sand drag bike

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drowe67
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sand drag bike

Post by drowe67 »

Hi,

I'm building a electric sand drag quad bike with some friends. The ideas is to win against a IC bike over a 200m sand course. I figure it's all about torque off the line and traction rather than top end speed. Ideal for an EV.

At this stage we are planning on starting with 72V kit, with something like 30kW peak. The bike with rider will mass about 200 kg. My EV masses 900kg and I can get 40kW peak so I figure the bike will have 4 times the acceleration of my EV off the line.

We'll start with some old AGMs 12V batteries (heavy) that I have lying about to get us moving then try something lighter in future. Range is not an issue, 5 mins run time over a day would be ample. Peak current is an issue, figure we need 600A off the line at 72V.

One thought I had was parallel banks of model aircraft batteries, some of those have short circuit currents in the 100 - 200 A range.

Any suggestions for parts or general design for drag racing would be much appreciated.

Thanks,

David
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woody
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Post by woody »

Peak Current off the line is a controller issue, not a battery issue.

I.e. your controller delivers 600A at 20V? = 12kW to the motor - it only needs to draw say 200A at 72V = 14kW from the batteries.

Full Battery power only comes in at speed - i.e when your motor speed comes up to the point when you give it full battery voltage - 72V @ 400Amps = 30kW both sides of the controller.

---

Can you give some ideas of the course and speed for sand drag racing?

For car drag racing 400m has a few issues to balance:
1. traction
2. delivering power at low speed for acceleration
3. delivering high power at high speed for continued acceleration

How much fuel do the quad bikes use on the course - this will give you some idea of how many batteries you need.
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drowe67
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Post by drowe67 »

Thanks Woody. The course is 120m long and runs with ICE Quads appear to be about 8 secs. I think that means an average acceleration of 3.75 m/s/s, and a terminal speed of 30 m/s (108 km/hr). Hmm, quite fast on a quad that low to the ground. On a bumpy surface too. Not sure how to relate motor power to accleration.

Also I can see we might have a problem maintaining acceleration over such a wide speed range (stationary to 108 km/hr). Could need 144V.

With my road EV I can see about 400A max thru my batteries when I plant it off the line. That then drops off as speed increases. So I am getting max power of about 40kW at low rpm. I do have a small X4001 ADC DC motor, so maybe the armature resistance is such that it can handle the full battery voltage at low rpm. Or maybe the motor current is much higher. Anyway - thats the sort off the line power I am looking at.

Cheers,

David
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woody
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Post by woody »

This Post mentions a "Moroso Speed Calculator" which is a heuristic to convert 1/4 mile times and speeds into engine power.

So going over your maths:
120m in 8 seconds is 15m/s = 54km/h average.
Assuming constant accel from standing start = 30m/s = 108km/h finish.
Assuming constant power ignoring drag is probably fairer and results in a slower finishing speed, but the maths is a bit harder, plus you can't apply any power at zero starting speed, you have to limit the torque somewhere.

e.g. accel 0-17m/s in 2 seconds (17m) (1G!) and constant 17m/s after that = 119 metres in 8 seconds. 17m/s = 61.2km/h

How to relate acceleration to power:
1) F = ma : Force = Mass x Accelleration
2) W = Fd : Work = Force x Distance
3) P = W/t : Power = Work / time

So Accelerate 200kg at 3.75m/s = 750N Force
over a distance of 120m = 120m * 750N = 90kJ
in 8 seconds = 90/8 = 11.25kW average

But over the line you are pushing much faster
4) P = Fv : Power = Force x Velocity
750N * 30m/s = 22.5kW.

These power figures are just for accelerating the mass, you need to add losses for all the sand and air you need to push out of the way, and any bumps + hills you have to climb, suspension which gets compressed, etc.
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drowe67
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Post by drowe67 »

Thanks Woody thats great information. Starting with 30kW peak is probably a reasonable starting point.

Some brainstorms:

1/ An idea re two motors and one controller. Assuming the motor current drops off with speed, perhaps I could switch in (with contactors) a 2nd motor in parallel with the orginal motor half way through the run. I guess a mechanical gear box might do a similar job.

2/ I'm thinking of using a 48V ME0909 motor at 72V. I recall the killer cycle uses two ADC 140V motors at 300V, so I figure for $500 we can take our chances with an 8 second run (its rated at 15kW for 2 minutes). I am thinking that two in parallel would give us some top end compared to a single larger motor. Guess it depends on the torque versus RPM curves.

- David
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Johny
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Post by Johny »

Even better than carrying that dead weight is to have the two motors in series initially. At a given speed switch them to parallel. You would need some kind of sequencer to cope with the contactor switching.

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