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The Problem
I truly love working on vehicles and especially customizing mechanical systems on them such as the drivetrain, steering, brakes, and suspension but a project vehicle is currently prohibitively expensive for me.
A riding lawn mower on the other hand, has mechanical systems which can be modified all the same but for significantly lower costs.
I have tackled a similar project before where I modified a lawn mower to drive at a top speed of 30 mph (header image). This mower however, had seriously compromised steering, no clutch and highly ineffective brakes. Its lack of safety was limiting top speeds, so I embark on this project again with a blank slate, safety approaching my #3 priority, and a higher speed goal of 40 mph.
Solving...
Mower stripped and ready for washing
Phase 1: Teardown and Cleaning
The mower was purchased after sitting behind the previous owners shed, untouched, for the past 5 years. There was an excess of rust, grass and mud on the deck, frame, and entire underside of the mower. Additionally, all 4 tires were dry rotted, and far from their days of holding air pressure.
After getting the mower home, the seized deck was removed, along with the engine, transmission, and battery such that the frame could be pressure washed.
Coroded and siezed mower deck
Phase 2: Transmission Pulley Swap
With a belt drive set up, speed can be traded for torque by either increasing the diameter of the engine pulley, or by decreasing the diameter of the transmission pulley.
Knowledge from my previous mower came in handy since I already knew that the constraint on space in the front of the frame (near the engine) limits the possible size of the front pulley. So, in order to unlock more speed, the transmission pulley needs to be downsized as well.
The transmission is driven by a 10" pulley with a 1/2" shaft. My spare pulleys, however, all had 1" hubs, so with access to RIT's machine shop I turned down an adapter. After welding it to the hub of a 4" diameter pulley, it was mounted to the transmission.
Since the pulley was now smaller, the belt needed to be shorter. Unfortunately, a belt of the desired length was not cost effective, however, the belt didn't necessarily need to be shorter if an Idler pulley could be added to extend the length of the path the belt must take.
Idler Pulley with custom riser welded to frame
10" Motor pulley to replace stock 4" pulley
Phase 3: Motor Pulley Swap
Testing with the reduced diameter rear pulley provided a top speed of 20 mph. Improved from original, but not fast enough! With limited space for the motor pulley, 10" was the maximum permissible diameter to ensure the frame would not need any cutting.
Conveniently, the transmission pulley removed in Phase 2 was 10", but inconveniently, it had a 1/2" bore which is incompatible with the engines 1" crankshaft.
Off to the RIT machine shop again to bore the pulley's hub out. Then, the bored pulley was slipped over the existing hub and welded together.
Oops! All Neutrals
With the two new pulleys geared at 2:5 (originally 5:2), a test drive was in order. Following a top speed run of 34 mph, I downshifted in anticipation of a turn. Immediately following the shift, the transaxle made a cluttering sound followed by the 4 forward, and single reverse gears all operating as neutrals. A sure sign of failure.
An investigation into the transaxle quickly revealed that the side gears (see figure below) in the differential had shattered. Replacements can be found on ebay for a sum of $30 with 3-5 day shipping, but with no patience to wait a week, I set to make the transaxle operate again without ordering anything.
Opting to compromise on the mowers turning ability (afforded by an "open" differential), Henry Badt and I designed modifications to create a fixed differential, bypassing the need for side gears.
With a fiber optic laser at RIT, a layered hub was made with 6 sheet metal profiles of the ring gear, which were then stacked and joined with socket head cap screws.
On a traditional lathe, an axle extension was turned, which bridged the gap between axles (left by the absence of differential) and which also allowed the axles to be welded to the hub.
Ring Gear
Layered Hub
Axle Extension
Fixed differential Assembled
Back to Solving...
Phase 4: More Idlers
Test runs with the newly welded transaxle were disappointing. Nearly every time the mower came to a stop and the clutch depressed, tension would be removed from the belt until it fell off.
Assuming the belt's tendency to fall off was a result of poorly constrained travel, Henry and I built 2 more idlers and 3 belt retaining posts. As shown below, the three idlers were placed in positions with two things in mind. First, the locations must permit appropriate tension in the clutches resting state and reasonable slack when in its depressed. Second, they must prevent the belt from rubbing on frame components.
After welding the idlers to the frame, the 3 custom idler posts were mounted through a hole in the frame. These were positioned close to the edge of each idler to prevent the belt from falling off with tension relieved (clutch depressed).
Phase 5: Foot Operated Throttle
At the speeds the mower is starting to reach, the dash-mounted throttle became a safety concern. With two hands occupied by steering, the throttle was tricky to adjust during driving. A foot operated pedal it is!
The throttle pedal was designed to be operated with the right foot while tugging the throttle cable on the left side of the engine. Utilizing components from the deck-height adjustment system, a setup like pictured to the right was constructed. With the grey supports attached to the frame, the rider may depress the throttle pedal which will rotate the shaft and pull on the throttle cable.
Success!!
Following the swapping of engine and transmission pulleys, repairing the differential, overhauling the belt system and installing a foot throttle, the mower was finally ready for some more testing.
Work concluded just in time to attend the weekly Car Club meeting AND the community car show the following day.
The club meeting on friday went surprisingly well. The 2 mile ride to campus was nearly effortless and the belt didnt fall of a single time (a large improvement compared to previous attempts)! It also led to some great photos included in the carousel below.
Once the mower was back home, some further testing began. Over 10 total miles of testing (and shenanigans), the 3 riders hit top speeds of 38, 34 and 31 mph. Additionally, across those ten miles, the belt managed to stay on the entire time!
Finally, the main event. The car show. Poor weather led to a small turnout in comparison to previous years (especially motorcycles) but the mower was robust enough to handle a bit of water. As excitement rose around it, a race was set up between the mower, a hydrostatic tractor and a customized electric Power Wheels car. The mower took second place twice but with lots of potential to race better with more driving practice. To top off the successful event, the mower was awarded best motorcycle since no real motorcycles attended. That's a win in my book!
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