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Discussion Starter · #1 ·
This spring I posted about a mistake I made: purchasing a Billy Goat Contour 24" high wheel mower.

To try to get it to work better I decided to increase blade tip speed. The listed speed is 12180 fpm with a 5.5" pulley above the blade. I changed to a 4" pulley and a belt 3" shorter, which should raise bts to 16748 fps, assuming no decrease in engine rpm. It cuts much better, but certainly not well enough to recommend it.

Should I go to a 3.5" pulley? How fast is safe? If there is no decrease in engine speed (not likely) bts would rise to 19140, but I would bet it would really be around 18000. :confused:
 

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What can it hurt to try? Most commercial mowers are 18-18500, 18500 being the maximum allowed for factory specs but alot of guys turn up RPM or change pulleys, so they would be higher than 18500. I say do it and if you don't like it take it back off right?
You HAVE checked deck level right?
 

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Discussion Starter · #4 ·
I installed the 3.5" pulley, and the cut with the regular blade is good. I can finally use second gear.

The mulching blade still leaves an unacceptable cut. The deck is probably too shallow and the mulch blade ends up pushing the grass down.

I tried a 52" belt, but it is too short. With the 3.5" pulley the 53" belt is too long. It works, but the deflection due to the tensioner pulley is too much: belt wear will be affected. I can't believe the difference between a 52" and 53" belt can be so much. I compared and installed each numerous times. Off the mower you can barely tell the difference, but installed it is amazing. I have looked for a 52.5" by 5/8" belt on the web, but no luck so far.

If you are stuck with a BG Contour, do yourself a favor and change out the blade pulley with a 3.5". It makes a big difference.
 

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Originally posted by FrankenScagMachines
What can it hurt to try? Most commercial mowers are 18-18500, 18500 being the maximum allowed for factory specs but alot of guys turn up RPM or change pulleys, so they would be higher than 18500. I say do it and if you don't like it take it back off right?
You HAVE checked deck level right?
Sorry, I think you're a bit off base. Most mowers are under 18500. Those that are around 18500 are bigger 60/72" machines, and guy's aren't increasing them further.
The main machine to speed up is the 48" WB. Furthermore, since I've been playing with speed ups on different machines, most of the WB's don't have enough horsepower to pull doubles and higher tip speed. And, guy's that "do" speed up their machines, without enough Horsepower, end up "below" the stock speed, as soon as they start mowing anything but very thin, light grass.
In other words, unless you've got a lot of horsepower, "in operation", a speeded up machine will actually cut at a slower speed.
Put it this way, as soon as you engage the blades on a 17/48 WB with double highlifts, the rpm will slow down, but you even start cutting grass.
At the current time, the SWB (23/48) is the only WB that will turn around 19,000 tip speed with highlift doubles and hold it through most grass. Yes, it's been sped up, and I'll have full info tomorrow.
 

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Pete, as far as I know all eXmark w/b's are 18000-18500 FPM stock. i know most don't have power to turn alot faster but a freind has a 17/48 Hustler w/b and changed pulleys to increase speed and he said a math wiz friend of his told him what the final speed was, it was close to 18500 FPM. It has plenty of power and does a fine job. I do think once you got much over 18500 or 19000 you would start having problems on most machines.

Just checked my 2003 eXmark catalog, all walk behinds (eXxcept Metro 21") have 18,500 blade tip speed eXcept the 36" Metro HP (belt drive floating deck). So I guess you're wrong, and the REAL commercial mowers do have at least 18,000 FPM BTS. I was right haha. check www.eXmark.com if you do not beleive me.
 

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Originally posted by FrankenScagMachines
Pete, as far as I know all eXmark w/b's are 18000-18500 FPM stock. i know most don't have power to turn alot faster but a freind has a 17/48 Hustler w/b and changed pulleys to increase speed and he said a math wiz friend of his told him what the final speed was, it was close to 18500 FPM. It has plenty of power and does a fine job. I do think once you got much over 18500 or 19000 you would start having problems on most machines.

Just checked my 2003 eXmark catalog, all walk behinds (eXxcept Metro 21") have 18,500 blade tip speed eXcept the 36" Metro HP (belt drive floating deck). So I guess you're wrong, and the REAL commercial mowers do have at least 18,000 FPM BTS. I was right haha. check www.eXmark.com if you do not beleive me.
OOPs, I forgot I'm trying to have a conversation with a Kid.....OK, so whatever you say is fine.... I'm wrong............
 

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Power has nothing to do with BTS. You can take a 21" walkbehind (push) mower and easily crank it well beyond the imaginary 19,000 FPM BTS. 60" Dixie Choppers come from the factory exceeding 20,000 FPM BTS. Figure it out for yourself.

Dixie Chopper recommends 3750 engine RPM
20.5" blade
1 to 1 pulley ratio
 

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Originally posted by Richard Martin
Power has nothing to do with BTS. You can take a 21" walkbehind (push) mower and easily crank it well beyond the imaginary 19,000 FPM BTS. 60" Dixie Choppers come from the factory exceeding 20,000 FPM BTS. Figure it out for yourself.

Dixie Chopper recommends 3750 engine RPM
20.5" blade
1 to 1 pulley ratio
OK... so try to even spin those blades with 12 horsepower at 3750 rpm (unloaded). It cannot be done.............Not enough horsepower.
Are you kidding????
So what are you trying to say??? Power has everything to do with blade speed. Sure you can spin a 48 WB with a 12 horse motor at 20,000 tip speed, but as soon as you put in into heavy grass it will practically stall. So, to cut grass, you need power. Please explain what you are attempting to say.............:confused: :confused:
 

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You sound like you know prety much what yer doin but I'd think twice about altering the manufacturer's specs (especially for liability reasons if you are going to use it on other's property). I leave research and development up to the engineers. If it doesn't do what you need, you bought the wrong product. But your post is interesting and I hope you accomplish your goals.
 

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Many Exmark Z's are governed to 3800 RPM with a 20.5" blade. Are drive (engine) and all deck pulleys the same diameter? I've never measured mine. They must change something, as the 72"ers would be flying!
 

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Originally posted by TLS
Many Exmark Z's are governed to 3800 RPM with a 20.5" blade. Are drive (engine) and all deck pulleys the same diameter? I've never measured mine. They must change something, as the 72"ers would be flying!
I don't know that much about Exmark, but Kaw motors come from the factory at 3,600, and you really have "push" to get the motors to run at 3,800. With everyone worrying about engine warrentte, it would be interesting to find out if Kaw will warrentte their motors running "above" the standard factory setting.
Also, even with 25/27 horse on a 72 machine, I'd love to have someone tach one with a set of doubles on it, to see just how much they slow down from the drag. In other words, if a 25/72 machine with doubles will hold close to 3,800, I'll eat my hat.
At the moment, the only machine I notice able to "basically" hold its rpm with highlift doubles is my 23/48 WB. Even my 25/54 slows down with highlift doubles. I'm sure other 48 WB machines with over 20 hp will hold also, but a 13/14/15/17 hp 48 machine will not hold rpm and naturally neither will a 17 hp 52 WB either.
Please keep in mind were talking double highlifts and not stock or low lifts..........
 

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Pete's wright!
I can take a diesel and stall it it in tall wet grass with double high's.
Pete
What it the bts on a 48/23 SWB out of the create.
Thanks john

Little one
I think what pete is trying to say thats its the HP that KEEPS the bts UP to where its when the blades are engaged.
John
 

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I run doubles on my 72" Chopper. My top set of blades are 48" Gators. I get the added benefit of another set of blades but I don't get the drain on the engine. The 25 Kohler probably would bog with 2 full length 72's on it but without the gators even the long blades miss some bahai at times.

Some of you Lazer operators ought to give this a try and see if your cuts affected.
 

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Pete,
I'm sorry, I gave you....
Wrong info.....

The 26 and 28 EFI, and the 27 LC Kawi, come set at 3750 RPM's, while the 27hp Diesel, and the 31hp LC Gas all come at 3800 RPM.

This comes from their 2002 Bid Specifications sheet. It has a lot of nifty specs that aren't in the brochures.

I don't know about the other engines, as this only lists the EFI, LC Kawi, and the Daihatsu's.



However, MY LC Kawi Came set at over 3800 RPM! I backed it down to 3750 the first week, and am now back up to 3810 free load.
 

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Originally posted by TLS
Pete,
I'm sorry, I gave you....
Wrong info.....

The 26 and 28 EFI, and the 27 LC Kawi, come set at 3750 RPM's, while the 27hp Diesel, and the 31hp LC Gas all come at 3800 RPM.

This comes from their 2002 Bid Specifications sheet. It has a lot of nifty specs that aren't in the brochures.

I don't know about the other engines, as this only lists the EFI, LC Kawi, and the Daihatsu's.

However, MY LC Kawi Came set at over 3800 RPM! I backed it down to 3750 the first week, and am now back up to 3810 free load.
Well, I certainly don't know everything:p . I like that 31 hp on a 72" machine. Sounds really great. Interesting to hear that Kaw seems to be increasing the rpm from the factory. It may be they are starting to understand what we need;)
One thing that's important is "not all highlift blades are the same".
 

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Discussion Starter · #20 ·
greasy_gun & ducky1,
I calculated BTS as follows:

Get the size of the pulley off of the crankshaft or drive. Calculate its circumference in feet (ex: C= pi x dia; 4" pulley = 1.05').
Get size of blade pulley and repeat (ex: 5.5" pulley = 1.44').
Get circumference of blade (ex: tip-to-tip =24" = 6.28').
Find real working engine RPM (ex: 2665 rpm).
Blade tip speed (BTS) = drive pulley circ / blade pulley circ X RPM X blade circ (ex: 1.05' / 1.44' X 2665 X 6.28 = 12178 fpm).
(I have rounded these numbers, so doing the math on them will give a slightly different result).
If the drive pulley size increases, so does BTS.
If the blade pulley size decreases, so does BTS.
I made a simple spreadsheet in Excel to test out what would work best. It will take you a few minutes using the info above.
Keep in mind that RPM probably drops with higher BTS, and changes when driving the wheels and cutting thick grass, so any BTS would need a real working RPM measurement.
It is also probable that as pulley sizes decrease you will see an increase in slippage. Higher BTS might cause enough additional drag to cause slippage also. So accurate BTS would require some kind of sensor on the blades, and not rely on math alone.
 
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