1. Become a member of the BLMRA and get along to a race meeting.
You will be sent the hanbook containing rules and regulations. It will also contain contacts for getting your new donar machine homoligated. Also by coming along to a meeting you will get a much better idea of whats involved and plenty of ideas for your build.
2. Arrange homoligation of the donar machine.
Before cutting your new donar machine to bits you need to get it homoligated, its not as painful as it may sound! You will arrange with one of the BLMRA technical committee to get it measured. This will then allow them to present you with a sheet that gives you all the detailed build specs such as width at front/back and a whole host of other dimensions. They will also likely advise ways to get round particular problems you may be presented with. Even if you think you have seen your model of mower racing we still need to measure it as there can be many variations.
3. Prepare the chassis
Unfortunately you are not going to be using most of the original machine you have, the following will need removing, rear axle, gearbox, engne, front axle, pedals, seat and any drive system.
4. Strengthen the chassis
Work out how you plan to strengthen the chassis, when racing its going to be under a lot of pressure and anything not up to the job will soon break. Remember you are going to need to plan how you are going to mount the front and rear axle.
5. Rear Axle
Unfortunately the old gearbox is worthless and can be scrapped, a new T Drive will need to be purchased. They are roughly £100 and if setup correctly are bomb proof. In that T drive you are going to need an axle, 25mm with keyways for rear hubs, brake disc and the crown wheel which sits inside the t drive. The whole lot will need mounting securely in the correct location and with adjustment bult in to be able to angle it correctly for lining it up when you get to the drive part of the build.
Brakes are going to be needed, many use the kart components system which does the job and can be picked up on ebay or Rons Bits.
6. Front Axle
The front axle can be quite tricky, you need to take caster, camber and ackerman principles into consideration. You will need to make a strong cross member to mount stub axles either side. Ideally make it removeable so it can be worked on or replaced if need be. Stub axles also need designing, you see many new people start with 17mm axles but i have seen this sheer so it may be worth starting with 20mm and spending the extra time converting the front hubs to take 20mm bearings. Other things to thing about are how to keep the centre of the wheel as close to the king pin as possible to try and reduce bump steer you will experiance.
The steering column will also need to be thought about, lawnbugs generally run the shaft over the engine and group 4 under the engine. But you dont want this getting in the way of anything else that needs to be put in place such as engine or exhaust. Although with one or more universal joints you are flexible to route the steering mechanism.
7. Drive System
You now need to work out how you plan to get the power from the engine to the rear axle. Unless your donar machine used otherwise you are limited to belt drive, either V or toothed. Each has pro's and con's.
V Belt - Strong and pulleys generally cheaper, allows you to use a slip clutch system which is a nice easy clutch to setup, more forgving in pulley misalignment, but there may be some power loss in belt slip.
Toothed - Most people who use toothed use the H section format, running inch pulleys and belts. Obviously belt slip is now out of the picture but you are unable to run a slip clutch. A mechanical clutch will need fitting, such a motorbike clutch. The CB250 being a popular choice. This can provide some very fast clutch reactions but will require some machining and brain power to get it all fitted and working spot on, but when it does work it works well. Pulleys will need aligning perfectly otherwise you will drop belts.
You will realise the engine shaft may sit lower than the top shaft of the t drive, there are a couple of routes to overcome this. You could raise the engine, not ideal as you want weight as low as possible. Many people go down the route of creating a top hat design that effectly allows the pulley to run around the outside of the gearbox bringing the pulley low enough to get a direct line to the engine. The other option is to fit a lay shaft, not ideal as can reduce efficency and you now have to worrk about aligning 4 pulleys and belts.
Remember you are going to need purchase pulleys to use at small spint meetings to pulleys used for large tracks such as the annual 6 and 12 hour races. Ratios are likely to be around 2.4:1 for the smallest and 1.6:1 for the largest.
Common engines are below, you have to use an engine of the original orientation your machine was homoligated with.
Honda GXV340 - Great at low revs, lacks power high up, not many being used as big ends have been known to pick up. But can be found on ebay every now and again for not a lot of money.
Tecumseh OHV 13.5 - Slower than the Honda to pick up but rockets once in the right power band. Please not they do not come with fuel tanks so something will need to be purchased, no fabrication of tanks is allowed.
Once your at this point your nearly there, small things like pedals, and the finishing touches will need doing. Take care when chosing a seat, what may feel great in the garage can often prove painful on the track.
10. Race It
The first time you take it out will be quite an experience and you will find that you will want to change almost everything, pedal locations, access to drive system, but its normal and just work through each one to get it feeling comfortable.