Human Electric Trike Thesis

Design of an electrically assisted human powered trike

Archive for November, 2006

Testing begins

Posted by Bob Dold on Sunday, November 19, 2006 8:58 PM

 Completed the first test runs under electric power today, the first test was about 2.6 miles and unearthed a coupled of mild problems:

Test 1:

1. Chain from motor fell off after about a mile, this was being caused by the mid drive cog assembly unthreading from its hub and pulling the chain out of alignment. I believe this is a result of the rear hub not freewheeling correctly and the rear swingarm twisting from the motor torque.

2. The right rear dropout twisted down 5 or 10 degrees, the torque from the motor and chain drive apparently loosened the bond of this tube causing it to twist down. I will fix this by restoring the tube to the correct orientation and backing up the bond joint with several rivets to keep the tube from twisting in the socket.

3. The right disc brake was rubbing and making a lot of noise, adjusting the caliper position should fix this problem.

I attempted to get a shunt resistor from Radio Shack to measure the current draw using my voltage logger, but they did not stock any with the required .01 ohm resistance and 9 watt capacity. I ordered several from eBay and should have them in a couple of days, in the meantime I will use the average and total current and power measurements from the Drain Brain.

Test 2:

After fixing the rear dropout and adjusting the brakes I ran a second test lasting 1.9 miles. I added Loctite thread locker to the mid drive freewheel thread in hope of keeping it from unthreading, it seemed to have work as I did not notice any signs of it loosening. The new, stronger lower a-arms have eliminated the flexing of the old parts, but there is still more body roll than I would like in the corners. The table below shows the results of these two tests:

The Peukert range attempts to calculate what the range would be by using the average current from the test and the Peukert exponents which are characteristics of the battery model. The Peukert exponent and capacity were obtained using Uve’s battery calculator and the battery’s reserve capacity and 20 hour rating from the Odyssey website. Using the overly simplistic Time= AH Rating/Amps doesn’t account for discharge rate, a more accurate formula is:

Time = Capacity Number/Current^Peukert exponent

Since the exponent is in the denominator, the smaller it is the less capacity it loses at HIGH, not low, amp draw. To get the range in miles rather than time, the formula is:

Range (miles) = Capacity Number/(Speed * Current^Peukert exponent)

The first two test runs were run up ad down my street which is as close to level as I could get without traveling too far, by going down and back the average grade should average to zero. The elevation and grade plot from the Garmin GPS logger of the second test run is shown below:

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Posted in Performance | 1 Comment »

New A-Arms

Posted by Bob Dold on Saturday, November 18, 2006 10:21 PM

The first test runs I did on the trike resulted in a bent lower right a-arm, I straightened the a-arm out as best as I could after each incident but it was apparent the a-arm was not strong enough.

The original a-arms were made from .405 diameter by .068 wall 6061-T6 aluminum with a yield stress of 15,000 psi. While the stress analysis of the arms did not show any problems, the testing must have produced a load case not accounted for in the analysis. I also noticed the right rocker arm was not moving freely which may have restricted the shock movement, putting undue stress on the a-arm. Whatever the cause of the bending, it was apparent the arm needed to be stronger. A search of McMaster found a .500 diameter by .120 wall 2024-T3 tubing with a yield stress of 45,000 psi – this will be more than 3X stronger than the original tubing. The only downside is this tubing is much more expensive than the original tubing, almost $10/ft vs. $1/ft. Because the OD is close to the old tubing size I will be able to use the existing pushrod mount by boring it out to .500 diameter and bonding the new arms in.

The above pictures show the new a-arm on the right compared to the old one on the left, you can also see how much thicker the wall is on the new part.

Posted in Components | Leave a Comment »

Assembly nears completion

Posted by Bob Dold on Sunday, November 12, 2006 8:27 PM

Spent yesterday doing final assembly of the trike it is now just about ready to test under electric power. I bent up new handlebars out of .875 diameter 6061 tube using a conduit bender, these put the controls within easy reach while sitting in the seat. The left handlebar has the motor throttle from the X1000 scooter and the left brake handle, the right bar has the deraillleur twist grip shifter and the right brake handle. Both handlebars have rear view mirrors on top to aid in rear visibility as it is very difficult to see behind while reclined in the seat.

I also installed the three Odyssey batteries to make up the 36 Volt battery pack, the batteries were wired in series by connecting the positive and negative leads of the batteries using copper strips. The controller power was then taken from the first battery’s negative terminal and the third battery’s positive terminal. The batteries were secured bay riveting aluminum angle the the battery support tray to keep the batteries from sliding fore and aft. The frame rails restrain the batteries from moving side to side.

I also put new front tires on the wheels and put new brake and derailleur cables in order to prepare for the first test ride. The first test ride was under pedal power only since I am waiting on the freewheel adaptor for the motor to be completed – this part which should have been simple has been one of the most difficult to fabricate – it took three machinists and four different lathes to get the LH metric thread correct to fit up with the LH freewheel. Problems uncovered during this first ride include the following:

1. Can’t shift more than two or three gears on the rear cogs due to the sharp chain angle from the mid drive cogs to the rear wheel – two possible solutions, either shift the larger sprockets to the right and give up a few gears to allow a reduced chain angle or switch out the rear cogs and derailler for an internally geared hub. I will shift the cogs for now to allow testing to continue and order an 8 speed hub which will eliminate the problem.

2. The chain running from the mid drive up to the pedals rubs on the underside of the frame, to fix this I will use the idler pulley from my Sun tadpole to run space the chain out from the frame. I will have to make up a bracket to attach the idler to the rear swingarm.

3. When the trike rolls backwards the chain backdrives causing problems with the chain jamming and coming off the sprockets. I am not sure why this is happening, the rear freewheel pawls should disengage when rolling backwards but for some reason they are not – hopefully changing to a internally geared hub will fix this.

4. Trike leans severely when cornering, I need to check if this is due to springs that are too soft on the front, or incorrect suspension geometry.

Remaining items to be completed are down to the motor adaptor and the idler bracket, both should be completed tomorrow by Pete in the shop.

Posted in Updates | 1 Comment »

11/10 Update

Posted by Bob Dold on Friday, November 10, 2006 8:26 PM

Fabrication is nearly complete, actually was able to ride the trike for the first time the other day using pedal power, still need to work out some chain routing issues, but for the most part it worked well. Based on my very short test ride a few things became apparent – #1, I am very happy with the suspension, it really smooths out the bumps and makes it more comfortable to ride. #2, the steering realy scrubs when pushed into a tight turn so I need to double check the geometry to check if the Ackerman compensation is correct. #3, running the motor unloaded produces a lot of noise, hopefully this will dissipate a little when it is hooked up to the drivetrain.

I tried instaling the handlebars from my Sun tadpole, but they are too short as seen in the pictures, I ordered some 7/8 aluminum tubing and hope to bend up some longer ones using an electrical conduit bender.

The drawings I got my disk brake mount locations from was incorrect so I had to space the mounts out about a 1/4 inch using a stack of washers as shown:

 

As with any prototype there are a number of things that have popped up that I would change if I had to do it over again, but that is all part of the learning process. Here are a few of them:

Adhesive bonding of aluminum tubes work well in some cases and not so well in others. The tubes I bought were not very round, so even though the blocks they go into were made very accurately, the tubes themselves were not round, causing uneven gaps which the Loctite adhesive did not like. On the block that holds the boom tube to the fram this was especially apparent and I had to reglue twice. It still is not a good joint and I had to rivet it to back up the glue bond. To fix this I would make the block longer to add more glue surface area, force the tube round using a mandrel, and add some sort of backup fastening system.

If this trike were to be manufactured, it would have to go to a welded contruction, the bonded construction requires too many machined components and requires precise tubing for the bonded joints. The welded construction would also produce a lighter weight frame by eliminating most of the blocks.

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