I've recently completely turned this mower into a new build. If you've read this before, proceed to step 12 for the latest updates. Otherwise, start reading below for the original build. Thanks to everyone who has commented before. As always, feel free to ask questions and I'll do my best to answer. Click Here to proceed to step 12.

In this demo, you'll get some ideas of how you can make a real racing riding mower used in national events. Have fun turning what used to be the family lawn mower into a fire-breathing high speed racing machine. Also- I'm constantly making changes and modifications to the final machine so check back to see what I've done. The next plans I have include steering upgrades.

Please read the following paragraph before proceeding.

Before we start, there's a bit of safety to discuss.Yes, racing lawn mowers from an outward perspective is sort of funny ( which it is!)But its important to realize that racing mowers such as these are heavily modified to handle much greater speeds than the original mower was designed for. Many of these mowers go 50MPH or more.Making a race mower isn't as simple as taking a stock tractor and making it go fast without any alterations. So its important that the frame, brakes, steering, engine, and wheels are modified or altered to handle this additional speed.So to make this point doubly clear, it is NOT a good idea to take a bone stock mower and make it go fast. You can, and will get hurt if you do so, and trust me- I've seen enough people wreck due to this very reason. So play it safe. Secondly, if you do plan on racing, make sure and check out the rules for your chapter and wear appropriate safety gear such as a helmet ( motorbike), gloves, boots, and long pants.My mower is built using ARMA ( American Racing Mower Association) rules and regulations. Lastly, your mower must have an approved jet ski/snowmobile style safety tether switch. If you fall off ( which we often do) the mower must automatically shut down or it'll keep right on going! Racing mowers might seem silly, which it sort of is, but you can get hurt if you're not careful. So be safe!

Ready, let's get started! The 'victim' I chose for this build is a late 60's Grants mower. Tiny little mowers like these were produced back when riding mowers were still deemed a luxury. They're little more than a seat sitting on top of a mower deck. Most used smaller engines. The advantage of using such a little mower is that you can reduce the weight dramatically by simply having a 'legit' riding mower complimented with a larger engine, hence a higher power/weight ratio. Don't get attached to it. When its done, there won't be much left of the original.

The first step is to strip the mower down to the frame. Modern mowers usually have a single stamped piece of steel. Older mowers like this one have frames made of square tubing or slabs of steel. This will give you an idea of how much of the mower is actually usable and how you can lay out the drive, steering, and brake components. Besides the hood, what's leftover to use isn't much. The rest are worthless such as the stock wheels, steering wheel, and transmission.
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Sorry Pal, ain't no mower gone 100 mph yet.
I see nothing to indicate this "mower" reached 100 MPH and sustained that speed. Bobby Cleveland driver and racer, sponsored by and employed by Gold Eagle went to the Bonneville Salt Flats (twice) and still was unable to reach 100, but he was close. The video can be seen on the USLMRA website. My brother, Chuck Miller, was there as part of his pit crew for the attempt. Bobby's speed was documented by the Salt Flat organization. So a claim is only a claim unless you have bonafide documentation to back it up. Video of a machine running illegally on a public road doesn't cover it.
I just realized though, you probably have to use a stock engine that has been upgraded, not transplanting a monster of an engine into them, correct?
No one has gotten a lawnmower to do 160km/h? Seriously? I just picked up a 110hp sled engine from a 2004 Arctic Cat Sno Pro and was planning on putting it in a lawnmower to motor around on. If you can get a sports bike to hit 200km/h with less then that I think I can get a lawnmower to do 160km/h. Clearly, no one has really put in a good effort to get a lawnmower to go balls out. I think I could get a lawnmower up to 200km/h easy with 110hp.
i have found the best way to reinforce the frame is to use 3/4 square stock along the top corner of the frame. this will make the frame very strong but not add to much weight
i have looked all over for a ragb and can only find either reducers or 1:1 ratio boxs, but most are only rated for 2hp and at a costly $250. could you please give me the specs on your ragb and where you got it.
go on ec distributing. go on mower parts and the very right bottom link will say right angle gear box. or get a peerless 1100
go on heymow.comgo on peerless.com. find a p-91
i was wondering if i take the pulley from the back and connect it to the engine and the pulley on the engine to the back would it make it go faster
aswracer3 years ago
i have a old mtd with 6 speed stock i wanted to know how to make it work for speed?
go on hey mow.com. otherwise just take out the factory grease and reverse chain and add some realy good grease of gear lube
rabrantes3 years ago
Also where did you locate your fuel tank and how big is it?
rabrantes3 years ago
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!
McClellan414 years ago
i was goin to do this to mine but never did and now wish i had!!! but might have to down the road!!
McClellan414 years ago
i have the same thing but not spring loaded its stationary and it has worked out great so far!!
McClellan414 years ago
so which ratio has a better acceleration (power) 6:1 or 2:1??
McClellan414 years ago
ya hi i was wonderin what size sprocket i should use on the input to the ragb..i have 17t on my clutch and the ragb is a 1:1 ratio???

chivesg56 years ago
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 (author)  chivesg56 years ago
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.

  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
  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
  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:

Nice Instructable.

  Ken Chevy
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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 (author)  gschmidt-24 years ago
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.
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 (author)  gschmidt-24 years ago
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.
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.
would this ratio apply to mine im running 4500 rpm with8to1 i dont know the tranny ratio yet
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.


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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.

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:


If, for example, you have 4,500 Engine RPM/8=562.5 engine RPM
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
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.


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.


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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 .
thanks for the equation !
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 (author)  JOHNTHEREDNECK4 years ago
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.
im following the guide lines from NELMRA im going to run in the prepared class when ever i finish this project.
mowerracer (author)  Ken Chevy5 years ago
Thanks for the math Ken. As mentioned- my math skills are not exactly great.
there is a speed calculator for go karts, or anything really, if you go to http://www.diygokarts.com/
  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.

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rebel91514 years ago
hey bro i goy a murray lown mower with the same size engine now if u didnt wanna buy parts but wanted to make it go faster what would u do
You can make it go faster just by taking off the gonvener and adding a throttle cable, But I dont suggest doing just that, there is a lot more to it. I have been racing for about 10years now and only have $200.00 in my mower at our local track, Top speed on it is 63mph but on the track we run the cops clocked us at 40mph in the corners and the best they could get was 57mph in the straights. I advise to strip the mower and start with the frame before you do anything, I have built 3 murrys but preferr the craftsman all modified and bought nothing but pulleys, belts, bicycle brake cables, and spark plugs just to get started
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