Electric Racing Chainsaw Build, Help Please

[stan.distortion]

Nope, DC. Starter motors are series wound, the more it digs in, the more torque it puts out. Over reving will be something to watch out for, series wound will rev it's self to death if you give it enough volts. If it wasn't for that and the sheer kick on strtup then all you would need is a relay to make it go (hence the geared starter suggestion to reverse torque reaction). Circuits aren't my strong point but a simple SRC circuit might be all you need for soft start and rev limiting, for sure a DC setup would be a fraction of the cost to same output with brushless.

 

[flat tire]

Starter motors are garbage compared to a Zero Z-force or Emrax.

 

[speedmd]

Luke could shed some light on the zero motor - hop up potential. I think it's way more than needed for a comparable hot saw power plant. Its rotor certainly looks to be able to take 12000 -14000 rpms. Controlling that both electrically and physically may be a challenge. Tesla's S motors were reported to run comfortably to 18,000 rpm some years ago after some factory software mods. Model 3 motors are reported to rev even better. A well designed rotor should not have much of a issue with these RPM's. Not in ten second duty cycles.

A simple audio analysis app for your smart phone can pick out the fundamental exhaust notes from the saw while it cuts. No need for a fancy blue tooth device. I have used spectriod and audizr with some good results before but at lower rpms. Should at least establish a loaded rpm target to try to beat.

 

[John in CR]

A DC motor candidate might be a motor for driving the landing gear for aircraft. Reliable and well built, but designed for high power intermittent use as well as light weight as opposed to an industrial motor. I have one that probably is too small for the job, ie a 4hp rating, and it does something like 11krpm on 12V and 21rpm on 24V. It weighs about 10lbs.

With such short run times, there's just no way a gasser could come to what's possible with the right electric motor of the same weight If the battery must be carried too, then the advantage could go to the gasser. The problem will be finding the right motor.

The OP gave us chain speeds, but we need to know the rpm required, so what is the tooth count of the sprocket and the pitch of the chain?

 

[Hillhater]

.....
The OP gave us chain speeds, but we need to know the rpm required, so what is the tooth count of the sprocket and the pitch of the chain?

The OP also gave us this....

9000-11000 max rpm
290 feet per second chain speed
50-60 hp
torque ???

Chain - We are using a specialty chain made by stihl called 404 high tooth. It is a .404" pitch that has a higher tooth than regular 404. teeth are cutting more wood than the rakers can clear.

Sprocket - The sprockets we are running are 16-18 tooth.

As i said, if you intend to match the 2T saw performance, you will have to have at least the same chain speed and “power”. ..preferably using similar sized sprockets or you will be in unknown chain handling zones.
The indications suggest that higher chain speeds are giving an advantage, !
And we know there are cells available (Lonestar) that have been proven to deliver 20+kW/kg, for 20-30 secs. So feeding a suitable motor is not an issue.

 

[Ianhill]

Hp is torque × rpm
We got rpm 11000 max
And we have Hp of 60 so if we reverse the formula we should have around 40 Nm.

The issue is the gearing harder the ratio more losses expected and the 2 stroke will not be geared so hard as the zero motor from stock.

But like speedmd mention luke got one of those to high rpm so maybe a 144v chainsaw overvolting the motor will get the rotor speed fast enough and as a side product the extra omph would see the stated specs smashed if it holds together could see 70hp 10krpm + at the rotor and even more torque for the 12seconds needed it would destroy the petrol equivalent.

Be a bit like the VW IDR just enough to climb pikes peak 12 miles no more in this sence the saws the same the 12 second rule kind of makes all this come together it has to be done.

 

[Ianhill]

Here's a crazy thought a flywheel chainsaw could just be the ticket heavy mass accelerated to mental speed with a clutch or something even better and drop the load in the chain all at once the chain speed will proberly go super sonic, enough energy stored in the flywheel to get the run time.

They used to run buses like this in the 50's granted it was huge design but the saw wouldn't need to have the accelerating motor onboard.

 

[Punx0r]

Battery can be backpack mounted if needed so isn't a constraint or a technical challenge.

I think it's a given the system needs to match the two smoke saw's speeds. Electric motors can be over-revved. Iron losses will either become excessive or the rotor will fly apart, but this is drag racing so such things are par for the course.

Direct drive would make things so much easier.

Brushed would also be easier, as long as the brushes don't burn up in under 10 seconds.

John's aircraft motor sounds worth a try if one can be found 2-3x larger. A series wound motor is more traditional for 1/4 mile drag racing but I'm not sure if a starter motor will be up to the job construction wise? They're already designed to be cheap and operate in short bursts, so maybe not much meat left on the bone.

Got to figure on getting 5-10x the power out of a continuously rated motor for a ~10sec burst.

 

[Hillhater]

Got to figure on getting 5-10x the power out of a continuously rated motor for a ~10sec burst....

Sounds like an opportunity to push an 15kW Agni well past its limits !

 

[serious_sam]

For a while now I've thought that going electric could be a game changer for a few reasons.

Just wondering, how would an electric saw fit in with the current regulations? I'm assuming the current regs limit engine capacity and fuel type, along with many other things.

Would this electric saw be just for fun, or are you looking to start a new category?

I ask because I'm in a similar situation with superkart racing. I'd like to move away from 2-strokes and build an electric superkart, but there are no regs that would allow it to race, currently.

In my understanding, coming up with a competitive design is only half the problem. The other half is developing a solution that could be consistently built for (or by) other competitors, with commonly available parts, easily inspected against a new set of regs, that is competitive with the current running costs. Otherwise you'll be the only one in your new category.

I'll get shot for talking about an ICE on this forum, but anyway, WRT your 2-stroke saw, not sure on the regs (if there are limitations on specific engines or if it's just capacity based), but 250cc single cylinder superkarts are running in the 70+hp range (Gas Gas is leading here atm). The modern 250cc twins run well over 100hp. I run an "old" 2005 Honda RS125 engine and it's pushing close to 50hp at 13k. Dirt bike engines are good for broader power bands, but if you want outright power, you could look into road racing (superkart) engines.

WRT an electric chainsaw:
-An electric motor will be so much smoother than the 2-smoker, you could probably target a higher chain speed (higher rpm, faster cutting) with less chance of throwing chains.
-Targeting to run at higher rpm with electric is a win-win anyway. If you change your mind and decide to slow the motor down, it will produce more torque. Unlike a tuned 2-stroke which drops out of power band and loses torque if rpm gets too low.
-Suggest looking for a motor that can spin the target rpm without gearing up. Gearing means losses in efficiency. But more importantly, faster spinning motors make more power with less weight than slower torquier motors (speed~power).
-You don't actually need a "60hp" electric motor. Your requirements are unique: being very short duration. A much lower power electric motor can produce much higher power levels for short durations. It would depend on the motor, but you might be able to get a 15hp motor make 60hp for 6 seconds without cooking (just pulling random numbers, but you get what I mean). Petrol engines are rated for the maximum power they can output. A 60hp engine can only flow so much air and fuel. You need to modify something to flow more and make more power. But electric motors are (inconsistently) rated based on how much power they can output continuously without overheating. But they can easily put out much higher power for shorter durations just by pumping more current. No modification necessary. Similar situation with batteries and controllers to a certain extent. It's all about heat production/transfer and temperature limits.

 

[serious_sam]

Got to figure on getting 5-10x the power out of a continuously rated motor for a ~10sec burst.

Beat me to it!

 

[stan.distortion]

Battery can be backpack mounted if needed so isn't a constraint or a technical challenge.

I think it's a given the system needs to match the two smoke saw's speeds. Electric motors can be over-revved. Iron losses will either become excessive or the rotor will fly apart, but this is drag racing so such things are par for the course.

Direct drive would make things so much easier.

Brushed would also be easier, as long as the brushes don't burn up in under 10 seconds.

John's aircraft motor sounds worth a try if one can be found 2-3x larger. A series wound motor is more traditional for 1/4 mile drag racing but I'm not sure if a starter motor will be up to the job construction wise? They're already designed to be cheap and operate in short bursts, so maybe not much meat left on the bone.

Got to figure on getting 5-10x the power out of a continuously rated motor for a ~10sec burst.

I'm not sure on the starter motor either but it would be very easy to find out what kinds of current they'll take, stick on a clamp meter and try to turn the engine over in gear and I wouldn't be surprised if a large truck starter will draw over 500 amps. I'm very curious about the aircraft unit too, sounds an interesting unit

 

[DogDipstick]

I'm not sure on the starter motor either but it would be very easy to find out what kinds of current they'll take, stick on a clamp meter and try to turn the engine over in gear and I wouldn't be surprised if a large truck starter will draw over 500 amps. I'm very curious about the aircraft unit too, sounds an interesting unit

I got a typical starter dyno sheet right here. They are there when you purchase refabs. This is a Nissan 6Cyl Xterra.

I honestly think beating the energy and/or power density of a 2stroke 60 hp ice is gonna be tough. There is alotta btu in a tank of fuel,,,.. and .... you guys know the rest...

 

[John in CR]

290ft/sec of .404" pitch chain and a 17t sprocket is 290*12/.404/17*60 = a sprocket speed of 30,401rpm , so unfortunately direct drive is out unless you can run a drive sprocket more than double the size. You also have to consider that an electric motor's peak power point is when it is loaded down to where it is spinning at 50% of it's no-load speed at that voltage.

Also, direct comparison to a gasser's hp is very unfair to the gasser, because they might make close to that hp at some point during the cut it's unlikely that it's making anywhere close to it's max hp rating during much of the cut. The much wider power band of an electric enables you to put max hp right in the range you want it run, so an electric motor making 1/2 the rated max hp of the gasser is still probably plenty for the electric to win. I could be wrong about this part for the special case of competitive chain saws, since the motor rpm during a cut is more constant, unlike a car drag race.

 

[flat tire]

I could be wrong about this part

Yes, you are wrong. Competition gas saws are designed to put out max power during the entire run. That's sort of the point.

 

[speedmd]

Chain is .404" pitch with a drive link every other link. 2X pitch = 0.808" drive link separations to my understanding. 18 links for a 18 tooth is .808x 18 = 14.544" per rev x 12,000 rpm =174528 "/min or 242 ft/sec
10000 rpm = 202 ft/sec
16000 rpm = 323 ft/sec

 

[speedmd]

Competition gas saws are designed to put out max power during the entire run. That's sort of the point.

Big issue as I see it, is the constant desire for the chain to grab more than the power plant can torque through. Power by itself is not what keeps it from bogging down and keep it singing in the sweet spot. Operator has to constantly adjust and feather the contact force based on how the chain is hooking up. OP writer can certainly tell more. Having the broad even and greater torque - band of the magnetic drive would make this much less of a issue. Anyone having run through a mud bog on a 125 trying to stay with open class bike understands that peak power is a useless measure when it starts to bog a smidge.

 

[John in CR]

I could be wrong about this part

Yes, you are wrong. Competition gas saws are designed to put out max power during the entire run. That's sort of the point.

I watched some of the Hot Saw competitions and they look like garage built, with little change in sound from freewheeling chain to cutting. I don't don't those noisy things can make high power, it seems like they're probably way over-spec'd so as not to bog down under load. I'd be surprised if they get even half the rated max power of the motor to the wood. Essentially I think they're putting the biggest motor they can handle the weight of, because it's 3 cuts of a log...down, then up, and then down again. The upward cut is how they prevent someone from putting the biggest motor possible.

Speedmd,
From what I can find, chainsaw chain pitch and sprockets are like anything else and the pitch is the distance between drive links.

 

[John in CR]

An electric version would be interesting for sure. I see they start with the saw on the ground, so the unit would have to include the battery too as part of the saw weight.

Something that will be different is that the electric motor will have a lot more gyroscopic forces due to more spinning weight at a much bigger radius. The gyroscopic forces might even be a problem.

 

[speedmd]

hard to read but drive link spacing is twice saw chain pitch

 

[speedmd]

found a clear one

 

[Hillhater]

hard to read but drive link spacing is twice saw chain pitch

Yes SMD, you are generally correct, except the drive pitch is not quite 2x the actual chain link pitch since the “drive links” have different pin spacing (usually less) than the chain links and cutting links pitch.
The quoted “0.404” pitch is a theoretical dimension and is not the actual chain pin spacing on the links.
John needs to understand that chain saws do not drive on the chain roller pins, but on the “drive tangs” on the bottom of the chain.
John also need to understand that a 2 stroke can be tuned to make peak power at whatever point in the RPM range the tuner wants to set. ...so yes , those saw motors can be making 60bhp at 11,000rpm.
Also, electric motors do not have a constant torque output over their rev range.
For instance, the much lauded Tesla “S” motor runs up to 18,000 rpm,...but at that point it is only producing 20% of its peak torque , which starts to reduce above 6000rpm !

 

[Ianhill]

Looking at the chain it's optimised for a poor torque production rather than any specific rpm i think,
There's a cutting tooth , a spacer link then a cutting tooth on the opposing side, so the spoil has had 3 links to clear, so every forth link makes a cut on the same side so there's a break between cutting forces ideal for the 2 stroke deleivering torque in ripples but if those cutting faces are compressed there's then a constant cutting action on the chain, no problem for the electric motor alot less ripple in its torque production, I think these chains are developed for 2 stroke in mind and when applied to electric there's so much space in time between cuts the rpm drops very little until its really stressed, the motor is delivering current like a hammer I imagine if it was being measured its peaking as the chain grips and slips.

If the cutting faces could be compressed the benefit of a slower chain speed would have the same cut rate at half the speed but more torque right up electric street and then realizing the achievement of getting it to lack of a gearbox would just make it shine a real relentless cut but there needs some R+D on the chain cutting faces.

This is how the 36v saws can cut so well I think the way the torque is delivered rather than the amounts, I can only speak on the ones I've used and the 36v just plows on while a 2 stroke can be grabby but the chains they use are exact same.

 

[Hillhater]

Ian, ..your analysis of chain cutting action ignors the fact that there are multiple teeth (20 ?) engauged with the wood at any instant , so any load variations are small.
You might be surprised to know that most “professional” chainsaws have fewer cutting teeth than your average farm/garden variety,..(known a “skip tooth” chains ), with an extra drive link , or two, between cutting links, and used for their much faster cutting ability......at the cost of finish roughness, wear rate, and power needed.
So, the problem is still finding a motor with enough POWER at 10,000 rpm.

 

[Ianhill]

I think your right on the torque analysis I've neglected the fact there's lots of cutting faces but it doesn't stop the fact there's possibility of reducing the chain speed with added cutting faces.

Skip tooth chains are slower cuts better extraction of spoil, have a look at fire fighters saws chains they use carbide tips and have the extra link for increased cutting speed there's also anti kick back built into the extra link.

I bet any competition would spec a chain like racing says a tyre compound to stop someone coming along with a diamond/carbide and having an advantage.