Another DH Project

[motomoto]

Well, I stayed at the shop until 11:30 last night to get something done. I made the
main crank sprocket but realized that it is probably going to be too weak. It is going
to take up to 5 hp at times and it will only be spinning somewhere below 90 rpm.
that is a lot of torque in my book. I think I will compute that out and see how much
force that creates. Anyways, I took some pictures.

View attachment 2

 

[recumpence]

Very cool.

You are my hero.

Matt

 

[Miles]

It is going to take up to 5 hp at times and it will only be spinning somewhere below 90 rpm.
that is a lot of torque in my book.

Yes! More than 400Nm....

 

[Whiplash]

That seems not too bad really? 147lbft of torque would seem fairly easy standing full weight on a 175mm crank if someone weighed 200ish pounds? Hell a Wallmart bike can do that no? I stand with my whole 195lb body weight on one pedal all the time?? Hell, I even jump on it sometimes!

 

[Miles]

I make it about 500 pounds on a 175mm crank....

 

[motomoto]

Very cool.

You are my hero.

Matt

No Matt.

You are my hero and a lot of other people's. You carved the path for the rest of
us to follow, and you did it with competence and communicated it well. Thank
you from all of us.

 

[Whiplash]

I make it about 500 pounds on a 175mm crank....

Do you mean that is what I am putting to the crank at 195lbs on the pedal? Basically that was what I am saying, the 400Nm is way less than I could do with body weight alone, right? Or no?

 

[Miles]

I make it about 500 pounds on a 175mm crank....

Do you mean that is what I am putting to the crank at 195lbs on the pedal? Basically that was what I am saying, the 400Nm is way less than I could do with body weight alone, right? Or no?

No. 500 pounds at 175mm is equivalent to 400 newtons at 1 metre. So, you're not even halfway there

 

[motomoto]

So on a sprocket with a 3 inch radius, that is a lot of force?

 

[Miles]

About 1166 pounds.............

 

[Whiplash]

I got a bad calc on google, sorry, but because 400Nm is only 295ftlbs. if you multiply my 195lb weight by the lever on the crank, I don't know the math, but you should still get way more than 295ftlbs of torque?? I am just saying the sprocket should be fine since it looks to be the same as any other bike sprocket structurally and they take more than 400Nm of torque if anyone more than 200lbs was to pedal hard.... If I am wrong, show me how to figure it, because I want to learn! Thanks!

Hope I am not ruining you thread! Sorry!

 

[Miles]

Yes 295 ft pounds is equivalent to 400Nm. But, if you decrease the radius you need to increase the force to get the same torque. As 175mm is less than 1 foot, you need to apply more than 295 pounds.
305 / 175 = 1.74
1.74 * 295 = 514 pounds

Torque is force times radius.

So, your 195 pounds * 0.574 of a foot gives 112 foot pounds torque...

 

[Whiplash]

I see your point, it just seemed like it would be more than that, after using torque wrenches all my life, it seems like it would be a lot more than that... Doe not seem correct to me. If that were true, for me to put 65ftlbs on a bolt with less than a 12" breaker bar I would have to exert MORE than 65# of force? I can easily do that with one arm, and I am pretty sure I could not curl 65# very easily. Not saying its wrong, it just seems there must be more to it?? I don't know maybe I'm crazy...

 

[Miles]

Ref: http://www.roymech.co.uk/Useful_Tables/Human/Human_strength.html

 

[motomoto]

I said

So on a sprocket with a 3 inch radius, that is a lot of force?

Miles wrote;

About 1166 pounds.............

It looks like I have a lot of force transferring between the two sprockets. The sprocket that transfers
power to the rear wheel will have 5 -1/2 inch diameter bosses plugging into the other sprocket, which is good.

I think I will take my chances and see if it twists the sprockets into a pretzel. That would be cool. I have
a crazy reduction of 6,000 rpm at the motor to 90 rpm at the crank. Let's see what happens.

 

[Miles]

I was just following your 5hp figure as an example.

It looks like we will have a way to control phase current and therefore motor torque, soon. This should make it easier to keep a handle on things. So, a known maximum torque output from the motor, multiplied by 67, less the losses, will be your maximum torque at the bottom bracket.

For reference, the peak torque rating (60 sec.) for the Astro 3210 is 3.5Nm.

3.5Nm * 67 = 235Nm less 9% losses is 214Nm.

 

[Miles]

Maximum power out for an Astro 3210 10t on 44 Volts is 2600 Watts (3.5hp) at 3000rpm - that's 8.3Nm. Stall torque is 16.6Nm

 

[Whiplash]

WOW, they can be turned that slow now? I thought they were like 10,000RPM motors? That makes it a VERY easy install with such a low RPM, what is the KV of a 10 turn?

 

[Miles]

Max. power out is at half no-load speed.

Kv for the 3210 10t is 135rpm/V

 

[Green Machine]

subscribed...

 

[Whiplash]

Max. power out is at half no-load speed.

Kv for the 3210 10t is 135rpm/V

Ah, I see...

 

[motomoto]

Sound like I have this thing geared way wrong. I thought you want to keep it
up towards the maximum rpm all the time for the most efficiency and power.

 

[Whiplash]

I was under the same impression appro 90% max rpm was best efficiency

 

[Miles]

At maximum rpm you have no torque at all.... The usual thing is to gear for around 85% of no-load speed. You can see efficiency against power potential and speed, below. Maximum continuous current is about 40 amps. Maximum efficiency is the point where the copper losses and the parasitic losses are at parity - below that, the efficiency is pretty much dependant on torque output.

 

[motomoto]

Miles, thanks for the help. The graph is awesome. What is Eta?