Wider TC belt = Higher Top Speed?

[Bosch]

This sounds like a rookie question, but apparently a wider belt yields a higher top speed?

My Murray Explorer came with a Comet 30 TC (a clone actually, but you get my point..) It had a 1/2" wide belt labeled "Oregon 75-368". It served us well, but eventually needed replacement. So I got online and ordered this:

https://www.amazon.com/gp/product/B0018U47RI/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1

As you can see in the product description, it's 3/4" wide. Not 1/2" like the old one. It's definitely a taller final drive ratio now. I can feel it in the way it takes off, and my phone bicycle app clocks me at a blistering 38.5 mph top speed. With the old narrow belt, I regularly saw nothing faster 32 mph, and 33 mph (once).

So here's what I can't get my head wrapped around... If its climbing higher towards the edge of the driver clutch (because its wider) that will produce a taller ratio. I get that. But... back on the driven unit, it shouldn't be seating as close to the center at speed (again, because its a wider belt). Right?

Apparently it's simply gaining more on the driver than what it's losing on the driven, yes?

 

[Joe-405]

Won't work. It's false. The belt will be too wide and you won't have a neutral setting or it will burn the belt up.

 

[pRoFiT]

If you know you basic values you should be able to calculate max MPH. are you running about 20" tires? or high rpm engine 5000+ with say 14" tires?

Looking at the comet 30 i think i remember seeing a 0.9:1 ratio at high speed. So im guess a 6:1 ratio with the added tav2 gives you about 5.4:1 at MAX.

Is it possible the 1/2" was the wrong size to begin with and now you have the correct size belt?

I kind of agree with joe. A larger belt would cause wearing. Looks like it fits good on yours though?

 

[Bosch]

Is it possible the 1/2" was the wrong size to begin with and now you have the correct size belt?

That's what I'm wondering too. Someone else with a Murray needs to jump in here.

I drove it around and put about an hour's worth of heavy time on it, and nothing ever got hot or showed wear. So maybe the 1/2" was wrong. But is a farm kart really supposed to be pushing 40mph?

Honestly, that's a little too fast for my 11 and 13 yr old.

 

[pRoFiT]

Yah thinking governor is removed on the engine? Probably hitting 5k RPM. Then maybe 15" tires on back? could hit 40 maybe. With little kids, oh yah. With you maybe 38 Me maybe 32

 

[itsid]

if the belt doesn't come as a comet series 30 belts.. it's impossible it has the correct sidewall angles..

by luck you can install a v-belt that has the overall included angle about'ish correct (it'll NOT be perfect) and in that case..
a wider belt cannot tip as much as a narrower belt would.
which indeed might tilt too far... with the straight sheave being nearly vertical.


Anyways.. the beltwidth makes a difference in case you use the incorrect belt.
and a wider than std belt might make sense (provides a bit more contact--thus can provide more torque and as a result could shift a hair higher)

But frankly.. that's the same as saying a butterknife is better to remove screws than a scalpel blade...
it's both crap if you can instead use a proper screwdriver!

Buy a Comet TC specific belt and see how well IT does!

'sid

 

[Bosch]

Well if the Oregon 75-368 belt is wrong, it's a really good wrong! The last one lasted for the full 2 1/2 years I owned it plus whatever time the previous owner put on it.

I'm not questioning what the top speed is, that's a known value. I don't need to calculate that. I know for sure my phone app is accurate. My method of measuring it is consistent.

What I'm questioning is WHY? Any change in geometry (drive ratio) on the driver should be offset by a loss on the driven. But clearly that's not the case.

I'll keep using it till it's used up, but that'll be a while.

---------- Post added at 05:46 AM ---------- Previous post was at 05:43 AM ----------

a wider belt cannot tip as much as a narrower belt would.
which indeed might tilt too far... with the straight sheave being nearly vertical.

That's a possibility, I hadn't thought of that.

 

[KartFab]

What I'm questioning is WHY? Any change in geometry (drive ratio) on the driver should be offset by a loss on the driven. But clearly that's not the case.

We are missing important information. Assuming your app is accurate, you will need to provide the following info to see if, in fact, the belt produces a higher top speed than a genuine or knockoff 30 series belt.

1) RPMS - tachometer, those cheap ones on ebay work great
2) torque converter jackshaft tooth number
3) axle sprocket tooth number
4) tire size

If you can give me that info, ill let you know what the theoretical top speed is.

If you want to compare both belts, you would need to set up an experiment keeping RPMS equal at top speed using the two different belts.

So its obvious that the crappy belt you had sucks in comparison to the 3/4" width belt because 3/4" top width is what the belt is supposed to be at anyway, so that is kind of a mute point.

Also, the assumption that drive unit riding higher will make driven not ride as far in because of width is misinformed. Good idea though. Its called a DRIVEN unit for a reason. How far the belt rides in is determined by how far the DRIVE unit pushes the belt out and the center to center distance. Because the drive unit is pushing the belt out 'further' , the driven unit actually takes that belt in further to the center of the driven unit, which may actually cause it to rub on the crush rivets, ripping apart the belt slowly. This occurs when you have a belt with a circumference that is too small. IT can still ride high on drive, but the driven eats it up. The driven is forced open beyond its normal operating range because there is nothing stopping the wear tabs from going further up the cam slopes.

So my general thoughts here are, you had a really really really crappy belt on there to begin with, and putting a belt on there that is the right width (not necessarily angles, or size) makes a gigantic difference.

I think you should focus less on the difference between two wrong belts, and just put the right one in with the proper alignment and center to center distance. After all that, then you will get the performance that you don't even know about, because you have never had it.

 

[Joe-405]

I thought the pic was the old belt. By looking at the pic the one on it is the correct one. Somebody before you could have put the wrong one on it or put a narrower one on to slow it down for the kids or just installed it cause that's all they could find.

So I'm in agreeable terms with these guys.

 

[Bosch]

Those are good thoughts, KartFab. Thanks. If you want to go through the math, I'll post the requested data later in the week. I'm travelling too much for work at the moment.

 

[Bosch]

Ok, time to look at this again...

We are missing important information.

1) RPMS - tachometer, those cheap ones on ebay work great
2) torque converter jackshaft tooth number
3) axle sprocket tooth number
4) tire size

1: I don't know true rpm's, but it's a Predator 212 with the governor removed. So... 4,900 rpm? Maybe someone else here can firm up that guess.

2: Ten teeth.

3: 60 teeth (If I counted correctly).

4: Actual tire size is 17.5 inches.

 

[Joe-405]

Maybe 4900 with a extreme tail wind and downhill. If all you did is governor removal. And I'm assuming it was the correct way by taking the engine apart. No way its getting that much rpm's. And if you wanna slow it down go bigger on the rear or smaller on the front sprocket.

I just checked 4400 is 38 mph with that gearing. @ 6:1

 

[chancer]

What?
Any 212 with a removed Gov should be getting at least 5000 possibly 5500.
and
Geared 6:1... No problem seeing those RPMs.

 

[Joe-405]

What?
Any 212 with a removed Gov should be getting at least 5000 possibly 5500.
and
Geared 6:1... No problem seeing those RPMs.

I've never had or seen one with only a governor removed do over 4500. If it is then he's geared so far off it dragging the engine down really bad then. And still has alot more mph left !

 

[chancer]

Just playing with the calculator
I have 12" tires at 6:1 ratio. Briggs 206 OHv, with intake and exhaust. Gov. Removed
Maxes out at 32MPH that is 5400RPM

I do not have a tach but reverse calculating the speed, tire height and gear ratio, Gets me 5400.

 

[Bosch]

I did remove the internal parts as well.

I forgot to mention that I also installed a stage 1 kit on it, which includes the air intake, exhaust, and 8 degree timing advance. Or is it 7? Can't remember now, but anyway, Sorry. Forgot to mention that.

 

[Joe-405]

The timing is your extra rpms. Put a regular timing key in it and I guarantee it will slow it back down. I add 6* to my engine and gained 500+rpms by just doing that only and an extra 10 mph.

 

[Bosch]

So what's the theoretical top speed?

 

[Bosch]

So anyway, I stumbled across the speed calculator elsewhere on the site, and it says 38 mph is entirely possible at upper 4,000 rpm's. I didn't think belt width would do that, but the Murray definitely goes better with the wider belt.