LITTLE B' - David Brown bike engined mini tractor

Hi all,

Thought i would put up a few pics, showing what is going on under the skin on my wee David Brown mini tractor.  As i had mentioned this was built with no rule books or production chassis as a basis, i kinda built the tractor around the engine, for the purpose of eliminating a big unknown i used a 150cc pit bike engine as that was an engine i was pretty familiar with.  The thing took somewhere between 6 - 8 months to complete, (its difficult to say when exactly i finished building and started 'tinkering'), power figures are based ion manufacturers claims of 15-018hp, although i have done a few things to the engine and ignition, the dry weight is about 98kg.  



This is the earliest photo i have of it on my current phone, so really the start point was the engine and it's cradle.  i borrowed my housemates 140cc engine, (which was toast :/), to do mockups with, the red frame is the engine cradle, which was the first item to be made, this braces the engine casing and gives it a solid interface to bolt to the chassis rails by.  The cradle has 4 slotted holes that connect it to the chassis rails, these allow the whole engine/cradle to slide back/forward, there are 2 tensioner screws on the back of the cradle to tension the chain before tightening the 4 main bolts down.  This pic also shows the start of the front axle and rear axle mountings.



this pic shows the front axle pivot setup pretty good, the axle is supported at the front by a spherical bearing and at the rear by a rose/heim joint, by using 2 spherical bearings like this i can account for any mismatch or misalignments between the 2 mounts, the axle allowed for about 10" of crossaxle but is locked out with rubber bushes on the finished tractor, this is safer at speed but also allows the rear inside wheel to lift enough under heavy cornering that the tractor turns well even with the wide locked rear axle.

next we have the steering, the 2 stub axles are mounted on pairs of heims, fairly standard issue on race mowers over here, the 2 track rods and drag link are hollow tube with threaded ends welded in, there is a central bell crank mounted to the back of the axle.  i went with the bell crank/draglink setup as the axle was a swinger instead of a fully locked one and i didn't want to risk any bumpsteer, in hindsight with the amount of crossaxle i limited myself to in the end it really wouldn't have mattered if i has went the direct steer route, (where the 2 track rods mount to the bottom of the steering column like on a go kart).  The steering column is hollow tube with lathed up ends welded in, it mounts into another spherical bearing at its base and a go kart top bearing at the top.



This picture shows some of the sweeping bends i had to make on the chassis, i did them by 'lobster backing' the tubes, its pretty effective at getting a smooth curve with a bit of practice.  The hood profile was done in 16 and 20mm hollow box section, i had to make my own panels and hood so i figured the best way to do it was to build a skeleton like on a stock car to rivet the panels to.  Here the tractor also has ag tyres on the back, we ran it like this once, the handling was too weird, it understeered like crazy on acceleration and oversteered on deceleration, now we only run matching tyres front/rear and never really any more aggressive than turf savers... so far

ill post up more tomorrow

Sweet framing on the front end setup.

Looks very interesting! Pretty neat that you built the frame like an actual tractor where they build them around the engine.

Pretty neat frame setup. Those engines have a clutch and 5 speed in them? What's the top speed with this thing?

Mint! Looks great with them chevrons on her

Thanks guys, the front end took a bit of thinking because i wanted it narrow like a tractor but it needed to be wider at the rear to support the axle and to allow for the engine.  This one is a 4 speed but 5 speeds are available, (also somehow there is a 3speed with reverse available although i have never seen one or how it works as apparently it uses the same casing??).  The clutch is integral and is a cable pull deal, top speed I've had it to is 40mph, although with the current gearing i know it hits 36mph, there is potential to make it faster again but i like the current ratio for blasting about

mattd it may end up with the chevrons on yet if we get some snow this weekend lol



looking a bit more together in the garden here, the seat bolts directly onto the top of the engine mount, also i've fabbed up the shift lever and rest of the bonnet structure.



And here it is a while later with the proper engine in place, mk1 exhaust, (a straight thru which was unbearably loud!), fuel tank mounted with exhaust can straps and the electrics, at this stage we were testing the engine no drive in place yet and no pedals either haha

So it's manual or has that auto clutch thing that when you let of the throttle it takes the load off the gearbox to allow you to shift? I got a 110cc engine like that

its a multi plate manual, you can only get the semi auto boxes in the smaller engines like the 80cc, 110cc and maybe the 125cc, you don't really need to clutch going up or down the box if you rev match good but its advised the most i use the clutch for is for getting the back end to step out , its a tricky one to launch, but I've a fix for that ill show you guys some pics of here soon

Thats sweet, cool seeing something way different from the usual. tires on the back look real aggressive and the bare bones look is cool. Doing fenders or sheetmetal work?

Dude that thing is a awesome build! Would love to see it play in the snow!

redlinemotorsportts wrote:

Thats sweet, cool seeing something way different from the usual. tires on the back look real aggressive and the bare bones look is cool. Doing fenders or sheetmetal work?

Thing is already complete, these are pics from the build.

Top notch fab work man, I love this thing!

Thanks for the encouragement guys, Redline, we actually ran it for few months with no tinwork at all when sorting out all the fine tuning of the other systems, i really liked the way it looked then.  The snow is supposed to hit us tomorrow so i hope to get it out for a blast if it does, I've another set of wheels and tyres i can fit with a bit more grip but the chevrons aren't on them wheels, ill make a call later on tonight as to whether ill get them fitted ready for tomorrow, otherwise its turf tyres and go kart wets instead of the slicks.  i wanted to get a set of triple rib or aggressive multi rib tyres for the front to match up to the rear chevrons but none were available for a 5" kart rim that i could find  



Theres the chassis after we did the frames for the fender and running boards/footplates, i had modified the dash framework also by this stage, i must have been halfway through modifying the pedal mounts here, originally there weren't going to be footplates, it was going to have 2 pegs for your heels like a Ferguson TE20 we have on the farm, (TO20 on the other side of the atlantic).  Anyways we twigged that was gonna be excessively dangerous after the first few test drives haha, so the mower style footplates were added.



And here is the finished frame getting painted, there is now mounts for the fuel tank, oil cooler at the front, brake master cylinder, pedals, stays for the fender and a location for a 3rd rear axle bearing, it was really satisfying painting it up after all the time welding it

Yeah Jon heard about that snow. Hopefully I get my Westwood going by then

Intense.

If you get a lot of snow, might wantto think about front skis and a skidplate. I live in the area that got hit with the 3' of snow and i cant take anything anywhere. Front end digs in and then it bottoms out.

Doug Fackler wrote:

Intense.

If you get a lot of snow, might wantto think about front skis and a skidplate. I live in the area that got hit with the 3' of snow and i cant take anything anywhere. Front end digs in and then it bottoms out.

Well we ended up with a few inches of snow at the farm, but it was that nasty wet stuff, but still, a bit less friction makes for a bit of fun first time out for the 'little b' in the snow, i borrowed the wheels and tyres i had bought for the tractor i'm building next, the 'super b', (go kart wets and turf savers on the same wheels),  it felt sweet

i had never used it with the turf savers before and found they didn't affect the balance of the tractor too much, it still breaks loose at the rear when provoked, tips into corners when you want it to - only now it feels a lot faster! The only downside seems to be that the front wheels now throw snow and water right into the carb so that kinda cut the fun short, but i know what to fix for next time!

A couple more snaps here of the build, i guess one of the weirdest parts of any build seems to bee all the little pieces you end up building that take aaages but are only small and kind of blend into the build unnoticed, i mean i had all kinds of issues with 'standard' parts and stuff that i bought in that needed modified, i don't think many standard 'off the shelf' items involved in this build ended up on the finished tractor without first having to be modified.  And then you have all the small turned components, made to measure etc.  The steering/throttle assembly shown here was one of my favourite assemblies because of all the small parts like this.



So I've got the steering wheel from a race car, below which it all gets a bit customised, I'm using a shifter kart paddle clutch setup, but have modified it to control the throttle, the biggest hurdle here was that the clutch paddle pulls 20mm of cable through in a stroke, but the carb needs to pull 30mm to go from closed to full throttle.  The solution was to use a clutch lightener from a moto trials bike in reverse, so thats what the grey metal box is, it took ages to find this thing, mainly because i didn't know it existed so i was looking online for ages for something to fit the bill and found it actually by accident in the end!  Finding half of the stuff seemed to be half the battle i discovered.  Theres bits from motorbikes, quads, mountain bikes, lawn tractors, golf carts, go karts, rally karts, shifter karts, monkey bikes, race cars and more in this.  The centre cap on the steering wheel is the top cap off a mountain bike, later i was able to swap it for one that was also a garmin mount so i could have a speedo.



This second pic is the new 'mk2' engine mount.  The first one died a horrible death in testing, we were doing full throttle launches on dirt with the chevron tyres, the thing was too quick, i always like the tyres to be the weak point of a driveline, (its better to smoke the tyres than break a transmission), the tyres were to grippy on that surface and the original engine cradle was the weak point, it got all bent up and twisted, the new one was made from thicker material and strengthened where the old one had broke, the holes are to offset the increase in weight from the old mount, i had set myself a weight limit of 100kgs for the whole build.  I'm glad to say this mount is still going well

Looks like fun! Im waiting for our snow to go down a little more before i attempt another snow run.

I hate finding obsolete parts. When I redid the sending unit on my truck, i needed a special 90 degree connector. Took it to 3 auto parts stores and scoured the internet but couldnt find a fitting term for it. That was back in September or October. Its now February and I just found a replacement connector lol.

We got some snow too, didn't get my Westwood out though. Seemed fairly dry where I am. Melted when the sun came up at about half 1.
I love seeing the progress pictures of this tractor! Didn't realize that the frame was so 'curvy'

Doug, tell me about it, my big tractor was even worse, but certainly it gets easier when you build up a network of dealers and suppliers you can trust and who know the parts well.

We'll get decent snow yet Matthew (fingerscrossed)

This time I've some pics from the initial rebuild after paint, this was the phase of the build where i thought i was like 95% completed, but all them small detail jobs then took over, pushing the build completion out further and further.

The first pics show the tractor frame with axles on and no engine, it looks unbearably clean lol, we had switched to the rear slicks to balance out the handling, this was the first time they were fitted luckily they suited the tractor somehow!





The next one here shows the brake setup at the rear, i had mentioned that there was a 3rd bearing on the axle, it is sandwiched between the rear sprocket and brake disc.  The mounting holes in the frame for the 3rd bearing are slightly oversize, the outside 2 bearings are located and tightened down first, then the centre one is tightened afterwards to avoid the bearings binding up.  the purpose of the centre bearing is to stop the rear axle bending under the tension of the chain, it also serves as a handy interface to mount the brake to.



lastly we have a drivers eye view, the gearstick on the left pulls back to go up the box and pushes forward for a downshift, the little number box on the lever tells you what gear it is in.  sounds a bit flash like, but it was essential - i found that the transmission needs to be spinning in order to select a gear, so first gear can be hard to find unless you know the tractor well, with the readout you know its in gear so don't risk breaking the box when you do a hard launch on the launch control... (yep you heard that one right lol )

How does the launch control work? I'm guessing if engages the clutch at a certain speed to allow a certain amount of slippage out of the hole.

Hi Rodney, i have a 2 stage rev limiter on the ignition unit, when i activate the switch on the ignition unit it will stop the engine revving any harder than a set rpm, (in this case 7000rpm, which can be adjusted), max revs is 13000rpm. i picked 7000 rpm after a lot of testing, it seems to be the sweet spot for a fast start - not too fast that it burns up the clutch, not too slow that the engine bogs and generally where the engine makes good power. it maintains the revs by intermittently cutting the spark to make a controlled misfire.

there are 2 switches that need to be pressed to activate the lower rev limiter, one on the steering wheel, and one on the clutch. The one on the clutch is on a mechanism that can be adjusted so that the switch disengages when the clutch just reaches the bite point, as soon as you let the clutch out past the bite point the circuit is broken and the engine revs freely.

So the start sequence goes; hold clutch in, select 1st, let clutch out slightly against the brake, (checking first is engaged), press the button on the wheel and hold the throttle wide open! it won't rev any higher than 7000, but will pop and bang, wait for the tree... then you let the clutch out -warp speed! the button on the steering wheel is pressed with your left hand so you have to let go of it before reaching for the gearstick and making the shift between 1st and 2nd, this way you don't accidentally death grip the button and reactivate the low end limiter when clutching for 2nd.

OK, rev limiter, I see how it works now. Pretty cool. I was thinking of a top fuel drag car.

Digging thru some more photos here, so we had reached the first full mechanical build stage, and the tractor was to be run for the first time on its new rear slicks...



slicks don't grip much on anything besides clean, firm surfaces, our driveway is none of that



However, with a bit of practice we learned how to make them hook it took a couple of near-backflips to work out that there was a particularly grippy piece of tarmac right beside the front door, (it was the bit where i weld usually so i always sweep it up afterwards - unintentional track prep!).  We also adjusted tyre pressures for grip and weight transfer gains, and learned the way the engine wanted to be driven.



so then he got a wheelie bar added! this is the mk1 exhaust, which was a straight thru pipe, it gave a lot of power high in the rev range, but it was just too loud lol, i wanted to be able to test out little things on it without p***ing off the neighbours, so in this pic it actually has tape over the exhaust exit, the tape actually killed a lot of power and made it nearly impossible to drive, so a mk2 exhaust was getting dreamt up at this stage.