Good or Bad Idea: aluminum frame for first-build commuter

[eli-f]

Update/Conclusion
Thanks everyone for chiming in. I have decided to find a steel frame mountain bike instead of my current bike. That way I go where some science suggests, can expand to a more powerful build later on, and can use wider tires for inclement weather. It seems a rear motor will be ideal for my commuting/hill climbing.
Goal
I want to build a commuter that goes around 20 MPH. I would need a range of <= 20 miles, pedaling the flats and throttling up hills. The 2 main hills in my ride have between a 6-8% grade. I intend to use a brushless hub motor on the rear wheel (undecided on geared versus DD). For cost reasons, I want to convert my existing hybrid (Marin Larkspur) into an ebike. The specs state it has an aluminum 6061 frame. Magnets do not stick to the frame and the drop outs are aluminum.
Actual Question
Is my current bike frame a feasible choice for this lower power commuter?
I have read that steel or chromoly is best , and to avoid aluminum:
http://endless-sphere.com/forums/viewtopic.php?f=3&t=52245&p=774823#p774823
http://endless-sphere.com/forums/viewtopic.php?f=3&t=53244&p=792780#p792780
http://endless-sphere.com/forums/viewtopic.php?f=3&t=41609&p=608862#p608862
Yet I still would like to use what I have.

 

[spisska]

The issue is that the motor sits in the wheel, and that the universe insists on enforcing the laws discovered by Newton et al.

When you give power to your hub motor, you also create an equal and opposite force on your frame, wherever your hub-motor is mounted.

In the standard front or rear mounting, you put extraordinary force on the dropout. It's a huge force in a tiny area, which is why every sensible person here says you need at least one torque arm, and probably two. That force has to be spread.

Steel is flexible and can absorb force, but heavy. Aluminium is light, but is also brittle and can shatter under force. Carbon is lighter, and while it can be stronger than aluminium, it can also be more brittle.

In a bicycle, weight is a big concern, and an aluminium frame can help save a lot of weight. Though even a pedal bike made of aluminium has a risk of breaking.

In an ebike, weight is much less of a concern. That is, the choice of steel vs aluminium vs carbon fiber frame makes far less of a difference to overall vehicle weight than battery chemistry. But the properties of the respective materials are very important.

There's simply no point in trying to save weight in the frame when you're measuring such savings in grams or ounces. Your choice in batteries will mean differences measured in pounds/kilograms.

 

[e-beach]

Hey eli-f!

Welcome to ES.

I have been running an aluminum frame for my bike for over 3100 miles now. I started with a chromoly frame but switched to aluminum because it had front suspension. My chromoly frame did not and it was too hard to find a 1 inch steerer tubed suspension fork for it so I went to aluminum. It has worked fine for me up until recently.

The thing about aluminum is that it doesn't stretch or spread like steel. And when it breaks it breaks suddenly. In fact a week or so ago I was riding my bike and heard a noise like I ran over a piece of metal only when I looked back I didn't see anything. For the next couple of days my steering felt weird but I thought that my front tire was under inflated. It turns out that I had a failure of the fork brace (the arched part of suspension forks that connects the left and right side fork tubes). It is so odd that I will post a thread on that one in a day or two when I have more time.

When adapting aluminum pedal bikes to motorized the dropouts are usually the suspect, but not in my fork failure. Just goes to show that a steel frame bike with an aluminum front suspension fork could have just as much problems as an all aluminum bike.

I am building a second bike now and it is all aluminum as well. But what I am paying close attention to are dropouts and torque arms.

Do a really clean and tight job of filing the dropouts to fit the 14mm axle and use a good steel torque arm, IMHO you should be ok on an aluminum frame on a 20mph commuter. Just know that the chain stays don't spread like steel one and you will need to periodically check for cracking at the welds.

Or, you could spend another $50 to $100 dollars USA and get a steel bike and adapt that instead.



P.S If you haven't seen these videos, go here and watch them. Especially the one on filing the dropouts. Widening the chain stays probably doesn't apply to your frame as aluminum doesn't stretch like steel.

http://electricbikebuilding.com/

 

[The fingers]

I chose an all steel bike to convert and generally I'm glad I did. While commuting, I had two rear rims disintigrate into spaghetti. Got clipped by a car passing me at 50 mph one night last year. Initially, I had to install the torque arm backwards to get it to fit. Even with all that, I don't worry at all about fork or frame flexing or cracking. If it does, I'll just have it welded for a few bucks. If I were an expert with a shop and knew what I was doing, maybe it would be different.

 

[d8veh]

Your aluminium frame will be no problem for a motor from 250w to 500w, which will be more than enough power to meet your objectives. You should use one torque arm as insurance against anything moving that shouldn't. The seven electric bikes I have and all those I've ever built have/had aluminium frames and forks. One of them has 3kw from the battery going through the back wheel plus another 2kw through the front aluminium forks.

 

[Samd]

+1. What D8VEH said.

 

[heavymetalthunder]

If you keep the speed to 20 MPH with a 500 watt system and not add any extra weight you should be OK. The problem being, once you have the taste of what a E-bike can do there is very little room for expansion as in more power, lights, or for cargo boxes for grocery shopping. Keeping that in mind when I did my conversion, I used a older steel full suspension mountain bike built using a 1000w 48v system with a rear rack, two cargo boxes, full lighting system, extra batteries, steel rims, center stand, and better tires. My problem is darn thing weighs about 100 lbs. ready to roll. By pedal power alone, the best I can only do 10 MPH on the flats, and big hills I could just forget it. On a recent ride on the local bike trail is was nice to power up to 18 MPH and add pedal power to push it a little over 20MPH for exercise. Some of competition bike riders in spandex zipped right by me, they must have been doing at least 25 MPH.

 

[Samd]

Which dropouts did you test with the magnets, front or rear? Test both.

 

[dogman dan]

If you have a decent torque arm, you can run a rear motor on an aluminum frame. To run with no torque arm, I'd recommend steel. No reason to say you cannot use an alloy bike for commuting.

But it will be easier to get the torque arm right, if you have a largish flat dropout, rather than one that is too fancy, or has not flat area.

This is one of my current bikes. Aluminum, and I ride it very hard on the dirt. I give it 1200w. Since the dropout was large and flat, I could simply bolt on half of an old design torque arms intended for front motors. drill one hole, bolt it on. easy.

 

[eli-f]

Thanks all for the comments and advice. I will definitely use a torque arm. Since the front fork is chromoly, I am considering placing motor in the front wheel as long as it will not dramatically reduce my hill climbing ability.
@Samd
I only tested the rear drop outs. The fork is chromoly rigid and the magnets stick.
@spisska,
Your argument clarifies the issue a bit more for me. This reinforces the idea to go with steel or chromoly.
@e-beach
Thanks for the welcome. I'll check out that link.
@the fingers
Peace of mind while riding would be welcome ( minus unaware/antagonistic drivers).
@heavymetalthunder
I hear ya. At this point I want to start tasting sooner rather than later so I accept the limitations my current bike will have.
@dogman. Looks good. For my first build I'd like to avoid drilling the frame.

 

[carsonwen]

Easier to install in rear and just buy one or two torque arms. They take like 5 mins to install.
A front wheel drive will reduce your hill climbing ability because of traction. When you climb hill, your body leans back putting more weight on rear wheel, less on front.

 

[dogman dan]

There is a universal rear torque arm that requires no drilling available at Grin Cyclery btw.

I just showed how to use a type intended for front motors that is commonly available. In a dropout plate that big and flat and strong, drilling the hole does no harm at all. If yours is weak and thin, or just funny shaped, then it's NOT such a good choice for a rear motor. There are about a zillion torque plate solutions for the rear, including homemade. In the for sale new, you can buy some custom made ones to bolt to the disk brake mounts of bikes.

As to the front motor, you wouldn't lose any ability to climb hills on pavement, unless it was really wet or icy. Even wet, you can still climb 8% grades with no problem at all. But a front only motor sucks in dirt, like riding trails. It can be fine on moderate slope dirt road. It's just not going to climb the really steep stuff in dirt well.

I liked front hub on the pavement enough to do about 10,000 miles of it. But you do lose ability to hop up curbs easy. My town has the wheelchair curbs everywhere now, so no big deal for me.

 

[MadRhino]

...My town has the wheelchair curbs everywhere now, so no big deal for me.

That is one good excuse to ride a front motor, for if it makes you wash out in a turn and you end up in a wheel chair, at least you will have the facilities to use it.

I still believe a front motor on a two wheeler is a nonsense, and only good for grannies riding very low power. About ALU frames, you know what I think: A good ALU frame is better than any steel frame unless you are willing to live with 3 times the weight. For stiffness, crash resistance and corner handling, you 'd need a motorcycle frame to beat a pro DH racing frame. Then, if you build cheap, build steel because cheap ALU frames are sh*t.