OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers Scott Bonnar Model 45 rear drum assembly

hi all ive taken apart a rear drum assembly for a scott bonnar 45/20" and the offside drum has a weight that is held in by 2 bolts im wondering if someone knows why im guessing its to counter weight the drive regards steve...

When you design a roller-type mower, you should end up with even pressure across the width of the 2-part roller, despite the fact that the engine is usually offset toward one side of the mower. To get even pressure, you need a corresponding weight on the opposite side. For safe, convenient handling - especially on cross-slopes - you want the mower's center of gravity as low as possible, which means just bolting a flat slab of cast iron under the platform opposite the engine would not be the best possible solution, though it would be fairly good. I guess one fairly neat solution would be to have a cast iron weight inside the roller-half opposite the engine-side of the mower. Because the mowers come in multiple widths, you could vary where you put the weight on the axle (inside the roller), to get the weight distribution you need.

Having said all that, I don't know the designer and I'm guessing what he/she was up to.

thanks for your reply grumpy u say to get even pressure a corresponding weight on the opposite side of the engine is needed im a bit confuzed as the weight in the pic fits inside the drum in my case the offside or the side below the engine kind regards steve...

I think a person with a Scott Bonnar, or Bruce, had better answer this one, since they can tell us whether their weight is in the same place as yours. My initial suspicion would be that yours has been taken apart previously and put back together backwards, but I have never worked on an SB so I'm guessing.

Hi Steve and grumpy, I just checked the Illustrated Parts List for the 20" Model 45, and part number 47, "Drum Weight" and part number 48 "Drum weight screws (2 off) are clearly shown as being located on the engine side of the machine....So Steve's machine is assembled correctly.
Just a thought, the reason may be to offset the weight of the third segment used on the 20" machine whereas the 17" Model 45 has only 2 segments in the rear roller.
The plot thickens!!

Thanks Deejay, I guess we are back to square one on what the purpose of the weight is.

G daY! all further more to the weight in drum debate 2 of my 3 s/b 45's have 4 holes in each drum, 2 small and 2 bigger holes. The other one only has 2 holes the bigger size. My first thought was its something to do with the manufacturing process! the first scott bonnar i bought the older style chassis the rear drums have 2 holes in each drum or did the weight only go into the 20"model 45-0431 i think not. Attached are 2 pics of the before said model regards steve...

Hi Steve and grumpy,
I have just spoken to the gentleman in Mildura, who worked for Scott Bonnar in Adelaide and later was a SB dealer/repairer, about the drum weight and the drum hole debate.

He stated that the "drum weight" was placed in it's position (offside) opposite the "driven" roller (nearside) to assist in the turning of the machine whilst in motion. He said that most machines that have a "split rear roller" have a counter-weight for turning purposes.

Re the holes, he stated (as Steve correctly thought) that they were part of the manufacturing process when mould casting the roller segment.
This guy has a wealth of information, and I thank him for his assistance helping with this issue.

great info deejay your friend in mildura would be interesting to talk to about the in's and out's of the finer details relating to scott bonnar ltd thanks for your help again cheers steve...

I was about to ask all of the above q's as I am in the middle of my resto process (SB45 20inch). My weight is on the right side and both drums (2 piece split roller) has the 4 casting holes, two big and two small.

I not sure if anyone these days anywhere would be able to find out the above information or similar except for this forum... I am amazed with the information this forum generates.

Hi Aussie, yes mate, we are indeed lucky to have some gentlemen still around that worked on or for Scott Bonnar , who can share some good technical info for our members here. Long may they live.

Since this post is over 12 months old, I will now close it.