OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers Scott Bonnar Engine Positioning on Frame?

Is there any reason why Scott Bonnar has positioned the motor towards the outer edge of the frame on the Scott Bonnar 45 series? Would it be better weight distribution if the motor location is positioned closer to the middle of the mower frame?

The weight dist seems fine....as the clutch is centred and the engine isnt that weighty and would balance/equalize all the gearing and chains that are on the other side. Engine,clutch centred and final drive gearing last....all seems to fit as a driveline should.....:)

Its definitely motor heavy on one side. Planning to put some scales underneath each side and finding out the real weight distribution.

Hi SBM,
I must agree with Mal (bluegmhtmanaro) on this issue.

You just have to mow with a carefully maintained and aligned Scotty 45, to see the finish on a fine lawn. To achieve this finish, the machine has to be well aligned and balanced.
To give you some history of this design:
The SB Model 45 was first introduced in 1968 and was designed by the company's design engineers, in the SB factory at Thebarton, South Australia.

It has been the most successful domestic cylinder mower designed and built in Australia by Scott Bonnar Ltd. and carried on successively by Rover-Scott Bonnar, Rover Ltd., and lately by Protea Mowers and Star Mowers.
Protea is the market-leader for professional-quality turf care machinery in South Africa....Click HERE

If you are thinking of weighing the machine on each side, you have to remember the the right-hand rear roller (looking from behind the machine) contains balance weights, which helps balance the machine whilst turning and also the differential action of the rear rollers.

I also have an ATCO Model 2058 (ie a 20" manufactured in 1958) which has a Mark 12KS Villiers positioned in the same area as the engine on a Scotty. This was the usual practice of those times. The petrol version of the SB Model 17 Supercut, designed by the excellent SB engineer Sid Bowditch, in 1950, also had the engine positioned the same as its protege, the Model 45.

Hoping this is of interest,

Hi Deejay,

Thankyou for your comments. while I agree the Scott bonnar has been a successful australian design and build mower, there has to be some design reason why these motors were placed off centre. I am not convinced.

As a experienced design Engineer and many years of mowing lawns with a professional balanced cutter wheel, the motor side does always cuts the lawn lower.

I am planning on modelling my scott Bonnar mower in 3D and analysing this weight balance difference. Great little project. Then build a prototpye to test.

watch this space!

The only reason I can think why the scott bonbar designers placed the motor over to one side is place weight over the one roller to help turn the mower.

Pretty much all walk-behind reel mowers have the engine offset on one side of the machine. John Deere, Jacobsen, even the Protea mowers which are based on the the Scott Bonnar range have offset motors.

I am intrigued by what you describe as a "professional balanced cutter wheel" Can you please post a photo of this device as I'm sure our members would be interested.

Here is a pic of the Scott Bonnar Model 17 Supercut that amongst other applications was used to cut bowling greens, tennis courts and cricket wickets....In reference to 'bowling greens' where the playing surfaces have to be cut precisely level, I find your statement "the motor side does always cuts the lawn lower" not well founded in practice.
Have a look at the set-up of Sid Bowditch's design here:

I rest my case.

I wouldnt rest my case Deejay just yet. Just starting my project lol

By a Professional balanced wheel cutter, I mean a "carefully maintained and aligned Scotty 45" by others (workshop)

This view is also shared by other owners who have their blades sharped and aligned by workshops that have been in business for over 30 years etc. Just because something is used in service does not mean it does not have its faults.

your thoughts?

Hi SBM,
I must admit that the description of a Scott Bonnar Model 45 as a "Professional balanced wheel cutter", the most extraordinary that I've heard, in my 40+ years experience with cylinder mowers.....
So from your last post, I gather that you are only concerned with a perceived fault of the Scotty 45.

As we go through the idiosyncrasies of my particular favourite machine, we shall inevitably condemn this thread to Tech Talk, a topic that we have for in-depth technical discussion.

Just so we are on the same page, firstly, alignment is the term we use when we are concerned with the machine chassis itself, and the components fitted to it.... eg., soleplate, rear rollers, front rollers etc.

It is not to be confused with cylinder reel to bedknife clearance, which is discussed in detail HERE

I have owned my Scotty 45 since new, and have never found my alignment to be an issue. Its chassis was aligned in the jigs at the SB factory, and has never required adjustment.
Mine has a G100 Honda (Japan) engine fitted, whose weight I do not know compared with a Kirby Tecumseh or Briggs.
As for the cut, with a sharp cylinder reel and ground bedknife blade, carefully adjusted, on my kikuyu lawn I have no problem....the finish is superb.

If your machine is cutting unevenly, I suggest you may have a chassis alignment problem with your machine.

Hi Deejay,

I do thank you for your comments. But my reference to the machine cutting unevenly was on my fathers new Scott Bonnar back in 1977. Not my old Scott Bonnar motor that I am rebuilding at the moment. Maybe my father Scott Bonnar had a chassis alignment problem from new? I don't know, But there are many more people out there reporting this same problem. To a trained eye, this uneven cut finish can be seen. Not a big problem to most, but to say the finish cut is even, this is not the true. This is why I have asking this forum regarding the motor position, to find out the design reason behind the engine being offset, as well as doing my own investigation to find out if it is a chassis misaligned problem as you put it in your own words is the problem.



Peter

P.s I have heard more extraordinary things in my 30+

Peter, why not just check the weight distribution? Shouldn't take any more than 5 minutes. All it takes is a set of bathroom scales, two similar diameter, fair-sized pieces of pipe 18" long, and a block of some kind the same height as the set of bathroom scales. Just a the pipe inside the sideplate, bridging from the front to the rear roller on each side of the mower, with the scales underneath the pipe on one side, and the space block proping up the other pipe to the same height so the weight is properly distributed. Read the scales, then switch the pipes, scales and prop to the other side. Read the scales again. If the scale readings are identical, balance is perfect. If not, we'd all like to know how much different they were. Then if you can get hold of the other two widths of machine and repeat the experiment, we'll end up knowing whether Scott Bonnar's engineer knew his onions or not. Be fair, though: the mowers all need to have the original engines.

Hi Grumpy,

That's what I am planning to do. Just have to find some time now and I will post up the results. Its not rocket science lol

Yes, the original engine will be used in this test.

Peter

Thanks Peter. It is important to post pictures so we can all see exactly how you do it - otherwise the questions and uncertainties will cause this thread to become the longest in Outdoorking history.

Once it is done and documented, hopefully others will follow with identical tests on the other sizes, and other models. SB45s not only had three widths and two different deck designs (solid and two-piece), they also had various engines over time. Some have also been retrofitted with other engines, especially Hondas, and it would be useful to see what effect this had.

If there actually is a significant side-to-side imbalance of the SB45 by design, we can continue this thread to talk about possible remedies. Conversely if there is no substantial imbalance but some members are having problems with uneven cut from side to side, we can consider possible causes of that (such as frame misalignment).

Has this post stalled????
Steve

Hi Steve,
It looks like it has stalled, seeing the last post was on the 10th June, and thanks for bringing it to our attention.
I, for one, would have liked to see the results of that test.

As this thread is over 3 months old without replies, I will now close it. If any member wishes to add additional comments, please PM a Moderator.