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jonathanone
03-09-2011, 08:56 AM
The 42 has 2 blades spinning at about 18,800 the 48 has three blades spinning at about 16,700 both decks are the Bob-Cat 'Dura' deck would anyone venture a guess as to what differences might be observed??

GMLC
03-09-2011, 09:43 AM
Good question and I dont know the answer. I do know a 42" deck is not that common, they are out there but kinda an odd ball.

davidcalhoun
03-09-2011, 09:46 AM
I think you are referring to blade tip speed not RPM.

Blade tip speed is: engine speed X blade length X 0.262

The 42" deck uses 2 - 21" blades where the 48" deck uses 3 - 16" blades

dishboy
03-09-2011, 09:52 AM
The 42 has 2 blades spinning at about 18,800 the 48 has three blades spinning at about 16,700 both decks are the Bob-Cat 'Dura' deck would anyone venture a guess as to what differences might be observed??

If the blades are not timed the two blade design will mandate a longer wheelbase and different offset between wheels on corresponding passes. Otherwise, if the ground is not level you should get a flatter cut with the three blade design if the decks are level. IMO a TIMED two blade design will cut better than both mowers in question but I don't that is what you are referring to?

jonathanone
03-09-2011, 10:02 AM
yes, tip speed , from searching tip speed here the consensus is that faster is better, some say much better ???? But could the two blade deck also turn out to be a screamer ?

gm, we are thinking along the same lines-if it doesn't work out the deck size may bite us trying to get rid of it..-IF we could get rid of it. Probably would not be much interest for the pro market, unless we gave it away ditto the home owner market also.

This is one of those times that if we buy it we really would own it.

JimQ
03-09-2011, 10:12 AM
My experience tells me the 2 blade 42" should be a better all around deck than the 3 blade 48".

The first thing you'll notice when you engage the deck will be the 42" deck will probably be quieter. Despite the higher blade tip speed of the 42", the 48" blades are actual turning more RPM's than the 42".

3300 (approx) RPM - 42" Deck
3800 (approx) RPM - 48" Deck

48" 3 blade decks sound like a buzz saw. You'll probably feel more vibration out of the 48" than the 42" (more rotating components and at higher speeds) due to the higher blade RPM. The spindle bearings will probably last longer on the 42" than the 48" because of the lower rotational speed. that translates to fewer rotation of the bearing for every hour the machine is used. Not to mention lower bearing temps.

Everything else being equal, the 42" should cut better than the 48"

The blades on the 42" (21.5" approx) will be longer than the blades on the blades on the 48" (16.5" approx)

Generally speaking, longer blades cut better than shorter blades as ground speed increases.

I'm not going to to go into the math (unless it would help make the point) but...

The higher the ground speed, the more grass is cut per cutting cycle of the blade. (2 cutting cycles per revolution) Think forward movement here, not vertical height or grass density. The closer to the rotational center of the blade you get, the closer the cutting edge speed gets to zero.

The 42" deck also has one less blade overlap to leave stripes when processing large amounts of material.

The 42" will probably be cheaper to maintain. One less blade to buy, one less spindle to replace, etc.

Manufacturers like the 3 blade 48" because it shorter from front to back and a allows the machine to be shortened. The 42" will physically take up more space under the machine.

That's about all I can think of at the moment. Let me know if anything needs clarification.

Q

JimQ
03-09-2011, 10:37 AM
...IMO a TIMED two blade design will cut better than both mowers in question...

I agree with you. When the blade tips pass directly by each other it disturbs air flow and creates noise.

Not sure if you've ever noticed it but at times, you can actually hear the blades going in and out of phase. It's that "drone" you can hear withe blades enagaged but not cutting grass ... loud LOUDER loud LOUDER loud LOUDER loud..... Ha! :laugh: I'm sure you get what I mean.

That is another reason the 2 blade will probably be quieter than the 3 blade. One less set of blade tips passing each other and creating noise.

Q

kilgoja
03-09-2011, 01:15 PM
personally i've never noticed a difference in the cut between a 42", 48" or a 54"...only difference is the width lol...just keep the blades sharp and all is good :)

you could get a 54" as the blade tip speed is high as a 42" and it has 3 blades

jonathanone
03-09-2011, 05:11 PM
thanks all;

dish/Jim, I found more info stating that the rpm for both are the same, 3550, both .250 thick high lift offset; and Jim you were real close on the rpm and blade size, 2 blade deck uses 21", 3 blade uses 16" as David stated-does this indicate anything as to what might change?

JimQ
03-09-2011, 05:25 PM
thanks all;

dish/Jim, I found more info stating that the rpm for both are the same, 3550, both .250 thick high lift offset; and Jim you were real close on the rpm and blade size, 2 blade deck uses 21", 3 blade uses 16" as David stated-does this indicate anything as to what might change?

Don't confuse engine RPM and blade spindle RPM.

You can always change the clutch pulley and/or spindle pulley to put the blade RPM where you want it. It doesn't have to be a 1:1 drive ratio. It's generally not.

I would bet the blades are longer than 21" and 16". If they weren't, you'd have no overlap and probably have stripes between the blades. ....or maybe there is overlap and they are not true 42" and 48" decks.

Q

jonathanone
03-09-2011, 05:30 PM
Just as I finished posting the great people from Bob-Cat returned my call, they really are incredibly responsive-and here is the info I got;
the blades on the 42" are not timed, what they did was to stagger the blades one being further back than the other.

when asked about noise the answer was that this was a tough one but he thought levels, pitch would be about the same, but did qualify it as this was his opinion.

When I asked if a staggered blade design would be inherently less of a load-one blade would enter the grass slightly before the other, on the engine the answer was in theory yes, but in practice ??

Thoughts.

davidcalhoun
03-09-2011, 05:36 PM
Not sure what your last question means in post #9. Does it mean you still don't why the 21" blade is rated faster?

With the same RPM, the longer 21" blade is moving faster than the 16" blade.

Here is the formula again: RPM speed (spindle) X Blade Length X 0.262

Blade thickness has nothing to do with it.

nitro121
03-09-2011, 06:27 PM
I use two small ztr hustlers. I do all residentials mainly...and the 42's are great cuts. Plus they barley fit through the standard gate these days. A 48" wouldn't get in or I'd have to remove a whole 6 foot section. Plus I can get down in ditches where a larger deck would hang or scalp. And I can still knock out my 3 acres in no time.

So anyway....42's are great cut.

Peace

jonathanone
03-10-2011, 07:27 AM
Dave, I wanted to point out that the rpm for both decks, according to BC, were the same, wasn't certain if the thickness (mass/weight) figured into the equation at all.
Does anyone know how unusual an offset deck design is and why either a timed or offset design would be selected-simply the whim of the engineer?

dishboy
03-10-2011, 08:02 AM
Dave, I wanted to point out that the rpm for both decks, according to BC, were the same, wasn't certain if the thickness (mass/weight) figured into the equation at all.
Does anyone know how unusual an offset deck design is and why either a timed or offset design would be selected-simply the whim of the engineer?


Most commercial mowers use offset designs because they are cheaper to manufacture, the sum of parts required are cheaper. You can time decks two ways, one is with gearboxes which is expensive, and with cogs and cogged belts. The problem with the belt method is if you hit a object belts and coggs break and it costs $ to repair. This can be a issue with lazy or careless operators.

Landrus2
03-10-2011, 08:29 AM
Every mower has its place. 21-36-42-48-52-60 just to name a few.
All these mowers if they are fine tuned they all give you good, results
Size of property is how you should look at these mowers.

davidcalhoun
03-10-2011, 10:27 AM
On a two blade off-set deck, as dishboy has pointed out, the mower deck will be longer to allow the off-set blades to have an overlap. This overlap is necessary to allow the blades not to hit each other but still cut the grass completely between the two blades. The increase in overlap (to a point) will help the mower to cut grass without leaving a uncut stripe in the middle of the two blades. This overlap comes more into play when you are cutting on curve or turning. Ground speed is also a factor.