Yet Another Chainsaw Dyno

[.404]

The question is, "Where is the rest of the CR 250?" Hahahahahahaha.

 

[dieseldave]

That was all that was left of it after I lent it to my brother in law... lol I have a (relatively) intact CR250 that I bought from a guy who sticks in my memory because he had a Husky 55- powered bicycle in his shop. It was a bit, ummm, unrefined but it obviously worked and I thought it was pretty cool. Should have taken a picture of it

 

[blsnelling]

Very cool Dave!

 

[wyk]

At first glance I thought the same about brakes heating up but after that running it you obviously didn't. One problem you run into that will give inconsistent results is the pads/rotor might glaze over

That bike brake can take far, far more energy than that chainsaw could every hope to generate.

 

[fordf150]

yeah after i thought about it i changed my mind and understanding how it works now i dont think the brake glazing will be a problem either. Just a case of me speaking without a full understanding of how this was going to work

 

[maico490]

Excellent work dieseldave. Finally people are coming around to the fact that power is torque multiplied by rpm and you don't need any fancy calibration. There are some minimal losses in the hub bearings but the drive chain doesn't matter as a chainsaw is driving a chain anyway. Don't know if you follow supercross but Jean Michel Bayle used to drag the back brake pretty much the whole lap to keep the rear end stable. I've also seen Dave Thorpe (ex World Champ) race locally and his back brake was a permanent blue colour. One of my friends went on a motocross school he was running and he recommended both power and rear brake for rough down hills. A brake that will stall a 50Hp mx bike isn't going to be much troubled by a chainsaw.

 

[nk14zp]

I like how you let the saw idle at the end.
It bugs me to see a saw shot off right after a cut/load.

 

[wyk]

that's a clever set up the problem you might have is as the pads build heat you will need to change your load on the brake leaver to keep the same load on the engine but nice job and looks like a nice set up to break in a fresh engine and set up the carb without cutting useless cookies of wood...

I will be stunned if the chainsaw can get those brakes even warmed up compared to the abuse they were originally designed for. 3-9 HP VS brakes designed to stop 100 MPH and 400+ lbs(bike plus rider)? These are machines that stop when there's a little bit of a pinch in wood, let alone from metallic brake pads.

 

[B-N]

Very cool set-up!

Even if you aren't able to compensate for the parasitic loses precisely you still have an excellent tool for saw comparison.

I'm looking at your list of saws and can't wait to see what the results look like for the rest of your wood shed!

 

[dieseldave]

Excellent work dieseldave. Finally people are coming around to the fact that power is torque multiplied by rpm and you don't need any fancy calibration. There are some minimal losses in the hub bearings but the drive chain doesn't matter as a chainsaw is driving a chain anyway. Don't know if you follow supercross but Jean Michel Bayle used to drag the back brake pretty much the whole lap to keep the rear end stable. I've also seen Dave Thorpe (ex World Champ) race locally and his back brake was a permanent blue colour. One of my friends went on a motocross school he was running and he recommended both power and rear brake for rough down hills. A brake that will stall a 50Hp mx bike isn't going to be much troubled by a chainsaw.

I like your handle! Do you have one, if so what year? I have an '81 495 KTM that I've had for 30 years now, with a wide and interesting assortment of bikes in between. I was aware of JMB's reputation as a brake dragger, but didn't realize that a lot of other top riders do it too. I did some brake dragging on my '90 CR250 along with some temp measurements and that convinced me that the dyno was a workable idea. I suppose that if you put a strong saw on there and ran it at full load until it ran out of gas it might overheat something, but I have no reason to do that. One thing that does concern me is spinning that hub at up to 4000 rpm. I did some rough calculations and it wouldn't ever reach 1000 rpm on a stock bike, maybe 1100-1200 on a geared up desert sled, so it isn't designed to turn that fast. The bearings don't worry me much, but I wonder about the structural integrity of the hub itself. Maybe I should put a guard over it in case it decides to explode like an old fuel car flywheel...

 

[dieseldave]

I like how you let the saw idle at the end.
It bugs me to see a saw shot off right after a cut/load.

I always like to give a piece of equipment a chance to pump some air through at no load to cool down. I even do it with my 4" grinder lol

B-N, I am interested to see what some of those saws do, too. I also plan to do before/after porting comparisons, etc. The problem right now is data acquisition- with both the scale and the tach being digital, it is nearly impossible to get an accurate reading with the #'s bouncing all over the place. An analog tach and scale would help a lot, but the real answer is a load cell to plug into the laptop

 

[B-N]

Could you put the scale readout beside the tachometer and take a video of it?

You could replay the video/slow motion/pause it to get a good approximation of what the readouts are. Just a short term bandaid until you refine the measurement system. You're right, analog gauges would be better, digital data acquisition would be boss.

 

[Mastermind]

What would you charge to build me one of those Dave?

 

[dieseldave]

Yes

 

[dieseldave]

Could you put the scale readout beside the tachometer and take a video of it?

You could replay the video/slow motion/pause it to get a good approximation of what the readouts are. Just a short term bandaid until you refine the measurement system. You're right, analog gauges would be better, digital data acquisition would be boss.

I might do that as a stopgap measure. I'd have to have an oilproof camera, though lol After seeing how well this thing works the lure of digital data acquisition is strong, and I've pretty much made up my mind that is what's going to happen as long as the cost isn't too outrageous

 

[Mastermind]

Perfect.

That's a very good price on some spinny stuff.

YES

 

[dieseldave]

Perfect.

That's a very good price on some spinny stuff.

YES

I find it's always good to be positive with a prospective customer and never commit to anything... Just remember to be positive when the costs skyrocket like a government contract- you know, the ones where they agree to buy something for $73,ooo apiece and they end up paying $1.5M

 

[Mastermind]

Like the $640 toilet seats, $7,600 coffee makers, and the $436 hammers?

 

[dieseldave]

Like the $640 toilet seats, $7,600 coffee makers, and the $436 hammers?

Right! And the really amazing thing about those were that they were all the low bid!

 

[maico490]

I like your handle! Do you have one, if so what year? I have an '81 495 KTM that I've had for 30 years now, with a wide and interesting assortment of bikes in between. I was aware of JMB's reputation as a brake dragger, but didn't realize that a lot of other top riders do it too. I did some brake dragging on my '90 CR250 along with some temp measurements and that convinced me that the dyno was a workable idea. I suppose that if you put a strong saw on there and ran it at full load until it ran out of gas it might overheat something, but I have no reason to do that. One thing that does concern me is spinning that hub at up to 4000 rpm. I did some rough calculations and it wouldn't ever reach 1000 rpm on a stock bike, maybe 1100-1200 on a geared up desert sled, so it isn't designed to turn that fast. The bearings don't worry me much, but I wonder about the structural integrity of the hub itself. Maybe I should put a guard over it in case it decides to explode like an old fuel car flywheel...

I had a '82 490 but sold it about 10yrs back to buy a modern bike. Still kicking myself now. A 495 KTM was one of the few bikes the Maico wouldn't outrun to the first corner but was a lot easier to ride for the rest of the race. As Dirt Bike said back in the day "the 495 has more power than anything else on the market. Sometimes it has more power than two of anything else!"
I wouldn't think the hub should be a problem as it has no spoke tension on it or any of the forces from a spinning rim. However the forces go up as the square of the speed so a guard would possibly be a good idea. Also if the chain did break they tend to go forwards where you could be taking measurements.
As you say electronic gathering of the data is the way forward but none of that stuff is on my list of specialist subjects.