DIY shark pack attachment and frame tube strength.

I got a very inexpensive frame to practice building an ebike for the first time. I had difficulty utilizing the bottle cage brazeons in the main triangle. Instead I went for attaching the pack under the down tube for lowest center of gravity (had to drill holes in the frame to do so). I did this with out accounting for fork travel. It is probably ok but not perfect. I want to ask the opinion of the group. Switch to non suspension fork and give up whatever benefits I might get from the worn and cheap suspension or relocate the battery to the MAIN triangle and have 4 small holes under the downtube?????? Please and Thank YOu!!

A hole in a tube creates a stress concentration. Not only does it weaken the tube, it creates a starting point for crack propagation.

A down tube is probably the most critical structural element in a bicycle frame so you are right to be concerned.

If your very inexpensive frame was overbuilt and made with straight gauge tube, it might be ok for pedaling. E-bike duty has higher speeds and potentially greater stress, it's hard to make that call.

If your frame was inexpensive enough, maybe buy another? It could be cheap insurance.

On the other hand, cheap frames don't seem to make for the best conversions.

On steel frames holes in a tube are not a problem, and they can be plugged with welding or brazing. Better using threaded inserts on alu instead of screwing directly in the tube though, for reliability. The old unused holes in alu can be plugged with epoxy.

Low center of gravity is not an advantage on most ebikes. Not at all on those that are fast. Low COG is good for slow technical riding. It does make weight shifting longer, giving the extra precision to maintain lateral balance when stopped or in very slow maneuvers. When you ride fast, you are better with higher COG, that is making the weight shifting shorter, giving the minimal movement required for optimal control when the inertia that is caused by speed would require extra force and displacement to move lateral balance.

Examples: Trial motorcycles are low COG, while racing motorcycles are high COG.

Are there threaded inserts that don't require a special tool to install them????

onetearless said:

Are there threaded inserts that don't require a special tool to install them????

Click to expand...

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

Available in a few different diameters. If you have one of the new anti-improved bikes with non-round, non-straight tubing, well, you'll need to be cleverer yet.

Also, when you mount a downtube battery upside down, it can often capture and retain water in a way that it will not when it's mounted right side up. One of my friends has killed two batteries this way, in rapid succession.

Keep in mind that you are the center of mass of your bike+rider system. The farther away you place a weight on the bike from your own center of mass, the more sluggishly your bike will maneuver.

Stress concentrations are worse on aluminum because aluminum doesn't really have elastic deformation. A threaded insert is an even bigger stress concentration than a plain hole unless you mean a rivnut but that's probably not going to reinforce a tube very much.

For aluminum, I would want to weld in a "braze on" to reinforce a hole in a tube. I'm not sure if you need to heat treat after that (I tend to avoid aluminum except in structures where I just need a light weight material to take up space instead of being particularly strong).

For steel, if you add a braze on to a hole, that's pretty safe. If you plug a big hole with just braze, that might take enough heat to weaken your tube. If you weld, you have a new heat affected zone to deal with.

These issues can be overcome but at some point the effort isn't going to justify the end result, especially if it includes a "?".

Chalo said:

onetearless said:

Are there threaded inserts that don't require a special tool to install them????

Click to expand...

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

...

Click to expand...

Yep. Clamps are the easiest, most robust way to attach anything to a tube.
Inserts are pressed like pop rivets, ideally adding epoxy before insertion. They require the tool, of course. They are exactly the same as the bottle cage mounts that are on almost every bike

An alu tube that is weak, thin or damaged, can be wrapped with CF repair kit tape, making it very stong.

You did not say what your frame is made of. If it's steel then the holes won't matter.

Volt_Ampere said:

You did not say what your frame is made of. If it's steel then the holes won't matter.

Click to expand...

That's a huge oversimplification. Steel will crack from a stress riser (like a hole) placed in a high stress area. Generally speaking, it's a bad idea to poke holes in any part of a bike frame.

If the bike is truly a cheap piece of junk, then it's likely to be made from thick tubing that has a surplus of strength (being weaker at the joints and dropouts).

I agree with smoke.
I'd actually rather just build some mounts that make the appropriate holes available, which wrap around the tube.. rather than concentrating the weight of a ~10lbs battery on two small bolts, which is already a weak design in the first place..

In lieu of that, just throw a battery in a case inside a frame bag and call it a day.

I agree with the statement that m5 bolts designed to hold a 1 to 1.5Kg bottle are not really suitable for a 4+Kg battery. But I really would like to see some hard evidence of broken frame due to additional holes drilled - whether it's steel or aluminum. Not talking about CF. I've seen cracks in the upper part of the downtube, near the headtube, even on factory drilled holes to accommodate light cables though this is the most stressed part of the bike and needs extra care. And than I've seen accidented bike frames (which I recommend to anyone interested into frame structure), but the forces which split a frame in those cases would have broken any frame, whether there is a hole drilled or not. While not really sanctioning wild drilling operations here - especially not on super thin oversize tubes - I think a solid frame can take a fair amount of modding before becoming a liability. Common sense is in order, and some experience of metal working. AFAIK creating a heat zone during a speedy weld is far more structurally dangerous in the case of aluminium frames than cold drilling a holes, as the HAZ is 7 times less strong the the rest of the frame...

clamp on bottle mounts:
https://www.ebikes.ca/shop/electric-bicycle-parts/battery-accessories/bottlebob.html
seem like the thing to do
I don't trust waterbottle mounts