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Chapter Twelve: A cheap gear swap, and it helped too
Submitted by Sturdly on Sun, 06/10/2007 - 10:01
Riding season is finally close to getting seriously underway here and I'm still waiting on Allan to send the new items to test. I've stated before that I need less top end and more climbing assistance so I was thinking. It's a rainy Saturday, so outside chores are on hold. All the inside stuff is caught up so why not do an experiment on the Blast with the motor gearing. My thought was that a lower assist gearing might get the motor more in it's sweet spot and help pull the hills. How to go about it? Whatever has to be reverseable should my logic be off and I put myself on a $40 budget.
The design constraints are that the 44t drive sprocket is the largest that will fit in the current location due to derailer clearance issues and the motor mount will only go so far outboard before hitting the inner chain ring. Hmm, I knew I needed to get a larger drive ring and or a smaller drive sprocket on. The current gear ratio is about 3.1 to 1 and I wanted to get it closer to 4 to 1, this after the motor gear reduction. To get near 4 to 1 I would need around a 52t ring (common size so it would be cheap and easy to procure a used one for the test) to go along with the 14t freewheel on the motor. This would get me about 3.7 to 1 and if it worked a 13t freewheel would net the magic 4 to 1.
With two Jacksons in hand off I went to the used bike parts store. Rumaging through the chain ring bin I found a decent 52t Stronglite sprocket $5.00 and some change, so far so good. Then a quick stop at the hardware store to get some allen head cap screws, nuts and a hand full of washers for use as spacers to get the ring out far enough to clear the derailer, $8.00 and some change. well within budget. Then home for the installation which started by removing the right crank and ring assembly.
Step one was to disassemble the assembly then center the new ring and match drill it, no problem. Two reattach the motor drive ring and crank then space the new chain ring with chain to clear the derailer. A little trial and error and that too was completed in short order. I then reassembled the entire crank assembly and reinstalled it. Now for the fun part adjusting the motor mount to align the two motor drive sprockets.
To start I moved the mount out as far as I could and still clear the innermost pedal ring. Then I ran it through the gears to double check the clearances. This is a place where those Esge stands pay for themselves, you can run the drive train without a bike work stand as the rear wheel is elevated when the stand is down. Problem was one of the cap screws holding the motor to mount interfered with the chain when shifting to or from the lowest gear, tossing the chain each try. Solution countersink the hole and use a tapered head screw to hold the motor. This of course required removing the crank assembly again to get room for the screw swap. With the mount all the way out the sprockets were still not in alignment so I got out my poor mans mill (round bastard file) and custom made a spacer from a washer that would fit between the keyed motor shaft stop and the freewheel. Rounding third and heading for home I thought, not.
The final step was to install the chain and adjust the mount to take up the slack. Problem....the mount when adjusted to it's max in either direction left me about a half link from an acceptable tightness. What to do? Half links are easy to come by for single speed chains and the like but I had to use a narrow 8 speed chain for clearance reasons. Time for some research, low and behold KMC makes a 3/32 half link around $4.00 or so.
None locally so time to order. OUCH those new USPS rates are brutal! $8.00 to get a $4.00 part delivered! I went ahead and pulled the trigger with an order. $28.00 into the project now so within budget but if I decide to go to a 13t freewheel and the special tool to install I'll be over by $10.00 or so.
There sat the Blast, a block from heaven with a droopy motor drive chain. I just couldn't take the suspense once the clouds cleared for a brief instance. So I mounted up and tried her out knowing that I would be throwing the drive chain if i didn't really nurse it. WOW the difference in pulling power is substantial, I think I'm on to something. Thinking that a 13t freewheel will net me about 15mph or so max with assist just might prove a fair trade for better pulling. As I read somewhere,when I want speed I have plenty of hills to go down.
What a stack of chain rings, 5 on the crank. Looks kind of Rube Goldberg. You can see the stock elationebike sprocket through the new 52t job. It's amazing just how quickly rings grow in diameter when you add teeth. I can still use stock elation gearing should I ever go to Kansas and want more top end by quickly swapping out the chain for a shorter one and running the smaller wheel. Of course adjusting the motor mount would be in order but the big wheel can remain installed as I made sure the clearance would allow that. I'll clean it all up and install the half link upon arrival and update this chapter with performance impressions.
Until next time........