Simplicity Broadmoor R/S Project

Ok, here we go............

I wasn't planning to build any more tractors in the near future. But with the Top Tractor Challenge going on, I started thinking about various ways to make a better machine and decided it was time to try some things I wanted to do in the past but so far have not. Full suspension was one thing I always thought about, never did it. Also with the GTR mini tractor project I originally wanted to try a turbo setup. Recently thanks to Creepycrawler, I was made aware that small belt driven superchargers exist. I've always wanted to build up a lawn mower engine but so far I've been too cheap to bother. Not only that but I didn't see much reason to since I never had any engines blow up before. Another thing I had planned on doing was fiberglass bodywork. I ran out of time during the build off and decided to scrap that idea. Nothing is 100% set in stone yet but I have a pretty good idea of what I want to do in mind.

The Simplicity Adventure Edition #64 is going to be the measuring stick with which to gauge the new build. It's roughly the same size and I would guess weight as the average off road tractor. The main problem with it being competitive is lack of power. Also, I think the center of gravity is a bit too high, possibly slightly higher than most other people's tractor's.
When I built it, I had the following requirements in mind:
1) Long range fuel economy
2) Deep water crossing ability
3) Easy to work on, easy to get replacement parts

The 8hp Briggs is nice and light to get off the machine, easy to take apart in the middle of no where and there are lots of used ones around for parts. Plus it doesn't need much fuel. But I went a bit too small, and even for it's original intended use, it's not quite enough. However, it works much better for cruising around at haspin and getting into tight places than trying to race around the courses marked out at the TTC. So rather than hack up the #64 and modify it, I figure the best thing to do is start again and this time build something specifically designed for the Top Tractor Challenge.

I'm a fan of the 1970's simplicity hoods, so once again, the plan is to build a simplicity. (Plus I already have all those orange shirts)
One thing is that I don't have a 1970's simplicity. And I don't plan on finding one either. Most of the machine will be custom built, but with a more or less conventional layout. I want to built something about as close to the limit as you can get while still using lawn mower parts. So I want to stay close to stock tractor dimensions if possible and also use a tractor engine and transmission. For example, I don't want to make it twice as wide and twice as long to fit a suspension in. Not only would it not fit through the courses at the TTC, but in my eyes it would be too far away from a lawn tractor. I think it should have the size and look of the original tractor, more or less.

Here's what I think needs to be done to build a winning machine...
1) Low center of gravity
2) More power
3) Good steering/handling
4) Reduce weight where possible
5) Acceptable ground clearance

In my Adventure Edition build thread there was a pic from when I was first figuring things out. I imagine the new build will end up looking somewhat similar. I hope to make my own copy of a simplicity hood and grill from fiberglass.




I have two of these engines but I need to take both of them apart to see how they look.


I'll update things as they progress. Questions and criticisms welcome.

I’m excited for this one, your build quality has always been top notch. Hopefully it’s done in time for haspin and you’re able to bring it

Sweet, I've been looking forward to seeing this thread come up!

I believe you said a Roper 633 was the choice for a transmission in this build? I'm definitely interested to see how the rear suspension setup works out!

Thanks. That's my plan to be ready for Haspin. I thought about entering it in next years build off, but that's what I did last time with the #64 and it was really close with how much time I had. I put in some long hours to get it done in time and I had it completely disassembled about three or four days before Haspin. If possible, I'd rather not have a repeat of that lol.

And yeah I'm gonna try to use the horizontal shaft opposed and a peerless 633 transaxle. Probably driving a live axle, with the rear wheels not actually being attached to the 633 axles. The modern style transaxles seem to have a lot of shift key issues. I feel the 633 is a tad more heavy duty in that respect. And although there's some issue with shifting on the fly, I think I can get that part to work better.

So we'll see how it goes. I'm currently trying to figure out the exact placement of the engine and transmission in relation to the wheels. I want the weight down low and also more forward. That makes things a bit of a tight fit while trying to keep the stock wheel base and width close.

In the past I just started with a frame or whatever and made things fit as I went. This time I'm actually trying to draft it out on paper first so I have a rough plan of action before I start getting parts.

Working on some scratch paper to get a rough estimate of how wide I'll have to go with the wheels to nestle to front of the opposed in between them. I'm figuring on 40 degrees of steering, not sure what is normal but that's fairly sharp. If I use 18 x 8.50 front wheels, I can get around 40 degrees with about a 41-42" width.

On the other hand, I'm going by a square drawn out with the tire dimensions listed online. For one thing, those dimensions may not be exact to the tire when I get it. And of course the tire has a somewhat rounded shape. So that may give an extra inch or two of clearance more than what I'm figuring on paper. Also I need to leave an inch or so for suspension movement. For 18 x 8.50 those tires, to be able to steer at least 40 degrees and to fit the front cylinder of the opposed in between the back of the wheels, I think your looking at at minimum 40" width.

That's actually where I would be at with the adventure edition if I used 8" wide wheels with the current setup. So that's not a horrible figure. Maybe with some more accurate figures I could get steering somewhere closer to 45 degrees for those tight spots, but we'll see.

An unfortunate thing is that the wider area of the engine has to be in the front. The narrower cylinder needs to face the rear. That is due to the 633 transaxle input shaft spinning in a counter clockwise or backwards direction and the engine needs to be placed this way on the frame to match it. If the engine could be reversed, then you would have an extra 2.5" to play with. But that's not a huge deal.

Spent some time today weighing the Simplicity Adventure Edition and also getting more figures on engines and transmissions. I bought two cheap digital scales from amazon and they seem to be fairly close to each other. I had to turn them to make sure they sat flat on the concrete floor of the garage and that's why you see the yellow outlines marked on the floor. That way I could be sure they were in the same spot when I checked.

For accuracy, I switched the scales (which I marked 1 and 2) from side to side ect. Just in case anything was off. After getting several readings for each wheel both with me off the tractor and sitting on the seat, I averaged everything to get my final answer.

In this pic, you can see the plywood I used to make sure the wheels without scales were at the same height as the scales were. Any tipping of the tractor would throw off the readings a certain amount. So the closer to level the better.


Recording everything. The numbers 1 or 2 next to the weights indicate what scale I got the reading from.


The final figures are....

Simplicity #64 Weight (No Rider)
-LF 116.6 lbs 23.3%
-RF 113.3 lbs 22.6%
-LR 125.8 lbs 25.1%
-RR 144.9 lbs 28.9%  

-Front Axle 229.9 lbs 45.9%
-Rear Axle 270.7 lbs 54%

-Total weight no rider: 500.6 lbs

Simplicity #64 Weight (136 lb Rider) (Yes I only weigh 136 lbs)
-LF 122.0 lbs 19.2%
-RF 119.5 lbs 18.8%
-LR 195.0 lbs 30.7%
-RR 197.7 lbs 31.1%

-Front Axle 241.5 lbs 38%
-Rear Axle 392.7 lbs 61.9%

-Total weight with rider: 634.2 lbs

Some figures might be rounded very slightly but it's pretty close. Probably has half a tank of fuel roughly. But it's only a 2 gallon tank at the most, so figure plus or minus 16lbs (located at the rear axle)

For comparision, the GTR 4 Zero was 192 lbs total weight. For the R/S project, I'll be trying to keep the weight low (low center of gravity) and also reduce weight if possible. So now I have some info to go by. Here's a few more figures on various things.

-42CI 18hp Briggs horizontal opposed twin (with steel mount bolted on) 102.4 lbs
-8hp flathead Briggs 41.4 lbs
-122 series transaxle (I've been calling it a newer style 633) 73.6 lbs (with aluminum hubs and pulleys attached)
-1970's 633 transaxle (no hubs, no pulleys) 67 lbs
-Sears suburban 12" rim with bearclaw tire (rear simplicity wheel with low tire pressure) 27.2 lbs

Been waiting for this build thread to pop up, sounds like a solid game plan. Love all the effort your going to build a quality machine.

Thanks, hopefully it will all work out. We'll see.

Nice details, this is going to be good.

Thanks! Hopefully sometime soon I'll be able to start on some actual construction. Just have a couple things in the shop to clear out first. I got my hands on a super budget work bench so I have a dedicated place to take the engines and transmissions apart. Just gotta brace it up a little bit.

Looks like the stage is set for an epic battle between our new 2020 builds mr. studabaker! lol

Looking like a good start mr.moo! If this build turns out anything like your other simplicity, that oppy will whip it around like nothing. Keep in mind the oppys make for a top heavy tractor though, so maybe it'd be cool to mount it really low.

Thanks, and yeah that's the plan. It's a horizontal opposed of course so I might be able to get it a bit lower. Also had the idea to possibly cut down the sump if possible because I see it's a separate piece on the horizontals. It's separate on the verticals too, but in my case, the crank doesn't go through it. So I'm thinking maybe I can gain another 1/2" to 1" drop. I'll see how it looks once I take the engines apart.

Also a billet flywheel is supposed to save 10 lbs over the stock one. That will be a help. Not much I can do about the crank shaft. That's probably where the bulk of the remaining engine weight is.

I figured it would be nice to have a dedicated spot to take apart engines and transmissions for this thing. I was going to build a work bench but came across this one already made. Now I can leave stuff partly disassembled and not have to put it all back every day as I'm working on it.


Had to brace up the legs a little with some plywood and then painted it with some budget paint I picked up at lowes.

I like it, needs some simplicity racing team decals or paint work though

Thanks and yeah my thoughts also. I was gonna paint it orange but that's not a real common color for interior paint. Not since 1974 anyway... So maybe I'll put some big simplicity S logos on the ends.

Wow let me start by saying I'm loving the ideas so fare. I got real excited when I saw you were doing math. I know this is gonna be a good one. There's a guy on YouTube called Hicklife Customs that has some solid builds. But anyway he started a build that involved full suspension and a 633a so if you think you could use alittle inspiration you could check those vids out. Best of luck man. Keep up the good work.

I never really thought about it too hard, but I would have thought heavier could be a good thing... maybe helping with traction?

I think power and traction are some of the requirements for a TTC contender.

Looking forward to this at any rate.

Thanks mowerjunkie I'll have to check that out. And beal, probably true that not enough weight may be a traction issue. But I think that I won't be cutting the weight down enough to effect it that much. My idea is to go for acceleration and maybe even be able to spin the back wheels to burn around some tight places. On something like the steep incline where I flipped over this year, I'm thinking I'll be able to rip up and over stuff like that without hanging around as long.

From my very rough estimates, I think if I'm careful I can stay in the 400-450lbs range with this thing. And have maybe somewhere around 30hp. I don't know exactly how that will work out but that's a rough goal. I just hope the 633 can hold up to some serious hammering. We'll see how it goes as things progress. I finally got half my build area cleaned out so soon things will start coming together a bit. Also picked up some essential golf cart parts I needed from ATLTF President Doug, so now we're ready to figure this thing out.

Well I hope this goes as planed. I know that is gonna be a rpught weight goal to meet. Completely reasonable for a LT but with a 633a and an oppy it might be hard. Even under 600 with 30hp that power/weight ratio is gonna be smackin.

The GTR 4 zero frame was around 22 lbs I think, so for this build I figured I can keep it at 100 lbs probably. The extra shocks for the rear suspension might add a bit to the total weight. I'm going to try to use aluminum wheels. I have my estimates written down somewhere.

After thinking about it I realized your weight goals are quite reasonable actually. The 633a should keep it from having to much front end weight bias(a problem with alot of mower builds). Do you have any idea of the weight of briggs oppys or 633a? If knew that I would crunch numbers for a future build I just have know way of weighing them.

Nevermind. I just read back and saw all the weights. Very helpful info!! The #64 has great weight distribution by the way.

Thanks mowerjunkie. The #64 isn't too bad. I just think the weight is higher than other tractors maybe by a little. So when it get's tipped one way or another, then it effects the distribution more than a tractor where the weight is lower. When flat on the ground, two could have the same percentages but be different when both tipped (if one has higher center of gravity of course)

Some slow progress on the RS project...
Bought a golf cart for parts from Doug Fackler. The spindle pivot point is right about in line with the inside edge of the tire. I revised things on my scrap drawing to figure 18 x 8.50 - 10" front tires, pivot point on the edge, and 42" outside width. It seems I should be able to make a 45 degree steering angle and still clear the engine if I want to do so.

There is some concern over the placement of the steering shaft between the steering wheel and rack and pinion. Either it has to go under the engine (moving the engine a couple inches higher would hurt center of gravity) or over it. I think if the engine is low enough, over it would be a good idea. I may be able to do it with only two universal joints. But that will need to be finalized later on as the building progresses. No way I can get that technical with the drawings. I'm just trying to get a rough idea of the top view for engine, wheel, and transaxle placement.

Next I'll be trying to figure out where to locate the front and rear axles. I want to try keeping it short for tight turns, but there are three areas that will dictate the shortest wheelbase I can use.

-First, I need to leave room between the engine and front axle center line for the rack and pinion (if the rack is located behind the center line. Also might need to have some room for control arms to be attached to the frame.

-Second, The engine and transaxle can only be placed so close together. I will have the 633 moved forward pretty close to the rear cylinder head on the opposed. I might be able to tip the transaxle up a bit more than stock if I really need to gain a couple inches elsewhere.

-Third and lastly, there needs to be ample room between the rear axle of the 633 and the live axle (think that's what you call it) The plan is to hinge a swing arm off the rear of the 633 so that suspension movement doesn't effect drive chain tension. Also, this will be somewhat lighter (hopefully) than If I made a separate mount for the suspension. It should be much more compact too.


I'm not too worried about figuring out the rear suspension, I think that will be pretty simple and I'm not looking for a whole lot of travel back there. The front suspension on the other hand, I'm not 100% sure on yet. I wanted to do IFS, but if I go with upper and lower control arms, it will take up space, add weight, and add some cost with extra joints. And it may not be possible to use the golf cart knuckles and attach them to some sort of ball joints. If I make an improved version of the GTR 4 Zero suspension, while it has some wheel angle issues with travel, that might be the most compact way to do it. If I only need around 3-4" of travel, I could probably get away with that.

Lowest priority is things like the gas tank and battery, which could go almost anywhere. I don't want to get overly complicated with the frame or anything if I don't have to. That would only make it harder to work on and take longer to build.

Okay first of all I think that running the steering shaft over the engine might be difficult and would give it some strange steering wheel agle for a GT. I do however see the benefits though. Have you considered adding a right angle box somewhere in the system? Ohh I'm gettin gitty just thinking about rear suspension on a mower with a 633a. I honestly can't wait to see this thing take shape. I'm curious what your going to do about getting a sprocket hub for that tapered shaft... If you have a lathe this isn't a concern I guess. About the front suspension I know you are considering a single arm system. Have you considered I-beam it is still a single arm but the increased length gives you less variable camber and reduces bump steer. Not as easy to set up though. Just a thought. Best of luck to you man!

I did think about running the steering below the engine to a right angle box and then maybe have a universal up near the wheel to tip it back a bit. I do have two right angle boxes from the mower deck of a 67 sears custom but they are a bit heavy. Over the engine might be tough yeah, the main issue I think would be the front top edge of the engine. Especially if the engine is shoved right up front, that could be a problem.

The tapered shaft on the 633 is only on the input shaft, so that shouldn't be a bit problem. The axles are where I'll have some sort of hub for the sprocket. But anyway I think you can get tapered hubs for things. I heard they are sort of a universal angle but I don't know, I haven't checked.

I also did think about making the arms longer (Or even crossing over themselves as in a twin I beam front suspension) Upper and lower arms would definitely be the nicer option, so we'll see as things progress.