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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 once before where I modified a lawn mower to drive at a top speed of 30 mph, but with seriously compromised steering, no clutch and highly ineffective brakes. The image in the header is of this lawn mower. Its utter lack of brakes and lacking steering led me to embark on this project again with a blank slate.
The goal with this project is to maximize the top speed of this mower with the original motor and transmission. An additional point of focus is to retain, if not improve, the functionality of the steering, brakes, and clutch.
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. Needless to say, there was an excess of rust, grass and mud on the deck, frame, and entire underside of the mower. To add to the state of disrepair, all 4 tires were flat, dry rotted, and had serious flat-spots.
After getting the mower home, the seized deck was removed, along with the engine, transmission, and battery such that the frame could be indiscriminately pressure washed.
Coroded and siezed mower deck
Phase 2: Transmission Pulley Swap
With a belt drive set up, speed can be traded for torque (which is in excess now since the blade system has been removed) by either increasing the diameter of the pulley on the engine, 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.
The transmission is driven by a 10" pulley mounted to a 1/2" keyed shaft. Spare pulleys that I had access to, all had 1" hubs, so with access to RIT's machine shop, I turned down an adapter and welded it to the hub in a 4" Diameter pulley. With a surprisingly perfect, light friction fit, the pulley mounted up 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 built with spare pulley, round stock and an unholy amolgamation of hardware.
10" Motor pulley to replace stock 4" pulley
Phase 3: Motor Pulley Swap
Testing with the reduced diameter rear pulley indicated a top speed of 20 mph - Improved from stock, 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 to 1-1/8"...
With some aggressive chatter, the pulley was made an oversized fit. It could now be slipped over the existing pulley 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. Despite never releasing the "clutch" (which is functionally a tensioner which can be released with a foot pedal), 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 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
Next Steps
Reinstall transaxle
New top speed run
Fabricate foot operated throttle
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