lower) like this:

rack rails are in white, cross pieces in red, shelf is green. Cross pieces attach to the rails with screws via p clamp/grin's rail anchor/Tubus clamp set/ or a DIY alternative (welding not an option with aluminum racks?). Shelf attach to cross pieces either by welding or by bolts and nuts.

Questions:

• Is the DIY option doomed to fail? I do ride over a lot of potholes and small stretches of gravel.
• For the DIY option: is my design reasonable?
• Is two crosspieces (red) enough or is three much better?
• Are there strong enough P clamps? Or is grin rail anchor much better? They are $15 for each pair...
• Steel is better right? Is welding the shelf to the crosspieces important?

Thanks for any feedback! Also I searched the forums for past posts on this sort of thing but didn't see them, let me know if I missed something.

 

[amberwolf]

Regarding racks themselves and potential failure modes and things you can do about that, you can look for my posts with the word "rack" or "racks" in them. There's probably quite a few and each one may have info others don't have.

Regarding supporting the battery "shelf", I recommend adding anything right at the same points other vertical and horizontal supports exist, or as close as possible. The further you put your own load on the horizontal tubing away from load-transfer points, the greater the load on the horizontal tubing, and the heavier the load is the greater the shock load when you are not riding on a smooth surface.

Thoughts on shock loads--if you hit a pothole or go off a curb, then depending on the height difference and how much your tire compresses and how long that compression takes, you could have a worst case of a couple hundred G's of acceleration for an 8" curb (or hole) and a high pressure tire that doesn't compress much, meaning a 10 pound battery could exert a couple thousand pounds on the rack for the instant of deceleration. If the compression is slower, and over more distance, it could be a lot less, say 20 times the static weight for a 4" curb and a half inch tire compression, so only 200lbs of shock load. (random g-force calculator https://vaultcanada.org/FallCalc#Calculator with formulas (i don't know how to use those; me and math...not friends)).

Based on failures I've seen (and had), I doubt most racks have ever been tested or engineered based on shock loads, only static, which is pretty retarded given that the bikes they're used on aren't on perfectly smooth surfaces.

So a rack rated for say, 20lbs, with that much load on it (and all the extra that people will put on them because *everybody* seems to think that ratings or limits of any kind are a suggestion, not a limit or a safety thing), when used under rough conditions, could experience loads ten to a hundred times as heavy as they were built for.

That's one reason why seatpost racks are usually a bad idea, because they count on not only their own structure handling this, but depend on the seatpost and the seattube handling it, when neither of those parts is designed in any way to take this sort of loading.

 

[saucerful]

Thanks @amberwolf ! What you said makes sense but I'm actually not sure what it means in terms of the following options for mountpoints:



The green is the height the shelf needs to be. For some reason my intuition thought that mounting to a horizontal rail (blue is closest) would be stronger than to a vertical rail, though the green points would be the closer.

 

[amberwolf]

The load you need to transfer is vertical...so if you connect to the horizontal rails it has to transfer thru that and then to the verticals, and the points you conenct to the horizontal then have to directly take that vertical load.

If you connect to the verticals directly then you bypass alll that and the horizontal rack can still do whatever you need it to do (minus whatever weight the battery takes up).

(BTW, if you're not using that part of the rack, then you should just put the battery *on* that, as tha'ts how the rack was designed to handle loads)

Also, since this rack has no lateral triangulation to prevent sway, the lower your horizontal interconnects of the verticals are, the greater the stiffening it does of the rack as a whole, lessening the "tail wag" effect.


Regarding the rack design shown, itself, I recommend looking at past rack posts I've made; i know i've critiqued that style before.

(and sure, many racks survive all sorts of misuse and/or conditions...but myself I wouldn't want to trust an expensive easily-damaged battery that could start a fire if internally messed up to a "maybe")

 

[saucerful]

Very interesting @amberwolf . I would have thought the horizontal rails were better because that is where panniers hang. I don't totally follow why a 8 lb battery would cause more trouble than 20-40 lbs of groceries in panniers which I regularly carry on my rack.

Does the fact that pannier clips (I have Ortliebs) have a little play help or hurt the shocks to the rack?

You have a lot of posting history on racks... Can you share an example of a rear rack design you think is solid?

I really like having the top of the rack to carry bulky things and "hiding" the battery under the rack...

 

[Papa]

I don't totally follow why a 8 lb battery would cause more trouble than 20-40 lbs of groceries in panniers

It isn't the weight of the battery, in and of itself, that's the concern, it's the weight of the battery's mounted location that's the problem. Any added mass on the rear of a bicycle should, ideally, be mounted as low as possible. Why you ask? The rear of the bike's longitudinal roll axis is at or near the rear tire's contact with the ground, so the higher you mount any added mass from the rear tire's contact patch, the greater it negatively impacts handling... especially at elevated speeds. That's why panniers are far superior for hauling cargo than elevated flimsy racks - because the actual loaded 'mass' within the panniers, is closer to the rear tire's contact patch.

 

[amberwolf]

Very interesting @amberwolf . I would have thought the horizontal rails were better because that is where panniers hang. I don't totally follow why a 8 lb battery would cause more trouble than 20-40 lbs of groceries in panniers which I regularly carry on my rack.

They don't. But the battery is a more expensive and hazardous risk. Most groceries don't cost as much as a good battery (though an entire SB-Cruiser-load (two carts full) is pushing that these days), and they don't normally explode in flames when they've fallen off and been damaged.

The panniers full of groceries, depending on the pannier design, may not place as much of a shock load if the panniers help decelerate them (if they are stretchy).

But their greater weight will put a higher shock load on such a rack, and more lateral sway-load on it.

If the rack works for you as-is, and shows no sign of fatigue cracking, etc., and you don't ahve to ever retighten it's mounting hardware, etc., then it works. Just keep in mind that shock loading does dramatically increase the stress seen by a loaded rack (and the frame it's attached to), and the cyclic lateral sway (even tiny amounts) does add up stress over time.



I've broken plenty of racks, bikes, custom solutions, etc., carrying cargo of assorted types (including big dogs) before I figured out the shock load issue, and some of the other issues I've posted about in other threads. This one never broke the rack, but the seat stay just forward of the last "e" in "electricle" cracked, eventually. Can't find the pics of it *really* loaded down.

Does the fact that pannier clips (I have Ortliebs) have a little play help or hurt the shocks to the rack?

Any elasticity to absorb the shock of the load will help, by increasing the stop distance (as noted in the previously linked calculator).

You have a lot of posting history on racks... Can you share an example of a rear rack design you think is solid?

If there was one, sure. There probably is one out there...but I had to build my own, and not all of those were successful either.

But every one I've seen so far has *some* flaw that would keep me from wanting to use it. Most of them because they don't have triangulation, especially laterally. Others, like the one you pictured in the replies above, have mounts to the seat stays that allow lateral wiggle, which stresses the rack's mounts and structure. And the mounts to the dropout area, while solid, depend on screw tension to secure them to the stays, and place the load out away from the bike frame, cantilevering those screws in shock loading.

This was for a friend that needed a really tough rack for a solid-tired "bugout" bike that had to carry all the supplies he'd need if the world went screwy enough for him to get out of the city, including giant heavy-duty side panniers he had to hang from it). Welded up out of 1/2" square tubing specifically for this bike, this was the first test fit, I think. Dunno how it worked out for him in the end, but he never reported any problems with it.

These days I use a trike with a cargo box integrated into it, and trailers for huge things like pianos and stuff.

 

[saucerful]

@Papa ah great point about the mass being lower in the panniers. Note that with 20" wheels even the top of the rack is just 25" above ground with standard rack or 21" with a small wheel rack.

@amberwolf I haven't had trouble with my current rack but it's made by bike friday and is steel and actually has some triangulation! The problem is that because of the triangulation the area under the rack top is too narrow to fit the em3ev battery...

Thank you both for your input. I ordered some of Grin's rail anchors, will try just putting the battery on top of a rear rack and ride a bit to see how it holds up and handles. I don't like that as a permanent position because I like the rack top for carrying stuff (bulky but lightweight like a jacket or towel). If it seems OK I will try to DIY a shelf underneath...

 

[99t4]

Jacket or towel go great into panniers.

 

[saucerful]

Jacket or towel go great into panniers.

Of course, though I prefer to put a wet towel outside of panniers. The second reason I want the battery below is to hide it. But maybe I need to get over that.

 

[Chalo]

I use racks with a snap-on adapter (Racktime 1.0 in my case, but Ortlieb, Topeak and others offer equivalent systems) and I attach the mating "cleat" to the bottom of my battery cases, so they click on and off. I can mount panniers on both sides, and I usually do.

 

[saucerful]

I use racks with a snap-on adapter (Racktime 1.0 in my case, but Ortlieb, Topeak and others offer equivalent systems) and I attach the mating "cleat" to the bottom of my battery cases, so they click on and off. I can mount panniers on both sides, and I usually do.

Interesting! Are your battery cases soft or hard? How do you attach the cleat to them?

 

[Papa]

Note that with 20" wheels even the top of the rack is just 25" above ground with standard rack or 21" with a small wheel rack.

Mount both panniers, then tilt the bike left & right about 40 degrees to check pannier to ground clearance.


I needed a quality rack for a build - so I bought this one... used, but no idea who made it. Rarely do I carry on top, just used for Ortlieb panniers.

 

[Chalo]

Interesting! Are your battery cases soft or hard? How do you attach the cleat to them?

I use various Pelican clone cases, plus a padded tool bag with a piece of 6mm Baltic plywood in the bottom for attaching the latched fitting.

The cleats come with screws and washers for attaching them, so I drill the pattern into whatever case I'm using and screw them down

 

[Papa]

The panniers full of groceries, depending on the pannier design, may not place as much of a shock load if the panniers help decelerate them (if they are stretchy).

But their greater weight will put a higher shock load on such a rack, and more lateral sway-load on it.

There is lower pannier hooks or straps which secure the lower portion of the panniers to the lower rack tubes or seat stays. Maybe yours sway laterally,... mine do not,.. nor does the rack itself sway.

 

[AnthonyC]

> However, I don't like where I put the battery (vertical on the seat mast) and want to move to rear rack.

That's actually a great place by comparison with what you're planning. The battery's right under your own C of G so the extra load is where the bike's built to take it and hardly affects polar moment on any axis: you retain a nimble feel and hardly know it's there. What's the big issue with it where it is?

Re a couple of your questions, dynamic loads are greater because a battery's one lump whereas groceries have 'give', they shift and settle slightly. Your Sketch-Up design has poor fore-aft stiffness so the aluminum cross pieces could fracture. If you can live with charging on the bike the battery could be suspended, bare and right-way-up, under the rack's top, dispensing with any shelf. But a battery can be more secure on a seatpost too since racks unfasten with 4 bolts.

 

[saucerful]

> However, I don't like where I put the battery (vertical on the seat mast) and want to move to rear rack.

That's actually a great place by comparison with what you're planning. The battery's right under your own C of G so the extra load is where the bike's built to take it and hardly affects polar moment on any axis: you retain a nimble feel and hardly know it's there. What's the big issue with it where it is?

Re a couple of your questions, dynamic loads are greater because a battery's one lump whereas groceries have 'give', they shift and settle slightly. Your Sketch-Up design has poor fore-aft stiffness so the aluminum cross pieces could fracture. If you can live with charging on the bike the battery could be suspended, bare and right-way-up, under the rack's top, dispensing with any shelf. But a battery can be more secure on a seatpost too since racks unfasten with 4 bolts.

Thanks, good point on rear/aft play.

When I get the grin anchors I'll try mounting to the top of my rack and see about handling, though if it were under the rack it would be 4" lower.

The issue with the seat post location is that the mating of the battery (em3ev shark) and its slider has developed some play-- but maybe this would happen in any position after 3000 relatively bumpy miles? I added a velcro strap recently-- might help. Also it's just aesthetically very visible which I don't like ::

So I was thinking horizontal would be better and rear rack is the only option. The replies make me wary of putting the battery on a DIY shelf, so I have two other options:

1) an existing double decker rack. I found one from Yose power which I have no idea if its any good and the battery slot is very thin, I'd have to change batteries. @Chalo led me to find that racktime has a nice double decker rack! Problem is it is for big wheels, 17" tall, compared to a typical 14" rack. I wonder if there is a way to shorten it...

2) put the battery on an actual small wheel rack, which puts the battery right over the wheel, and then DIY an *upper* shelf that I can put light stuff like a sleeping bag on. The engineering requirements would be *much* fewer. I could even take two identical racks (they are only $45) and use the shelf of one of them as an upper shelf:



Re security, yeah rear rack is not as good as frame. I was just going to use security torx screws :: and bring the battery in when I was worried about it.

 

[classicalgas]

If that pic represents your bike, what's wrong with mounting your shark pack on top of the top tube, ahead of the hinge? Only a little lower than the rear rack, but much more solid and central, better weight distribution.

You might need to fit a good electrical connection there ( XT90?) and put multiple rivnuts in the top tube, but that's still a lot less work than what you're talking about with a custom built rack mount.

More spendy, but easier and more secure, would be a pair of semi custom tube clamps from Terra cycle

...https://t-cycle.com/products/heavy-duty-frame-clamp

Their choices in rectangle tube clamps are limited, but you might get lucky.

 

[Diggs]

Just a word on caution. The aluminum tubular racks seem to fatigue under load. I've had a couple crack and break right in the main support area (see pic). Eventually I inserted and epoxied inside a solid aluminum shaft for more support and it has been good so far. I have a spindly steel rack on another bike that has been heavily loaded and abused forever and seems to be holding up fine.

 

[amberwolf]

Just a word on caution. The aluminum tubular racks seem to fatigue under load. I've had a couple crack and break right in the main support area (see pic). Eventually I inserted and epoxied inside a solid aluminum shaft for more support and it has been good so far. I have a spindly steel rack on another bike that has been heavily loaded and abused forever and seems to be holding up fine.

Part of the problem is probably HAZ issues around the weld, but the other problem is lateral rack sway. (there can even be forward-backward sway if the seatstay attachments are nonrigid flat plate types, whcih are very common, as they bend)

Sway (or tail-wag, or whatever you want to call it) happens most when pedalling, usualy pedalilng hard so that the bike reacts by laterally tilting one way then the other, which causes the load on the top rear of the rack to follow it but not at exactly the same time.

I am not sure that people that say "my rack doesn't sway" understand that none of these bolt-on (or clamp-on, etc) racks (possibly short of laterally-fully-triangulated ones, which I haven't seen) are *completely rigid* and I am certain from the failure modes I've seen that all of them *do* sway laterally to some degree, even if it isn't visible to the eye (it could be measured with the right tests; I'm not an engineer so I don't know that I could design that test, but I can envision the general idea).

The cyclic load from even teensy tiny amounts of lateral movement at the top rear of the rack can still cause fatigue cracking and failure of the parts farthest away from that point, where the rack mounts to the stays (seat or chain or both) or dropouts, etc., over enough time.

Sometimes they fail elsewhere, such as at the transition from top of rack to side supports, and some don't crack the rack but just have repeated loosening of the bolts holding all the bits together.

Some are built well enough to not fail in any normal usage lifespan (which could be decades)...but I have yet to find one of those myself (not that I've had much in the way of non-low-end racks).

Steel racks probably won't exhibit the fatigue crack problem (at least not as soon) as long as the design is good enough and it's securely fastened to the frame and aren't made of stamped-sheets...but aluminum and other similar-behavior materials...they tend to break. Depends on the steel it's made of and how it's made, too, as I've broken them, almost always at the dropout/chainstay mount, or just above it, usually because of them being flat stamped plates, or tubing that has been flattened to make a bolt-hole point.

 

[99t4]

OTOH, you may have to run your own tests:

1. Keep the real battery where it is for now.
2. Install the rack of choice.
3. Strap 8 lbs. of bricks (or equivalent)(roughly in the shape of your battery) to the rack. (Dummy battery)
4. Fill the panniers with your typical load.
5. Go for some extended road tests over your worst roadways. Do plenty of bumps, potholes, slaloms, etc.
6. Closely inspect the rack for any weaknesses or failures.

If it survives cleanly intact, maybe you are good to go? But add a rack inspection to your "Pre-flight Checklist."

 

[BionicOnion]

I actually mounted my battery off a rear rack i built specifically for it instead of on top of the spine (due to no room below) and i can tell you it definitely changed the handling characteristics of my bike. It took me quite a while to get used to it. I don't know how to explain it other than the whole bike felt much heavier and the handlebar feel was just.... akward

 

[docw009]

I've wrestled with the problem in most of my conversions, trying to fit a battery on the rear rack. My second conversion used a rack battery from Luna, but it didn't come with a two decker rack, so I just mounted it on a regular rack, which lost the ability to tie things on it. Later I moved it to a stepthru model which had no downtube room for a battery.

First bike was a mid drive, so I only had 8 lbs on the rack. Did not make the bike unwieldy, Second one has more weight in the rear. Same 8 lb battery plus 8 lb motor. Seemed OK for pavement cruising. The guy who eventually wound up with it told me he was going 22 mph commuting to work,



My wife and I also ride 20" folding bikes. I would give up the folding feature on mine to mount the battery on the main tube, They could always be removed to allow folding, I even tried the Frog battery, but didn't trust the case.


On my wife's Downtube FS8. which is full suspension, I mounted the rack to the swing arm and it carries smaller batteries in a battery bag, although downtube mounts have been used.


I used to have a 20" Huffy Oslo which has a factory rack battery, It's a small 20 cell 36V10Ah battery, but is reasonably compact. I was able to buy a double deck rack on aliexpress last year for $69 shipped, Normally shipping adds $100. And I have a 30 cell 48V10Ah battery that fits it,



I will be using that double deck battery on my next conversion, which will be a 26" step thru.

By the way, I've dropped too many batteries off the racks and have had a rack come loose at the front mounts abd tip backwards til it hit the ground, The smaller bikes benefit from smaller batteries when the range isn't needed.