Dragster MOC

[Hopey]

So I've got 6 M-motors and wanted to see how much go-fast I could get out of them. Here's the result.

Of course it's bloody yellow; *sigh*. There's more pictures in the brickshelf folder

Some videos to come. It's pretty quick. I was hoping that the weight of the batteries at the back would make it pop a wheelie when it accelerated, but ended up making the nose a little too heavy. It'll pop up if you reverse and then go, but not from a standing start.

I used portal axles to hold on to the wheels a bit better, and to make it easy to change the gear ratios, although in the end I went for simple 16:16, as I discovered that this is the highest gear ratio that will, fit; you can't gear up, as a 20T gear won't fit in the input bit. I also put them in upside down, as there was actually too much clearance if the wheels were aligned with the axles. I made sure to get them as close together as possible for the look of it; there's only a 3 stud gap between the portal axles; just enough to fit the knob wheels that drive it.

So the total gearing is 20:12. It's a bit hard to see the drive train in the pictures. At the front, the left two motors share a single shaft driving a 20T gear, as do the right two. These two 20T gears drive a 12T gear in the middle, shaft to a knob wheel, which drives a perpendicular knob wheel, which drives the portal axle inputs. The same thing is mirrored from the back (with only 2 motors, no shaft-sharing) driving the other side of the same knob wheel. 16:16 in the portal axles.

I tried running it with a single battery pack, and it went well for about 1 second before everything cut out due to too much power being drawn, hence two battery packs.

Mechanically, I'm quite happy with it. The drive shaft train is well-supported, and the whole back end is very structurally sound, although the battery pack mounting was done as an afterthought. The front section's kind ugly and flimsy, and the whole thing's a fair bit heavier than it needs to be. The cockpit's terrible, as I didn't really plan for it at all, and just bunged some pieces on at the last minute.

If I have another go at it, it'll be one that integrates the battery packs into the body instead of just bolting them onto the back, and maybe putting all 6 motors in line, all driving a single shaft that runs right down the center of the whole thing. Might twist a bit under load, methinks...

That or a crawler with an M motor on each wheel.

[DLuders]

Whoa, that's one LONG Top-Fuel Dragster! I always wanted to build a Lego Technic dragster, but I never could get it right. There's got to be a fine balance between Speed, Torque, and Weight in such a MOC.

If you could get some of those new Power Functions L-Motors connected to the "Version 2" Power Functions Receiver (that come with the Lego 9398 4x4 Crawler set), maybe you could get almost the same speed (and a lot more torque) by using fewer motors (and fewer battery boxes). This will decrease the weight and allow the dragster to accelerate faster. From Philo's Lego 9V Technic Motors Compared Characteristics webpage, you can see the relative properties of the PF M-motor and new PF L-motor:

[timslegos]

Nice dragster! How does it perform? Do you have a video?

tim

[Paul202]

I hear you man, I wish Lego would release more technic sets with panels that aren't yellow. And you really need to post a video.

[Hopey]

I'd love to do it with L or XL motors, but my budget's rather limited at the moment, so I'm trying to see what I can build with what I've got.

Videos, as promised:

The finished product:

Just the minimal back end:

This is why it's got the small wheels underneath the battery packs. I was hoping it would still drive like this when it was finished, but no such luck.

[Zerobricks]

If you have RC motors, stick two in there...Now THAT is fast!

[Aussie BJ]

Very nice Dragster Hopey. I've been working on one myself but never thought to put 6 M motors in it your mad as but I like it.

Mine Below

Bigger Photo

The biggest problem I found was the weight of the battery boxes tends to drop the middle of the car but with a bit more support I got it right. I'm still working on mine and hope to post it on here shortly.

But yours will win the quarter mile (if the batteries last!) hands down.

Brendan

[Bigfoot]

I think if you made it shorter then it would wheelie easier.

[TimH]

Cool Dragster!

[Thirdwigg]

Very well done. I looks to have great speed. Maybe you should take a little length off the front, and the car will be a little quicker.

[z3_2drive]

Did you use the regulator part I told you about in your other topic, since I see the custom batteries.

[Hopey]

Very nice Dragster Hopey. I've been working on one myself but never thought to put 6 M motors in it your mad as but I like it.

Mine Below

...

But yours will win the quarter mile (if the batteries last!) hands down.

Brendan

I'm glad I posted mine first :)

I built mine from a mechanical perspective; I had 6 motors and wanted to see how I could use all of them to drive a vehicle, and then built a rudimentary dragster body around it. No engine, dud cockpit, not much planning. Yours is clearly designed to be a dragster from the start. I like the details such as the engine exhaust.

What're you using to drive it? I'mm only using all the M motors because that's all I've got, I'm fairly sure that a single XL motor would be comparable to the 6 M's, and two XLs would blow it away, if geared properly.

As an aside, are you seriously from Maclean? 'Cause I'm from Yamba, although I'm living in the UK at the moment.

[Traktor]

Whoa, that's one LONG Top-Fuel Dragster! I always wanted to build a Lego Technic dragster, but I never could get it right. There's got to be a fine balance between Speed, Torque, and Weight in such a MOC.

If you could get some of those new Power Functions L-Motors connected to the "Version 2" Power Functions Receiver (that come with the Lego 9398 4x4 Crawler set), maybe you could get almost the same speed (and a lot more torque) by using fewer motors (and fewer battery boxes). This will decrease the weight and allow the dragster to accelerate faster. From Philo's Lego 9V Technic Motors Compared Characteristics webpage, you can see the relative properties of the PF M-motor and new PF L-motor:

After taking a look on the graphs of the XL motor, it seems like the Xl motor uses less current per output watt, than the L motor at their respective effect tops. On the other hand the xl motors weight more per output watt than the L-motor.

[Aussie BJ]

I'm glad I posted mine first :)

I built mine from a mechanical perspective; I had 6 motors and wanted to see how I could use all of them to drive a vehicle, and then built a rudimentary dragster body around it. No engine, dud cockpit, not much planning. Yours is clearly designed to be a dragster from the start. I like the details such as the engine exhaust.

What're you using to drive it? I'mm only using all the M motors because that's all I've got, I'm fairly sure that a single XL motor would be comparable to the 6 M's, and two XLs would blow it away, if geared properly.

As an aside, are you seriously from Maclean? 'Cause I'm from Yamba, although I'm living in the UK at the moment.

Yes living and breathing in Maclean. Small world that's for sure.

Mine is only using 1x M motor but after seeing the graphs posted here I might change it to a PF-Train motor I have and see if it's any quicker. Would like two XL's that's for sure but no one has them on Brick Link in Australia.

Yes designed from the start. It had steering for a while but that was just painful and it's a dragster, so it only needs to go straight anyway.

Brendan

[allanp]

Whoa, that's one LONG Top-Fuel Dragster! I always wanted to build a Lego Technic dragster, but I never could get it right. There's got to be a fine balance between Speed, Torque, and Weight in such a MOC.

If you could get some of those new Power Functions L-Motors connected to the "Version 2" Power Functions Receiver (that come with the Lego 9398 4x4 Crawler set), maybe you could get almost the same speed (and a lot more torque) by using fewer motors (and fewer battery boxes). This will decrease the weight and allow the dragster to accelerate faster. From Philo's Lego 9V Technic Motors Compared Characteristics webpage, you can see the relative properties of the PF M-motor and new PF L-motor:

Here's what I don't get. According to the tables on philos website, @ 9v the XL motor produces 2.21W of mechanicle power compared to 2.38W for the buggy motor @ 9V. However the graphs shows the XL motor being more powerful. So which one is more powerful?

[DLuders]

Here's what I don't get. According to the tables on philos website, @ 9v the XL motor produces 2.21W of mechanicle power compared to 2.38W for the buggy motor @ 9V. However the graphs shows the XL motor being more powerful. So which one is more powerful?

I'll ATTEMPT to explain. On his webpage, Philo wrote that "These [synthesis] charts [below] summarize the above curves. The most meaningful shows the various motors sorted by maximum power they are able to deliver at 9V. Because rpm/torque curve is linear, a motor provides maximum power when load slows it down to half of no-load speed."

Looking at the graphs above, the no-load speed for the Power Functions XL Motor (at 9V) is ~225 rpm, so half of that is ~113 rpm.

The no-load speed for the 5292 RC Buggy Motor (at 9V) is ~925 rpm, so half of that is ~413 rpm.

Looking back to the topmost charts on this post, one can see that the PF XL motor's torque is ~22.5 N.cm at ~113 rpm, and the 5292 RC Buggy Motor's torque is ~8.8 N.cm at ~413 rpm:

This definition of a "Watt" is: "One watt is the rate at which work is done when an object's velocity is held constant at one meter per second against constant opposing force of one Newton." In other words, 1 Watt = 1 Newton*meter/second.

For the PF XL-motor: (~22.5 N.cm) x (113 rev/min) = (0.225 N.m) x (113 rev/min) x (1 min/60 sec) = 0.42 Watt.

For the 5292 RC Buggy Motor: (~8.8 N.cm) x (413 rev/min) = (0.088 N.m) x (413 rev/min) x (1 min/60 sec) = 0.60 Watt.

Philo's chart "shows the various motors sorted by maximum power they are able to deliver at 9V....a motor provides maximum power when load slows it down to half of no-load speed."

@ Philo: Can you help explain why my calculations don't match your chart above?

Philo continues by saying "The following chart sorts motors by torque and by no-load rotation speed (of course this depends a lot on internal down-gearing of the motors!!!). Torque chart lists torque at half speed point." [The "Maximum Power" point mentioned by Philo above]

Edited August 3, 2012 by DLuders

[allanp]

Thankyou very much for trying to explain. I didn't think to calculate from looking at those graphs, so we potentially have a third set of figures for output power.

rpm/60 x (2 pi) x torque (in newton meters) = Watts

so the XL motor equals:

113/60 x (2 x 3.142) x 0.225 = 2.663 W

Seems kinda right (table on Philos page shows 2.21W @ 9v and bar chart for power shows about 2.55W)

So using the same methed buggy motor equals:

413/60 x (2 x 3.142) x 0.088 = 3.806 W

WTF?????

From personal experience the buggy motor is the most powerful, but 3.806 W ? That's very different from anything Philo said it was, so now i'm even more confused.

BTW i've made an ungeared XL motor (basicly just removed all the gearing). It's faster than the old 9v ungeared motor now, it's a beast!

Edited August 4, 2012 by allanp

[Kisvakond]

Hey, it seems to be not too slow, it has really quite good speed.. Nice!!

( If I would you, I will try the 4 RC Buggy motors configuration, with double RC reciever units.. )

[kieran]

BTW i've made an ungeared XL motor (basicly just removed all the gearing). It's faster than the old 9v ungeared motor now, it's a beast!

any pics of that floating around?

[Philo]

rpm/60 x (2 pi) x torque (in newton meters) = Watts

That's indeed the method I used...

So using the same methed buggy motor equals:

413/60 x (2 x 3.142) x 0.088 = 3.806 W

...clearly you're right, I bugged (pun intended) somewhere for the buggy motor, I look more closely and update.

Actually I should redo the tests for the 5292 motor, this beast was difficult to test with the setup I had at the time. Hopefully I could do better now.

Seems kinda right (table on Philos page shows 2.21W @ 9v and bar chart for power shows about 2.55W)

Indeed, there maybe a discrepancy between table values and bar chart. Table that gives power at some (non-optimal) load, it aims to compare the motors at various voltages.

[allanp]

any pics of that floating around?

It looks identical to the original.

That's indeed the method I used...

...clearly you're right, I bugged (pun intended) somewhere for the buggy motor, I look more closely and update.

Actually I should redo the tests for the 5292 motor, this beast was difficult to test with the setup I had at the time. Hopefully I could do better now.

Indeed, there maybe a discrepancy between table values and bar chart. Table that gives power at some (non-optimal) load, it aims to compare the motors at various voltages.

Thankyou

Just a quick note about the buggy motor and the V2 receiver. In the crawler the reciever has 2 L-motors on one port which equates to (again according the Philos ever useful website) about 1 amp at some load. Which means each port should now be able to handle one buggy motor at some load.

Edited August 4, 2012 by allanp

[Philo]

I went through the data used to create the charts, only minor adjustments... except for 5292 motor that was all wrong! Indeed, it is by far the most powerful guy, 60% more than PF-XL!

[allanp]

Fantastic! Thanks for taking the time to check

Edited August 4, 2012 by allanp

[Kisvakond]

...

Looking at the graphs above, the no-load speed for the Power Functions XL Motor (at 9V) is ~225 rpm, so half of that is ~113 rpm.

The no-load speed for the 5292 RC Buggy Motor (at 9V) is ~925 rpm, so half of that is ~413 rpm.

Looking back to the topmost charts on this post, one can see that the PF XL motor's torque is ~22.5 N.cm at ~113 rpm, and the 5292 RC Buggy Motor's torque is ~8.8 N.cm at ~413 rpm: ...

(I've just read that above)

What..?! Only ~925 rpm..?! Noo.. it is impossible or it may not the no-load speed.

Are you sure? Maybe it is the loaded rpm during the test.. (?)

I never measured concerning any of my RC Buggy motors less than approx. 1300 rpm on the last axle (this is the right no-load speed, isn't it?)

So, the half of it for the calculation may be ~650 rpm.

Please correct me if I'm wrong.

Edited August 5, 2012 by Kisvakond

[allanp]

I think you're right, I missed that. That would make the motor even more powerful. I guess that's why the chart now shows it's power to be about 4.25W. Well, I guess now we know what's best to use in a dragster (just to bring it back on topic )