Dropout design, feedback needed! (Cromotor, pictures)

[ress]

Hi,

I've bought myself a 05 Specialized Bighit for my first eBike build. My initial plan was to build a reasonable cheap and fast mid drive bike. However, I realized that I want more power! The new plan is to use a Cromotor.

The Bighit is quite outdated, I know. But it seems to me that it's a quite good fit for a powerful hub motor: It's durable, parts are available and affordable, it has 24" rear wheel, the rear suspension has a lots of adjustment options, the frame isn't too bad for fitting batteries and my bike has the mighty Marzocchi Monster fork.

There are one major problem though, it has 135mm non-removable dropouts:

However, it should be possible to weld a new 10mm 6061 aluminum plate outside each dropout, which should give me 155mm free space:

I've come up with two different designs for theese plates:

Top view:

The second one attach to the set stay as well as the dropout and should be more rigid. I don't know if that's necessary though, and it might be hard to weld the plate to the seat stay as it's probably is made of 3 or 4mm aluminum.

The utmost part of the existing dropout will of course be cut off to make room for the Cromotor axle. Regardless which design i choose to go with, it will be welded by a professional and I plan to heat treat it in order to regain T6 temper. A steel torque arm will be added as well.

I'd really appreciate input in theese designs! Which one do you think is the best? Will the two "indentations" make them weaker? If so, would it be better to grind down the existing dropouts?

One last crazy idea; Will the cromotor fit 135mm dropouts if I dont use a rear disc brake? As I'll have regen, a powerful front brake and a "emergency" rear V-brake might be enough? If so I don't have to cut, weld and heat treat the dropouts, instead I could bolt on steel reinforcement plates. What's your thoughts on this?

 

[Willow]

Cromotor will not fit a 135mm dropout.

Don't weld on torque plates (especially if they are aluminium - they just won't last). Get them lazer cut out of 12mm mild steel - to the exact shape you have there with your paper templates. Bolt them onto the existing dropouts and they should be fine. Make them clamping type torque arms - there is a hell of a lot of torque going through the axle.

 

[Chalo]

6061 aluminum gets really soft when you weld it. I would not use as-welded aluminum for any kind of torque retention. I'd be hesitant to use even fully hardened heat-treated aluminum for that job.

 

[ress]

Thanks for the replys!

Cromotor will not fit a 135mm dropout.

Don't weld on torque plates (especially if they are aluminium - they just won't last). Get them lazer cut out of 12mm mild steel - to the exact shape you have there with your paper templates. Bolt them onto the existing dropouts and they should be fine. Make them clamping type torque arms - there is a hell of a lot of torque going through the axle.

I'm aware that aluminum will not be strong enough to handle the torque, that's why I will add steel torque arms. Probably doctorbass' creation; http://www.endless-sphere.com/forums/viewtopic.php?f=31&t=29129&start=300 The plan is to bolt them to the aluminum plates.

It would be easier to find someone to make steel plates that bolts on to the seat stay and doc's torque arms... Something like this (rough sketch);

But the rear wheel will be heavy, can I trust a bolted on dropout? To me a welded on dropout with a bolted on torque arm should be stronger...

6061 aluminum gets really soft when you weld it. I would not use as-welded aluminum for any kind of torque retention. I'd be hesitant to use even fully hardened heat-treated aluminum for that job.

Hmm, you're saying that the torque from the torque arm will break the dropout? Like you're saying, welding the seat stay will weaken it. It should be possible to heat treat it back to T6, but there's always a risk that some part of it will be weaker. Do you like the bolted on steel dropout above better?

 

[MadRhino]

Drop out weight doesn't affect wheel weight, this adds to the frame weight and that is a very different equation in terms of performance.

Drop out torque plates should be screwed ideally, made of steel or better, Ti if you are really concerned about frame weight. Half inch each side will hold more power that can be fed a hub motor, the limit will be your frame resistance, suspension pivots, grade of bolts, etc...

 

[ress]

I guess you have a point, the weight will add to the unsprung mass though... But that might not make much difference as the hub motor is much heavier.

Anyhow, I'm redrawing the dropouts once more. This time made of 10-12mm steel so that they will act as torque arms as well. I don't have much room left for bolts after the existing dropouts have been cut off, but I should at least be able to get two bolts in there. I also plan to glue them in place with Loctite Epoxy Weld. I'm searching for someone to fabricate them for me as we speek.

 

[MadRhino]

...I don't have much room left for bolts after the existing dropouts have been cut off,...

I wouldn't cut off the existing drop out material. Just add 1/4 steel plates on each, this will hold more than 10 Kw and very hard riding.

 

[teklektik]

I wouldn't cut off the existing drop out material. Just add 1/4 steel plates on each, this will hold more than 10 Kw and very hard riding.

+1

Another alternative is to build these as dropout extenders instead of TAs. Use your basic design above (or modify for clamping) but run a 10mm hex or carriage bolt in place of the present axle - re-using the existing axle mounts as bolt-up mounting tabs. Then add the dropout slots, derailleur hanger, and disk brake mounts to the plates as appropriate. Since the 'axle bolts' will bear the primary load, you can add lesser holes in the manner of normal TAs to the plates to prevent rotation. For road use you could pick up several inches for a longer wheelbase.

It looks annoying, but this could be done by hand with a little patience or get Big Blue Saw on the job.

 

[ress]

...I don't have much room left for bolts after the existing dropouts have been cut off,...

I wouldn't cut off the existing drop out material. Just add 1/4 steel plates on each, this will hold more than 10 Kw and very hard riding.

Another alternative is to build these as dropout extenders instead of TAs. [...] For road use you could pick up several inches for a longer wheelbase.

That's actually a good plan, should be quite solid. My only concern is that the already increased weight of the rear wheel will be moved longer from the pivots, which should add stress the frame, bearings etc. But maybe I'm over thinking this

I've made a new design with horizontal dropouts (that doesn't increase the wheelbase), if it fits it will be easy to increase its length and add a hole for mounting it in the existing dropout. When I get back home I'll test fit it and update this thread with pictures.

Thanks a lot for your input, I really appreciate it.

 

[teklektik]

FWIW - This extender idea has been used successfully before by others - particularly with replaceable dropouts.

Example by oatnet:

View attachment 1
Replacement extended swing arms are examples of more extreme length mods (re: your bearing concern).

Example by Farfle


Just make a solid attachment point that transfers torque effectively to the existing dropout member and all should be well.

 

[ress]

Thanks for the refrences Teleknik, that's encouraging. That indeed seems like the best solution.

I test fitted a cardboard model of the latest version:

Fits like a glove! As it gets support from the shape of the seat stay even a cardboard prototype feels quite solid. A extended version will get 3 bolts to hold it in place. Should work!

 

[teklektik]

A extended version will get 3 bolts to hold it in place. Should work!

Ya - and easy to revert the bike back.

Incorporating the hanger and brake mounts into the plates should get you 'std installation' without the need for wonky spacers or adapters.

This illustration may help you get the left plate layout tidied up (any convenient rotation about the axle center will do):

 

[MadRhino]

Swing arm extension by longer drop out plates makes for much better stability when you build a powerful bike. It does make suspension tuning more complicated, and expansive sometimes.

 

[Ch00paKabrA]

Since your bike has 135mm drop outs, it may be mandatory to set the drop out back further to accommodate the rotor if you are going to use a 203mm rotor and also, pay attention to the chain line. When I made my first set, I lowered the axle and the chain wound up hitting the chain stay in 7th, 8th, and 9th gear. It sucks when you have to start over.

I know the cromotor will only have one gear but it is usually a lower tooth count gear.

Good luck, I think the Big hits look awesome and they are quite unbreakable so It is a really good choice. Can't wait to see it finished.

 

[ress]

Thanks for the caliper mount dimensions! Good point MadRhino.

I made new carboard models today:

These extend the swingarm back- and downwards in line with the seat stay. They are quite bulky and will weigh around 400-500g each. I'll might try to shed some weight off by adding some holes and making them slimmer. 10mm is probably overkill, but I want a big contact area around the axle.

Overall I'm quite confident this will work. I've been talking to two companies about cnc milling them. I'll probably get a chock when they tell me the price...

Feel free to comment on the design! I'll update this thread on the progress.

 

[MadRhino]

You can take off half the stock re-desinging, on the upper part especially since the caliper mount is way overkill. The lower part of your design is OK for strength, from there you could just trim 2 tabs to the IS mount, or make an open triangular design for that purpose.

 

[Willow]

you could trim a bit - but honestly, the few grams of weight will be meaningless.... when your talking about using a heavy and powerful hub motor. I like what you have done.

 

[ress]

OK, I ordered the dropouts today! They're similar to the last ones I posted, but they extend along the chain stay instead of the seat stay.


The price? $225 It's expensive to make custom parts! But as I don't have to pay for welding and heat treatment I guess that's OK.

 

[Punx0r]

I'd be reaching for the hacksaw, drill and file...

 

[ress]

Yeah, I'll might try that next time. I think it would have been very time consuming to work 10mm steel though.

 

[Arlo1]

Use pinch bolts. No mater how crazy good the drop out is it will always be a lot easier to keep an axle from moving with proper drift bolts.

Here is how I did it for my drift trikes.

 

[dnmun]

you could cut the slots with a diagonal grinder loaded with a thin metal cutting blade.

if you wanna do a clamp like arlo is talkin, and don't have a welder to add the ears on the end then you could cut the slots long like that and use a gear clamp or two around the ears as they stuck out behind the axle to clamp it.

but in terms of layout, it may be best to wait to do the final finish filing or grinding after these extensions are mounted in their screw mounts on the frame.

the slots may not line up exactly so you will have to do test fitting of the axle and then file one side on one end and the other side on the other end, so the slots are colinear and the axle is orthogonal to the frame.

otherwise the axle may not fit as tight as it could when you put it together and have to file the slots deeper than you would need to.

but screw clamps would get you the clamping force.

 

[ress]

Use pinch bolts. No mater how crazy good the drop out is it will always be a lot easier to keep an axle from moving with proper drift bolts.

Here is how I did it for my drift trikes.

That's a nice solution! Next time, I'll might try to make them myself.

I've left enough material to drill and tap a pinching bolt. Does it really make that much of a difference? If so, I could talk to a friend that have a proper pillar drill.

The manufakturer told me they will be done by next week, will update when I hear from them.

 

[Arlo1]

Use pinch bolts. No mater how crazy good the drop out is it will always be a lot easier to keep an axle from moving with proper drift bolts.

Here is how I did it for my drift trikes.

That's a nice solution! Next time, I'll might try to make them myself.

I've left enough material to drill and tap a pinching bolt. Does it really make that much of a difference? If so, I could talk to a friend that have a proper pillar drill.

The manufakturer told me they will be done by next week, will update when I hear from them.

If you work the axle back and forth with regen and power yes it makes a big difference. Remember I run high power with strong regen on everything.

 

[ress]

Finally the dropouts are fabricated! Got this picture from the manufacturer:

I think they look good! But I regret that I didn't round off the sharp edges. I'll might file them down a bit when I receive them, I'll have to tap threads the holes myself anyways.

Due to cost reasons I didn't add pinching bolts, there are plenty room to add them later though.