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TLS
12-06-2001, 10:26 AM
Hey Exmark guys!
Check out http://www.exmark.com then click on any of the Ultra-Cut deck links. It lists a new blade that I think is going to be a great success. It is called the Excalibur blade. Full length sharpened, and SOLID lift foil. This just may be the same as the old pre 1997 60" Dixie Chopper .203" thick high lift! Thin, full length sharpened, and a good high lift solid foil.

This will save me the time of grinding down the edge of my old OEM blades to go full length! And with a thin .203 thickness will not drag any horsepower and cut like a hot knife through butter in the thick stuff.

Cant wait till Spring!

Kent Lawns
12-06-2001, 03:31 PM
Not to teach my geometery class here, but...

The sharpened the whole length thing is over rated.

Here's why:
The center of the blade moves at 0mph.
The tip moves at 215mph.

Halfway down and towards the center, the cutting edge is moving at less than 100mph and not cutting jack diddly. PLUS, it's only a 12" diameter of each blade that we're talking about.

The blade thickness does not drag power as much as the air that each blade moves. (And when you're in grass, the cutting that each blade does.)

~This lesson however does not mean that the Excaliber isn't a great blade.

Richard Martin
12-06-2001, 03:44 PM
I have to agree, in part, with Kent Lawns. I've noticed week after week that only the first 2 or 3 inches of blade ever really needs sharpening. I've had the Dixie blades and I ended up only sharpening the first 5 inches of blade and letting the rest of it go.

TLS
12-06-2001, 03:48 PM
Kent,

I WAS a straight A student in Geometry. This is not geometry, its physics!

The center of the blade moves a lot more than O, but I get your idea! This is all the more reason to keep that area sharp. Its spinning slower and needs to be sharp to cut those stragglers. We all know that not every blade of grass gets cut the first time, thats a fact. If your theory were correct, all mower blades would only have a 1/4" of the tip sharpened. This would work up to a certain speed of the mower travelling forward. Any faster, and it would be skipping a lot of grass. This is why a Scythe is fully sharpened. Think of that and apply it to a mower blade. Its how much it can "bite" off each rotation.

PLUS, it's only a 12" diameter of each blade that we're talking about.

I dont know what you are getting at here, so I'll leave that alone.

As for the drag theory, I proved you wrong around 1998 when I noticed that the replacement OEM Dixie Chopper blades were thicker .203 vs .250. It lowered my peak RPM's and slowed them down more in thick grass. Its not a weight thing, its a frontal area aerodynamics thing.

Richard,

Blade dullness is partially due to foriegn objects being hit. They will usually be hit right away. However, I have had real bad gouges in the middle of a blade as well! I will agree that probably close to 80% of the cutting is done in the first 6", but that extra 20% is sure nice to have, and IS noticable!

Kent Lawns
12-06-2001, 03:59 PM
Thank-you for agreeing on the aerodynamics part anyway.

If you're counting on the middle of the blade to cut the "stragglers" what is going to cut the stanglers at the mid-point between the spindles?

scott's turf
12-06-2001, 04:01 PM
What was your grade in physics?

scott's turf
12-06-2001, 04:04 PM
Kent, that is true. Blades only overlap by about a .5" or so. This is because the tip if the blade cuts all the grass. It is just another gimmick.

odin
12-06-2001, 04:17 PM
First 2 inches does just about all the cutting

Ricky
12-06-2001, 04:52 PM
If the front of the deck is pitched down, and the tip the blade cuts the grass, as the mower moves forward wouldn't the blade be turning above the cut grass the further back it goes? Just wondering myself.

mdb landscaping
12-06-2001, 05:12 PM
i see they offer a striping roller now for the back of the deck.

Runner
12-06-2001, 07:04 PM
Originally posted by Ricky
If the front of the deck is pitched down, and the tip the blade cuts the grass, as the mower moves forward wouldn't the blade be turning above the cut grass the further back it goes? Just wondering myself.
Yes, the further back on the bkade, the higher the cut, but this is so minute, there is very little variation. No gras is cut at EXACTLY the same height, there are always little variables. Some are a bit visible (if you look real close), some are not. This is why it is recommended that there be no more than 1/4" pitch difference between the front tip height, and the back tiop height. It IS important, also, that the front be lower, otherwise the gass just get blown and layed down more, resulting in a shabby cut.

edward hedrick
12-06-2001, 08:54 PM
Willhave to try these blades on my Scag. I modified the deck.
Blades need to be sharpened in farther. Ed

TLS
12-06-2001, 09:28 PM
Scott,

Not that it really matters, but I got pretty respectable grades in physics as well.

Edward,

I would check your sizes the only Exmark blades that are interchangeable with SCAG are the 52" ones, and maybe the 72's.

The full length sharpening really comes into play with higher mowing speeds. Also, a fully sharpened blade is more aerodynamic than a blunt square edge. It just slices through the air. Runner said it right with the deck pitch. Keep it at 1/4". From tip to center hole your only 1/8" higher, thats nothing. Any more than 1/4" you will see a washboard effect from the fronts digging in deeper.

mdb,
I have that roller. It sure is a great addition. Anybody that wants feedback, just search YORK. There are other benefits to the roller other than just striping.

Kent Lawns
12-06-2001, 10:01 PM
Originally posted by Kent Lawns

If you're counting on the middle of the blade to cut the "stragglers" what is going to cut the stanglers at the mid-point between the spindles? [/B]

TLS
12-06-2001, 10:11 PM
Kent,

I dont really understand your question? Just reading it, I would have to say the blade!

Kent Lawns
12-06-2001, 10:24 PM
Half way between each blade spindle (where the blades overlap) and for a few inches to each side you have only blade tips cutting.

How are the "scragglers" cut in those areas?

TLS
12-06-2001, 10:40 PM
HMMM... :rolleyes: I just dont know how "scragglers" can get by here. Its the peak cutting performance area of a deck, perpendicular to mower travel, and being hit in both directions.

Kent,
Do you really want to know the answers to these questions, or are you saying these things just for spite? :confused:

Kent Lawns
12-06-2001, 10:46 PM
Certainly no spite.

I'm just trying to figure out how the sharped center section is going to help to cut the scragglers when it only covers a small portion of the deck.

The overlap area between the blades is small: 1/2" or so. One blade is going backwards to the cutting direction, so it is certainly not the peak cutting performance.

And yet, with just the tips cutting, (no sharped center) the grass seems to be cut cleanly....

TLS
12-06-2001, 11:00 PM
Kent,

I'm scratching my head as well. :confused: All I know is that it makes a world of difference. Most OEM blades have approx 4" of sharpened edge, I take mine out to approx 9" and see a noticibally better cut and more RPM's. I actually noticed this on my old DC. Got a "cheapie" set of blades once through mail order and they had a tiny edge (4" or so). I was in a rush and just decided to try them. I was very disappointed in the cut and performance. They were later fully sharpened and I was then happy with them. I wouldn't be spending the time to lengthen the edge if it didn't help. Now, they will come that way from Exmark.

;)

MIDSOUTH
12-08-2001, 09:23 AM
I've been using these for about 5 months now, my dealer has been calling them bahia blades. They make a big difference in tough grasses and weeds. They are avalable now.

deason
12-08-2001, 03:23 PM
Let me be the first to say that I am not a physics major. High school grad ( mid class) and 1 year college flunkee.

Now, having said that let me interject some common sense here.

All this talk about blade RPM and how much of the blade is actually doing the cutting is fine. But, is the mower sitting still or moving? Thats right its moving. So who is to say that if more blade that is available for cutting, it won't cut more?

Think about this -
Any one ever seen a fan? A fan blade is pitched from spindle to end. Now think if only the end of the blade was pitched - would it move as much air? No.

I know we are not talking about air movement as pertaining to this thread. But, the more cutting surface a blade has logically should make a difference on the quanity of grass cut while the mower itself is moving.

not an answer - just a thought

point
12-09-2001, 02:58 AM
Physics lessons,

Kent Lawns - you are quite right, the rotational speed of the blade at the center is zero.
if one side of the blade is coming at you at 200 mph then the other side is going
away from you at 200 mph at some point in-between the speed must be zero.
that's rotational speed if the mower is moving at 8 mph the so is the center ( probably won't cut much at 8 mph )


TLS - a sharp front edge does nothing for reducing aerodynamic drag.
To cut drag you would be better off sharpening the back of the blade.
The least aerodynamic drag shape there is, is an air foil which is round in front tapering to
a sharp point in the back ( like an elongated tear drop )
aircraft designers will do anything to reduce drag yet look at all planes the leading edges
are rounded not sharp in the front such as the wings, tail and nose but the trailing edges are all
very sharp. Look at the back of a business jet you will find nothing thicker than 1/8 inch.
At close to and above supersonic speeds you do need a point in front but that is a whole different discussion.

most of the drag comes from the lift of the blade it is doing work that takes energy, although the thicker
blade .25 vs. .20 will have more drag it is nothing compared to the lift drag.
I do not believe there is much if any cutting past the lift area because there is no lift, in fact I'll bet there is a
downflow of air inboard of the lift area, if your sucking air up in the lift area it has to flow back down somewhere.

You stated you have seen nicks towards the center.
when you hit a rock with a part of the blade going only 50 mph it will nick it, other blades under the deck
can throw rocks into the center of the nicked blade and nick it even if the blade wasn't turning at all.

Great site I have learned a lot.
mark

HOMER
12-09-2001, 06:49 AM
One thing I've failed to see anybody mention is that a longer edge will help "re-cut" the grass that has been lifted up into the cutting chamber therefore providing more/better mulching capabilities.

The foil is on the end of the blade to create lift...........it's not too long, probably as long as the actual cutting area on the other side of the blade. The rest of the sharpened area is hitting that lifted grass many times after it has been cut initially.

Me ain't no rockit siuntist neider.