My research on Speedometers and GPS

[Cann0n]

Anyone know how to fix radar guns? Mine is screwed up. Ill get more info later

[Clemson_Fan]

i did read all the thread, but i was just thinkin of a cool way to wire the bike speedometer i mean i didn't think it would be like anywhere near as accurate as a gps but it would be cheaper. but i mean as far as cheap and accurate, a radar gun sounds like it would do what i needed it to and there wouldn't be a chance of breaking it really

Actually bike speedometer is most accurate... until the point that your going to fast for it.

[Old Guy]

Actually bike speedometer is most accurate... until the point that your going to fast for it.

If you read the whole thread I think you'll agree that the bike speedometer is impractical because the diameter of a skateboard wheel is far outside the parameters programmed into the computer.

I've started working with the Pythagorean Theorem to get an idea of how much GPS is affected by inclination; I'm not great at math so if anyone who is wants to take that idea and run with it it would be helpful.

[Betterthanux5]

I've started working with the Pythagorean Theorem to get an idea of how much GPS is affected by inclination; I'm not great at math so if anyone who is wants to take that idea and run with it it would be helpful.

i'd love to. i'm good at math just tell me like what your idea is.

[Old Guy]

i'd love to. i'm good at math just tell me like what your idea is.

If you consider a hill as a right triangle then you're riding the hypoteneuse while the GPS receiver is reading your speed relative to the base; that's where the error comes in. If you have reliable information about the distance traveled (the hypoteneuse, c) and elevation change from start to finish (b) then you can use those figures to determine the length of the base (a). Once you know the base you can use it to determine the disparity between your actual speed and the speed the GPS receiver records. This assumes that there are no other variables, and I'm not sure how safe that assumption is. Even if there's some error it might still bring you a bit closer to knowing your actual speed.

Some people might think that this is making the whole issue too complicated; I'd have a hard time assailing their postion.

Edit: I'm personally not so concerned about accuracy that whatever the GPS receiver says isn't close enough for me; I just like kicking these ideas around.

[Clemson_Fan]

If you read the whole thread I think you'll agree that the bike speedometer is impractical because the diameter of a skateboard wheel is far outside the parameters programmed into the computer.

A bike speedometer is the only practical method. I think i computed a .2mph theoretical error for a 70mm wheel at 30mph (My math is elsewhere) using rotation based system. There are some bike computers that will accept parameters to the size of the wheels but the problem is a bike computer does not operate quick enough (I think it was 2MHz for 66mm at 60mph again my math is elsewhere).

But there are ways to get around that so any bike computer will work for any speed/wheel size a skateboard will encounter...

[Old Guy]

A bike speedometer is the only practical method. I think i computed a .2mph theoretical error for a 70mm wheel at 30mph (My math is elsewhere) using rotation based system. There are some bike computers that will accept parameters to the size of the wheels but the problem is a bike computer does not operate quick enough (I think it was 2MHz for 66mm at 60mph again my math is elsewhere).

But there are ways to get around that so any bike computer will work for any speed/wheel size a skateboard will encounter...

How would you mount it?

[Betterthanux5]

If you consider a hill as a right triangle then you're riding the hypoteneuse while the GPS receiver is reading your speed relative to the base; that's where the error comes in. If you have reliable information about the distance traveled (the hypoteneuse, c) and elevation change from start to finish (b) then you can use those figures to determine the length of the base (a). Once you know the base you can use it to determine the disparity between your actual speed and the speed the GPS receiver records. This assumes that there are no other variables, and I'm not sure how safe that assumption is. Even if there's some error it might still bring you a bit closer to knowing your actual speed.

Some people might think that this is making the whole issue too complicated; I'd have a hard time assailing their postion.

Edit: I'm personally not so concerned about accuracy that whatever the GPS receiver says isn't close enough for me; I just like kicking these ideas around.

potentially that could work but i think you would have to measure each length the c and the b before hand. it would take multiply runs, and you would have to draw it up on a literal right triangle (or i would at least) it will give you the length of a even if there are slight differences in elevation or curves. which yes could get you closer to your actual speed. your gps would also have to have a stop watch, or you would have to carry a separate stop watch to try and keep precise time to calculate your speed if you want to make it more accurate than the actual devise

[GhettoNinja06]

What iPhone app do you guys use?
I use Smart HUD and it seems to work pretty well. only about 2 or 3 mph off from my friend's Garmin.

[crazyfoots]

How would you mount it?

Well when I mounted my bike computer I fabricated a small attachment to the backside of my hanger. I was using a new gen. Randal R2 hanger and the small semi-cylindrical cutouts (that expose sections of the bare axle) actually made a great place to recess my set screw which keeps the entire sensor assembly in place. I then used a 1/16" drill bit to insert a high power magnet into the wheel. The magnet has threads so between that and the compression of the urethane you never have to worry about it falling out. It was slightly out of balance, but it was not enough to affect my normal riding. If I were going to take it up over 40mph I would probably balance the wheel with some sort of threaded counterweight.

Also I was using it on a dropthrough so I attached the computer display to the top of my truck baseplate with some aluminum sheets I made from cutting up the sides of old aluminum cans.... the metal is only 1/64" thick at most, so I had to use about 5 plys of it.

Sorry I would post pics, but the speedometer assembly is back in my college apartment.

I hope one day to find a wireless bike computer that is both compatible with my wheel size and has enough range to reach my wrist while standing on my board. This way I could attach the display to a watchband and have the speed readout on my wrist. If not I am sure I could boost the wireless transmission range of my computer with some sort of small amplifier... just might not be too pretty.

[Clemson_Fan]

How would you mount it?

Yea just mount it to the truck bolts in some ways... there are many ways to build a mount.

[Old Guy]

Yea just mount it to the truck bolts in some ways... there are many ways to build a mount.

If you think drilling holes in your wheels to insert magnets is part of a practical solution go right ahead, but I'll stick with my GPS receiver.

[craker]

good power supply for anything car-nav related: Bosch 12V Mini Li-ion with whatever car charger you can get your hands on. I've seen the quality of battery fed electronics dwindling quick if not sapped to a battery with high amperage these small drill and flashlight add ons are a world apart from any small cell Lead-Acid or Ni-Cad, and could actually improve the reception of connected receivers: Just a reminder for anyone who has to go big or go home.

[mattyoaks]

There isnt much useful info in this thread about using a cellphone as a gps speedo...

Has anyone used a gps enabled cellphone with much success? (not including iphone)