How to make a racing lawn mower (Updated!)

Step 9 Paint and finishing it up.

Now for the funnest part of the build: Painting. Now all of that hard work gets to get shown off. But before painting, you need to do some prep. The chassis is likely oily and dusty from welding. All of the welds are also likely not "clean" meaning they're splattered. You can quickly clean up the frame with sandpaper flap discs attached to a grinder. Clean all of the rust, splatter, and rough edges to a smooth surface.

I used a oxide primer primer, which is a good foundation for the paint. I used plain ole' black spray paint. I chose spray paint because the frame is very likely going to get pitted with rocks and future mechanical modifications. So it can easily be touched up. Plus its cheap. The fiberglass hood was painted with orange engine paint, which I find dried quick and smooth.

Now the entire mower is put back together. Wheels, engine, brakes, electrical, etc. The mower went back together surprisingly quick. Less than two hours was all it took.

With the mower put back together, all that's left is to bleed the brakes, install the battery, fill the engine with oil, and then give her a test run.

Wanna' see it run? Watch the video.

Anyhow, it was lots of fun making this mower and so far I've been in 3 races this season. Feel free to ask questions if you want to make your own. if you want to see me in action, visit my racing club's web site, http://www.pvmowerracing.com

Comments

[rabrantes]

Also where did you locate your fuel tank and how big is it?

[rabrantes]

I really like the design of the frame you built and I am interested in building one during this winter. Could you tell me a couple basic measurements of the frame? How long and wide it is? How far from the front of the frame to where it becomes two frail rails? And the distance from the bottom of the bottom frame rail to the top of the top frail rail by the rear axle. I would appreciate it, thanks!

[chivesg5]

I am using a ragb and balts what size poles should I use how fast would it go and from the ragb to the axle iam doing by chane what size sprocte shoul i use

[mowerracer]

I'm not sure what you mean by Poles. As far as speed, it depends on your ratios. On mine , the clutch has a 14 tooth sprocket. From there it goes to a 14 tooth sprocket on the RAGB input. My RAGB is a 2:1, meaning 2 turns in, 1 turn out. The output on my RAGB is a 12 tooth sprocket and from there it goes to a 40 tooth rear axle sprocket which gives me around 35-40MPH on avg. Making the sprockets bigger or smaller will affect the speed. For example, putting a smaller rear sprocket on the axle, like a 35 tooth sprocket would increase the speed. Its hard to determine exactly what the speed will be. Most of us just make a rough approximation then fine-tune by adjusting. I already went to a smaller rear sprocket to slightly increase my speed. Just be aware that most sanctioned racing organizations have a limit on what the final ratio is. I believe 8:1 is typical, but double check with the USLMRA, or ARMA organizations first.

[Ken Chevy]

  Engine/clutch 14tooth -> 14tooth on RAGB=Same ratio.
  RAGB has 2:1 ratio. Now engine turns twice for axle turning once. RAGB has a 12tooth and rear axle has a 40tooth.
   40tooth / 12tooth = 3.33 ratio x 2 (from 2:1 ratio in RAGB) = 6.66:1 over all ratio.  
  Tires are 11 inches in diameter.
  With that ratio your speed will be:
5000RPM=24.56MPH and
6000RPM=29.48MPH.
  Changing ANY of the numbers changes the MPH. If you change to a 36tooth rear sprocket then the ratio becomes 36/12=3:1x2(RAGB)=6:1 total.
Then the speeds become:
5000RPM=27.27MPH and
6000RPM=32.72RPM.
  These are TRUE numbers, IF the clutch is locked up and not slipping. Using belts, there may be belt slippage, making your speed a little slower.
  With a 40/12 ratio, changing rear tire from 11 inch to 12 inch diameter changes speed at 6000 from 29.48MPH to 32.16MPH.
  It would be easy to put all of that into a spreadsheet to show speeds for every 1000RPM and what happens when changing sprocket ratios or tire sizes.
 I made a formula over 35 years ago to figure numbers like that out, the formula is:
RPM÷RearRatio÷TransRatio×TireDiameter÷336.13524=MPH

Nice Instructable.

  Ken Chevy
080810 1316

[gschmidt-2]

OK, after looking at your build i see that you are using a chain instead of a belt. also you are using a cintrifical clutch. whats the deal??? the way we race is with a belt, double pulley clutch. the only chain we use is between the trans and rear axle. i was wondering what rules you are using?

[mowerracer]

I built this mower under ARMA rules for the Pro-X class which means I can use a single cylinder engine and am not limited to using a geared transmission. As such I chose a right angle gear box and a centrifugal clutch because its a much simpler setup. I always found gearboxes to be a pain as there's more to go wrong.

[gschmidt-2]

OK, that makes sense. we only have one guy with an f/x mower. what state are you in? I'm stuck in N.E. and if your not a farmer or rancher there's nothing to do during the summer but race. right now I'm finishing up a mower that i hope will go 100mph.

[mowerracer]

100MPH is probably not going to happen honestly. We have quite a few machines on our team that are running modified Briggs V-twins and even for them we're talking maybe 40MPH tops. During our races our average speed is usually around 30MPH. Trust me- that is going to feel very fast on a rough track without suspension!

Either way, good luck with your build.

[OleNavy]

You should check out Bobby Clevelands last run at the Bonneville Salt Flats. He didn't hit 100 MPH, which was the goal, but he got awfully close to it. He said it was a scary ride. The was a video on the USLMRA website. Bobby works for Gold Eagle and takes their truck to a lot of the USLMRA sanctioned races including the national championship race at Delaware, Ohio, sponsored in part by my brother, Chuck Miller. Chuck owns Miller Lawn & Power in Marion, Ohio and the USLMRA Hall of Fame is currently located in his showroom on East Center Street.

[JOHNTHEREDNECK]

would this ratio apply to mine im running 4500 rpm with8to1 i dont know the tranny ratio yet

[Ken Chevy]

You say you have a box that is 8 to 1.   It has two shafts coming out of it. 

If you hook the engine driveshaft up to one of the shafts, the other shaft will either turn 8 times faster or 8 times slower than the engine is turning.

You will probably be using it to gear the engine speed DOWN, so divide the RPM number by the larger number, that will tell you how fast the output shaft will be turning then.

Input RPM = 4,500 / 8 = 562.5  RPM output speed.

Ken

122810 2127

[JOHNTHEREDNECK]

sorry i wasnt really clear ive got pulleys that are 8 to 1 then they go into the transmission which is either going to be a 5 spd spicer or a 6 spd peerless from a poulan . im hoping on getting 45 to 55 out of it.

[Ken Chevy]

I can't read minds (yet - LOL) and can't see the parts you have in front of you.

So, you have to give me numbers.

Make a mark on the input shaft of the transmission and another mark on the output shaft.  Then put the transmission in 1st gear and count how many times the input shaft goes around to the output shafts ONE time.  If the 5 speed goes around 10 times to one time for the output shaft in 1st gear, then that is 10/1, and 7 times in 2nd gear it would be 7/1 etc.

You HAVE to have the numbers to figure out the ratios.

So you will HAVE to get both transmissions, mark the shafts and count the turns to get the ratio for each gear...OR that info (transmission ratios) may be available from a dealer. 

But you MUST have:

Engine RPM
Pulley ratios
Transmission ratios
Rearend ratio
and, last but not least the Tire Diameter.

Once you have those numbers, then input them into the formula I furnished:

EngineRPM÷RearRatio÷TransRatio×TireDiameter÷336.13524=MPH

If, for example, you have 4,500 Engine RPM/8=562.5 engine RPM
562.5/6
Rearend ratio of 6/1
Transmission ratio of 5/1
and a Tire Diameter of 12 inches

then we would do 4500/8=562.5
562.5/6=93.75
93.75/5=18.75
18.75*12=225
225/336.13524=.6694 MPH

Less than ONE MPH @ 4,500 RPM engine speed.

So, get rid of the 8/1 and make them 1/1 pulleys.
Then the MPH would be 5.355 MPH

Change the rearend ratio from 6/1 to 2/1 and then you have 16.065 MPH @ 4,500 RPM engine speed.

Then change the transmission ratio to 1.5/1 and your MPH would be:

53.5499 MPH @ 4,500 RPM's of engine speed with a 12 inch tire.

4,500 RPM engine speed (w/1/1 pulleys) + rearend ratio of 2/1 and transmission ratio of 1.5/1 with a 12 inch tire = 53 MPH

Put different numbers into the formula, John.  It will work every time.  It is simple, 2+2=4 every time.


EngineRPM÷RearRatio÷TransRatio×TireDiameter÷336.13524=MPH

I used to use that formula a lot when I was drag racing years ago. 

You can use a regular calculator, use the one in your computer works fine, and just do them one step at a time.    Try it, figure out some ratios, John.

And, like mowerracer said, 45 to 55 MPH is pretty improbable without having an engine putting out a LOT of horsepower!

And it would take so long to get up to speed too, because of lack of HP.

You might try it with a 3.5 rear ratio to start with and that will be about 30 MPH and see what that is like.

Ken

122910 1438

[JOHNTHEREDNECK]

finally got the ratio from the tranny 6 turns of the shaft = one revolution of the tire the tire is a 12.5 . so i think i got the math rite but im not so good a math either motor 4000 8to 1 means 32000 on the input shaft of the tranny = 5333 of the tire in 5th gear rite ? for every 1 turn of the motor im getting 8 out of the in put shaft on the transmission .

[JOHNTHEREDNECK]

thanks for the equation !

[JOHNTHEREDNECK]

thanks just trying to get an idea of the speeds im attaining ill be putting a 18hp briggs twin on it once i race the twelve horse for awhile

[mowerracer]

45 to 55MPH would be improbable with a mild modified build. The average speed that we attain in our group is roughly 27-33MPH and trust me- that feels FAST out there on the track. Just as a reminder, if you are building on of these you will need to adhere to the rules and regulations outlined by one of the sanctioned organizations like ARMA or USLMRA. These rules are created to make your ride safer.

[JOHNTHEREDNECK]

im following the guide lines from NELMRA im going to run in the prepared class when ever i finish this project.

[mowerracer]

Thanks for the math Ken. As mentioned- my math skills are not exactly great.

[mud-dawg96]

there is a speed calculator for go karts, or anything really, if you go to http://www.diygokarts.com/

[Ken Chevy]

  No problem.  I put the math there so anyone, including you, can follow the steps and figure out your ratio's and MPH@RPM.

  If you change your axle sprocket to a 24 tooth, which would make it  2:1, then you could obtain 50MPH at 6,111RPM if you had enough power.

  It looks like it would be fun.

Ken
080910 0820

[mossimo3]

Is it at all possible to make the mower run fast by still using only stock parts? At this point I'm not looking to make it an extreme racing mower, just something fun to ride around in that goes somewhat fast. Also: Would a 1/4 inch steel plate be sufficient in preventing harm if the flywheel explodes?

[mowerracer]

The short answer is yes, but I wouldn't consider doing it. The reason is simply because the brakes, steering , frame, and the rest of the mower is built to go 3-5MPH max. The stock spindles on these mowers have a tendency to split down the middle if put under lots of stress. I know because we had a guy who raced with a "semi-stock" mower using the stock spindles. The mower flipped over because the right spindle cracked in half. That can be really nasty if you're going fast. That and the brakes these come with are useless at high speeds. As far as the flywheel, some protection is better than nothing, but there is a tremendous amount of energy released when one of those suckers explodes, and a 1/4" thick steel might/ might not stop the shrapnel. I will say that so far, I'm the only guy in my club with a billet flywheel yet none of the guys running stock wheels have had one blow up. But that doesn't necessarily mean they won't either. Anyhow, you can get away with making a lower cost racing mower using go-cart components. Check out www.heymow.com and read some of the builds to find one that fits your budget. Good luck!

[ac-dc]

No way it's getting through 1/4" steel, BUT the mounting of that steel to support members, bolts or welding/etc is the weak point. Bits of metal may still fly out somewhere around the plate at high velocity.

[mowerracer]

Precisely. The bottom line is that no flywheel is entirely safe. There is always the possibility that a flywheel can explode- even the billet ones even though the chance for them is much lower. The problem with a scatter shield is that the only place to mount it is the engine and the engine being made of cast aluminum is not going to withstand the force of a exploding flywheel.

[traxxasman]

About how much did u spend building this not including the mower, im a freshman and am on a tight budget?

[mowerracer]

Altogether probably around $1,500-$1,800. But keep in mind this was a heavily modified, custom mower. You can build a milder version for under $500 in some cases.

[derekonderdonk]

What is your tire and rim size? Complete sizes. _X_X_. Thanks.

[mowerracer]

These are 6" rims using 11 X 5.5-6 grooved ties made by Burris. They are go cart wheels and tires. The tires and wheels can be bought here: http://www.tbrwins.com/catsearch/frsearch.mgi

[joecrap]

Was it these rims: "6 X 6" diameter VanK Wheel - Powdercoated Black Finish/2.5" Backspace"?

[pnelson_89]

i have a toro 910 idk what year? would that be a good lil runner sence its so small?

[mowerracer]

It doesn't really matter what type of mower you happen to find because by the time you get through with all of the structural and safety modifications, very little of the stock mower will be left. As mentioned, the stock steering, brakes, and wheels are worthless so you will need to do the proper upgrades.

[fossheim13]

do yuo know abaot som other easy tings i kan do to make it go faster

[fast-dude]

how fast does yours go?

[pnelson_89]

i have a toro 910 idk what year? would that be a good lil runner sence its so small?

[chivesg5]

i do not no i no it is 11hp

[mowerracer]

I've never clocked it. My guesstimation is around 35-40MPH. But this year I have made upgrades and will be doing test runs at our first group event this weekend. I'm shooting for 50MPH.

[fossheim13]

do yuo know abaot som other easy tings i kan do to make it go faster

[chivesg5]

you took the gavaner off that way did it not blowup