Electric Bikes - traction problems
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02-07-12, 03:26 PM
I recently finished my electric conversion on a 20" folding bike (no longer folds). I have a front hub motor (BMC 36V 500W high torque geared hub motor). It has the power to climb the hills between work and home. The problem is that it looses traction and the tire slips on even dry pavement.
I have never heard anyone else complain about front hub motors doing this (but most hub motors don't have the torque this motor has). I am looking for some experience-based advice.
- I am running a 90 psi, 20x1.5 tire with just a little tread. I'm wondering if going to a lower pressure tire will help. I'm thinking Big Apple.
- I am thinking about reversing the fork. The current wheelbase is about 40 inches and reversing the fork will reduce it by less than 2 inches and change the weight distribution.
I do not recommend reversing the fork. The bike as it is now should require very little effort to balance. (It should feel fairly stable, and need almost no effort from arm muscles, to keep the bike balanced while moving at 10 or 25 miles per hour.) If you reverse the fork, it will probably require effort to balance at 10 miles per hour. At 20 to 25 miles per hour, it might wobble in a way that would cause you to crash.
I do have other ideas to correct the problem you are having.
Like you said, use a fat tire and low pressure (for instance if you use a big apple tire you should be able to get away with 20psi if you weigh 120 pounds or 35psi if you weigh 200 pounds). The minimum depends on the bumps you encounter on your rides -- as long as you always avoid bottoming out your tire (hitting the rim against the ground) you'll be fine.
Other things that might work for you: lean forward when accelerating hard. replace your handlebars and/or change their position so that you are riding more like a bent-forward racing cyclist and less like a beach-cruiser cyclist, since that'll put more weight on the front wheel. Go easy on the throttle whenever moving less than 10 miles per hour. (when starting from a dead stop, don't go past the middle on the throttle)
02-08-12, 12:33 PM
Thanks for the feedback.
Regarding the fork, I flipped it last night but have not tried it. The wheelbase drops to 36" (which strikes me as a minimum). I do expect that the low speed handling will require more attention. At higher speeds I would think the increased Trail would create a bike that tracks a straight line well, but would be quite a bit more jumpy. I was trying to find info on the wheelbase of racing frames for comparison. I will be quite cautious if I try this out. I do sit upright (for health reasons) which puts my weight quite far back.
Swapping out tires is an easy solution which can't hurt. The physics of tire width and pressure has never been clear to me, though there seems to be adequate anecdotal evidence to confirm that wide low-pressure tires have better grip than narrow high-pressure tires.
02-08-12, 01:09 PM
In searching for wheelbase ranges I found reference to a folder with a 35 inch wheelbase, and the Dreamslide standing bicycle with a 32" wheelbase. I guess that is some indication of how short a wheelbase can be and still be ridable.
02-09-12, 03:26 PM
Put a basket on the front of the bike and add some weight (Maybe even an extra battery) and that will help with slip.
02-10-12, 11:46 AM
Yes, I agree. I did the math based on my assumed center of gravity while riding. I figured that shortening the wheelbase from 40 to 36 inches would shift another 5 lbs to the front wheel. Moving the battery over the front wheel shifts almost 10 lbs to the front wheel. The choice is clear. I also ordered a Schwalbe 20x2 Big Apple front tire and will try running it at 50psi or below. Robert
02-10-12, 12:12 PM
Reversing the fork actually makes the handling more stable but it makes the bike feel sluggish when turning. I've ridden my Bike Friday with a reversed fork and the main problem was that the bike was much more prone to lifting the rear wheel under hard braking. I agree with the choice to move the battery weight to the front instead, especially if you can mount it fairly low. Traction is primarily a function of the weight on the wheel and the tire compound. There shouldn't be a need to go with a bigger, lower pressure tire.
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