Large Batteries in Bottle Mounts

velociped

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Inspired by a build I saw here, I am about do build up a Surly Trucker with a BBSHD kit. I have a 52v battery that weighs around 15 lbs with a kit for mounting to the water bottle cage. I like the idea of keeping all the weight low and centered but I am worried about the structural integrity of this system.

Have other folks done this with success? Will the two bottle mount screws be sufficient? Or should I consider mounting on the rear rack instead.

Thanks!
 
Downtube mounting is way better for handling than rear rack. You could add more mounts with threaded rivet nuts, if the base has room for extra bolts. Anyway I recommend using straps in addition, something like this. Unless you think it spoils the look, of course :p
It makes taking the battery of a tiny bit more troublesome though.
 
Thanks for this. I do want to be able to get the battery off regularly. It will be a long distance touring bike so if like to be able to take the battery into cafes and other places where I can get a charge but might not be able to take the bike. If possible I would like to avoid drilling any jew holes in the frame. But obviously I'll have to make a compromise somewhere.

I might look into a click strap system then. Or maybe a triangle bag, although I wonder how stable those are with the weight swinging around.
 
I don't think two bosses are enough to support any but the smallest frame mounted batteries. The last time I had one (shark pack, about 10 pounds), I used three clamp-on bosses of this kind:

https://www.chainreactioncycles.com/mobile/us/en/dmr-hinged-clamp/rp-prod745

prod745_Silver_NE_01


For a 15 pound pack, I'd want four mounting points, but two of them could be the existing frame bosses (built up with washers to the same height as the clamp-on bosses).
 
Those clamps look like a good inexpensive solution, especially with a bit of rubber between the frame and the clamp. For what it's worth I installed this: https://ebikes.ca/triple-bob.html on a customer's bike and it was solid as a rock with their small (10S4P) shark pack, I'm sure it would be good for nearly double the weight.
 
Chalo said:
I don't think two bosses are enough to support any but the smallest frame mounted batteries. The last time I had one (shark pack, about 10 pounds), I used three clamp-on bosses of this kind:

Have you had a failure using just the water bottle mounts before? The shear strength of M5 bolts is high enough I'd assume the weak point is either the bosses or the mount-battery connection. On a Surly (steel frame) with brazed on mounts, especially a touring bike that doesn't see high shock loads, I'd be perfectly fine just using the downtube mounts to hold even a 15lb battery. Straps are always a good redundancy though.
 
COAR said:
Chalo said:
I don't think two bosses are enough to support any but the smallest frame mounted batteries. The last time I had one (shark pack, about 10 pounds), I used three clamp-on bosses of this kind:

Have you had a failure using just the water bottle mounts before? The shear strength of M5 bolts is high enough I'd assume the weak point is either the bosses or the mount-battery connection.

The effect of inadequate attachment that I've seen is looseness that occurs when the screws elongate due to overload/vibration or when the mounting rail becomes indented under the screw heads for the same reason.

Also, when a battery is attached only at two points near the rear end of the pack, it's easy to flex the baseplate down at the front end and remove a locked battery without unlocking it.
 
Thanks Chalo, those clamp bosses are very interesting!

This is a helpful discussion. For now I'm planning to put it in the bottle cage mounts first just to check out how it seems. I've ordered a triangle bag for actually riding. And now I think I will order a couple of those bosses to check out.

I'll be sure to post a build thread when I'm done. I should be done with the basic build tomorrow hopefully. Then after a couple of days of shaking down and dialing in, it's on to the conversion.
 
velociped said:
For now I'm planning to put it in the bottle cage mounts first just to check out how it seems. I've ordered a triangle bag for actually riding. And now I think I will order a couple of those bosses to check out.

how are you planning on fixing the battery if there is a triangle bag and the mounts - through the battery bag?

The reason for the question - I have a similar challenge with a Surly Moonlander build underway
 
COAR said:
Have you had a failure using just the water bottle mounts before? The shear strength of M5 bolts is high enough I'd assume the weak point is either the bosses or the mount-battery connection. On a Surly (steel frame) with brazed on mounts, especially a touring bike that doesn't see high shock loads, I'd be perfectly fine just using the downtube mounts to hold even a 15lb battery. Straps are always a good redundancy though.

I didn't pay that much attention in dynamics classes, but it seems like this isn't a pure shear force situation, but a combination of shear and tensile forces as the battery tries to twist when the bike is leaning on bumpy surfaces. Maybe I'm looking at this wrong, but I'd worry about two M5 bolts holding up if ridden offroad, and maybe even over time on the street, since I believe the Trucker is rigid (no suspension fork). At minimum, frequent checking of the bolts seems like a good idea.
 
velociped said:
Inspired by a build I saw here, I am about do build up a Surly Trucker with a BBSHD kit. I have a 52v battery that weighs around 15 lbs with a kit for mounting to the water bottle cage.
Do not use a water bottle cage. Use the brazeons to mount a real battery mount to the down tube. Then use an additional strap to secure the battery to the downtube just in case.
 
E-HP said:
I didn't pay that much attention in dynamics classes, but it seems like this isn't a pure shear force situation, but a combination of shear and tensile forces as the battery tries to twist when the bike is leaning on bumpy surfaces. Maybe I'm looking at this wrong, but I'd worry about two M5 bolts holding up if ridden offroad, and maybe even over time on the street, since I believe the Trucker is rigid (no suspension fork). At minimum, frequent checking of the bolts seems like a good idea.

Tensile strength is pulling while shear is perpendicular to the shaft. They're all related anyway - if I recall shear strength is generally ~60% of the tensile strength. This site lists the various force resistances by size and grade. Even the lowest grade M5 bolt is rated for roughly 900 lbs of tensile strength. https://eurocodeapplied.com/design/en1993/bolt-design-properties

I'm no mechanical engineer but I would not imagine the battery has a ton of twisting (torsional) forces acting on it.

Personally, I have my 6lb battery mounted to the seat tube bottle mounts with 2 M5 stainless bolts and it's held up great over 100+ miles of singletrack riding. I generously applied blue Loctite though, which I would say is mandatory. I did a test ride on some gravels paths while finalizing the battery holder design with no loctite, and the bolts not super tight, and one had vibrated completely out after ~2 miles.
 
COAR said:
Tensile strength is pulling while shear is perpendicular to the shaft. They're all related anyway - if I recall shear strength is generally ~60% of the tensile strength. This site lists the various force resistances by size and grade. Even the lowest grade M5 bolt is rated for roughly 900 lbs of tensile strength. https://eurocodeapplied.com/design/en1993/bolt-design-properties

I'm no mechanical engineer but I would not imagine the battery has a ton of twisting (torsional) forces acting on it.

The way I see it, the mounting surface is curved/round, while the mounting plate is flat, or at least not matching the curved surface (albeit, I've never examined those mounts in person before), so the battery isn't support from twisting left or right, so more than just a sheer force. But like I said, didn't have that much interest in the subject, so just thinking out loud.

Great link.
 
E-HP said:
The way I see it, the mounting surface is curved/round, while the mounting plate is flat, or at least not matching the curved surface (albeit, I've never examined those mounts in person before), so the battery isn't support from twisting left or right, so more than just a sheer force. But like I said, didn't have that much interest in the subject, so just thinking out loud.

Great link.

I think that might still be shear, albeit in a different direction. There may be torsion/twisting on the battery, but that wouldn't really translate into torsion on the bolts themselves (unless one bolt is loose enough to allow the whole battery to rotate around the other one).

I've never seen the mounts in person either but from what Chalo mentioned earlier it sounds like there's definitely some give in them. Some bottle mounts are a bit proud of the tubing, so unless the mount is designed to go around them, it will considerably reduce the support.

I had to account for that in my DIY battery holder - without insetting a hole for the mounts to rest in and putting a support in between them, it was not very stable as there was a gap between the frame and the battery. Just looking at shark pack mount pics online, it's hard to tell if they'd accommodate the taller bottle mounts. Or that might just a steel frame "issue". The mounts on my Surlys are much taller than my Specialized aluminum frame, which are nearly flush.

CATRbTb.jpg
 
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