Grassroots Motorsports

Register Log in to GRM
- Community

Birthdays
Recently on the Classic Motorsports forums

Subscribe get a full year for only $23.95 Sample Free Test Drive—no strings attached

Login to post Forums » Off-topic discussion » Why this Message Board Rocks!

Dec. 21, 2011 8:26 p.m. Spitsix Reader

I hijacked a model railroading thread with a personal request of a needed model train part that had just broken. I had checked our local train shops and the web for a replacement with no luck. ddavidv (who does not know me from Adam except that I am a fellow GRMer and have a broken train) posts and puts me in touch with John Weigel at "The Peterboro Railroad" (http://www.peterbororr.com/). John (who knows me even less) answers my email even though is away at a graduation. After an exchange of pictures and descriptions, he asks for my address and ships out the part with the request that I send him just the shipping cost after I get the part (yeah - like that's all I'm going to send him.)! Part arrives and it is perfect! I can't wait to get to work tomorrow and solder it up!

There was no way I ever would have found John on my own.

No matter what the topic or question, someone on this board will step up with a suggestion or an opinion.

I want to thank ddavidv, the GRM community and the GRM staff for providing this discussion board.

Scott Lowden
Dec. 21, 2011 8:33 p.m. Gearheadotaku Dork

...and that's why I hang out here....
Dec. 21, 2011 8:37 p.m. griffin729 HalfDork

Me too. And, I'm just glad that once or twice I've been able to throw out some esoteric bit of knowledge that has been helpful. Great place, great community.
Dec. 21, 2011 8:39 p.m. Maroon92 SuperDork

Sorry to threadjack, but...Anyone know how to make an aerodynamic shell for a 100cc motorbike aiming to go 100 miles per hour?
Dec. 21, 2011 9:25 p.m. JThw8 SuperDork

Maroon92 wrote:

Sorry to threadjack, but...Anyone know how to make an aerodynamic shell for a 100cc motorbike aiming to go 100 miles per hour?

with fiberglass! Seems like a reasonably simple challenge from a construction standpoint, but I dont have a wind tunnel or advanced knowledge of aerodynamic theory, but if you can design it we could build it :)
Dec. 21, 2011 9:30 p.m. stroker HalfDork

In reply to Maroon92:

I think all the WWII drop tanks have been used up...
Dec. 21, 2011 9:48 p.m. jrw1621 SuperDork

Lightweight carbon fiber may be needed w/ only 100cc
Dec. 21, 2011 10:07 p.m. MrJoshua SuperDork

Maroon92 wrote:

Sorry to threadjack, but...Anyone know how to make an aerodynamic shell for a 100cc motorbike aiming to go 100 miles per hour?

I think lightweight heat formable plastic is what you need. I had sites bookmarked on another computer but of course they are gone now. Google DIY RC bodies, and DIY stormtrooper masks.
Dec. 21, 2011 11:15 p.m. EastCoastMojo SuperDork

I love this board.
Dec. 22, 2011 12:19 a.m. Appleseed SuperDork

Vacume forming? Click me!
Dec. 22, 2011 5:28 a.m. ddavidv SuperDork

Hey Spitsix,

Glad my idea/lead worked. I thought it may be a long shot, but if anyone could get that part to you, it's Uncle Weigel. If anyone is looking to get into trains, you could do far worse than buy from him (and his advice is always free). He ruined my hobby turning me from N to On30 scale, and I still thank him for it.
Dec. 22, 2011 7:29 a.m. MrJoshua SuperDork

Appleseed wrote:

Vacume forming? Click me!

This is what I was thinking about and explaining poorly.
Dec. 22, 2011 7:41 a.m. Curmudgeon SuperDork

Now if we could just get rid of those damn political threads...
Dec. 22, 2011 7:46 a.m. N Sperlo SuperDork

In reply to Maroon92:

Definitely carbon fiber. Make a mold out of plaster. Fill er with enough carbon fiber to make er strong enough.
Dec. 22, 2011 11:32 a.m. imirk Reader

Common Guys the Maths will tell you :D

The power required to overcome the aerodynamic drag is given by:

100 MPH=44.7 m/s

Cd of a streamline body is .04-.09

so lets call it .1?(there have to be other losses and wheels and such)

Mass Density of Air at Sea level at 20C is 1.2 kg/m^3

so it comes down to a function of power and cross-sectional area if you know the power out of your 100cc engine then you can determine how large it is(or the degree to which you must streamline), Mass really only matters in determining how long it will take you to get there. This also disregards surface drag, which is not insignifigant if this is on the salt-flats.

Lets take a stab at the size so we can get the required power...

In a recumbant position I bet your height can get down to about 1 meter (39in, 1in shorter than a GT-40) width would be maybe .5 meter for an area of .5m^2. to get smaller I think you'd need a skeleton/suicide head-first position

Power=.5*(1.2kg/m^3)(44.7m/s)^3(.5m^2)(.1)=2.68kW=3.6HP

Some googling suggests a 100cc engine is good for about double that HP figure meaning you only have to be half as aerodynamically efficient although that doesn't count drivetrain losses.
Dec. 22, 2011 11:53 a.m. N Sperlo SuperDork

In reply to imirk:

Haha... Long cylinder.
Dec. 22, 2011 12:29 p.m. Maroon92 SuperDork

imirk wrote:

Common Guys the Maths will tell you :D

The power required to overcome the aerodynamic drag is given by:

100 MPH=44.7 m/s

Cd of a streamline body is .04-.09

so lets call it .1?(there have to be other losses and wheels and such)

Mass Density of Air at Sea level at 20C is 1.2 kg/m^3

so it comes down to a function of power and cross-sectional area if you know the power out of your 100cc engine then you can determine how large it is(or the degree to which you must streamline), Mass really only matters in determining how long it will take you to get there. This also disregards surface drag, which is not insignifigant if this is on the salt-flats.

Lets take a stab at the size so we can get the required power...

In a recumbant position I bet your height can get down to about 1 meter (39in, 1in shorter than a GT-40) width would be maybe .5 meter for an area of .5m^2. to get smaller I think you'd need a skeleton/suicide head-first position

Power=.5*(1.2kg/m^3)(44.7m/s)^3(.5m^2)(.1)=2.68kW=3.6HP

Some googling suggests a 100cc engine is good for about double that HP figure meaning you only have to be half as aerodynamically efficient although that doesn't count drivetrain losses.

wow...

Oh, not salt flats, airplane runway. Salt flats are too far away.
Dec. 22, 2011 12:30 p.m. EdenPrime Reader

It took me about 15 minutes after joining this forum to realize this place is heaven.
Dec. 22, 2011 12:49 p.m. 4cylndrfury SuperDork

Spitsix wrote:

No matter what the topic, someone on this board will step up with a suggestion or an opinion.

Yep, someone will ALWAYS post a comment or their opinion on the topic at hand...even if you didnt ask for it, especially if you dont want it, and most definitely if its in direct opposition to your own...

...I miss Iggy
Dec. 22, 2011 12:59 p.m. RossD SuperDork

Most tires will have a rolling resistance associated with them too. IIRC the wind resistance becomes the majority of the effective resistance around 50-60 mph (for a typical car tire that is...)
Dec. 22, 2011 1:03 p.m. Stealthtercel HalfDork

OK, I'll bite: why is the Cd of a "short cylinder" roughly 10% worse than that of a cube?? On the chart, the height of the "short cylinder" looks about the same as that of the cube, so I [very dangerously] assume that the diameter of the cylinder is about the same as its height. So why does airflow hate it so much?

(And, Yes, this board definitely rocks.)
Dec. 22, 2011 1:38 p.m. imirk Reader

I'd wager it is because it is shorter than it is wide. A flat plate/plane is Cd=1.28 so that is about as bad as it gets.

You'll need to log in to post.