Downhill Terminal Velocity Calculator

[IRJohnnyRotten]

Version 0.2 now available.

http://spreadsheets.google.com/ccc?k...ogGm-36A&hl=en

Calculates your approx. terminal velocity subject to your riding position (frontal rider surface area in m^2), altitude above sea level at the start of the run (in m) rider's weight (in kg) and the gradient of the slope (in &#37. Please note that on the right hand side there is a conversion into imperial units.

If you fill out these FOUR fields (IN YELLOW), the spreadsheet automatically calculates your terminal velocity, and time and distance from start point at which you reach it from stationary with no push off. It also assumes that you are skating in a straight line (affects air density and hence the drag), wearing leathers (or clothes with low drag) and skating on a reasonably smooth surface with a constant slope

May be interesting to some.

Comments welcome.

8)

[longboardertwan]

damn...bored and an engineer/engineering major? That's cool though...

[IRJohnnyRotten]

damn...bored and an engineer/engineering major? That's cool though...

HAHA! Yeah! Crisis at work!

Majored in math...

[JayIV]

This is sick man.

Thanks a ton.

[T.V.A.R.]

That's pretty sweet, but you really need to add air resistance to your spreadsheet. You also need to factor in changes in altitude to make that one correct. For instance, there is a hill in Colorado that you can go over 75mph. However, it is at 8000ft where the air is almost 30% thinner. At sea level, that hill would not be nearly as fast.

[IRJohnnyRotten]

That's pretty sweet, but you really need to add air resistance to your spreadsheet. You also need to factor in changes in altitude to make that one correct. For instance, there is a hill in Colorado that you can go over 75mph. However, it is at 8000ft where the air is almost 30% thinner. At sea level, that hill would not be nearly as fast.

Air resistance already built in hence you reach terminal velociy... It's one of the parameters that you can modify already I just didn't include it as one of the inputs. Had a play with it and unless you are going down, say from the top of mount it won't be hugely affected. Temperature also is a factor, but I was too lazy to model that. May do in the next revision.

Keep the comments coming.
8)

[T.V.A.R.]

Air resistance already built in hence you reach terminal velociy... It's one of the parameters that you can modify already I just didn't include it as one of the inputs. Had a play with it and unless you are going down, say from the top of mount it won't be hugely affected. Temperature also is a factor, but I was too lazy to model that. May do in the next revision.

Keep the comments coming.
8)

Air resistance is HUGELY affected by different altitudes. I've skated roads at 12,500ft and roads at 12.5 ft, and trust me, there is a difference. For instance, I have gone nearly 60mph on a 3% grade, however that was at over 11,000ft. At that kind of altitude, one guy can be standing up airbraking while another tucks with no difference in speed. Read up on the physics of our atmosphere. Same reason cars are less powerful at altitude: Less Air. Less Air = Less Air resistance = Higher terminal velocity.

[sk8norcal]

not according to this cycling spreadsheet...
Bicycle Speed And Power Calculator
(set power to 0 and slope to negative %grade)

even at 12k ft,
there is a significant change in speed due to body positioning...

and 60mph at 3% grade doesn't sound right at all,
possible error in measuring of grade?

[SeanML]

Aha, a fellow math major!

You factored in drag AND air resistance? looks like someone has a physics background...

Also, this would require a consistent grade, would it not? constant-slope hills are hard to come by.

nit-picking criticism aside, great work! keep improving it!

[IRJohnnyRotten]

Air resistance is HUGELY affected by different altitudes. I've skated roads at 12,500ft and roads at 12.5 ft, and trust me, there is a difference. For instance, I have gone nearly 60mph on a 3% grade, however that was at over 11,000ft. At that kind of altitude, one guy can be standing up airbraking while another tucks with no difference in speed. Read up on the physics of our atmosphere. Same reason cars are less powerful at altitude: Less Air. Less Air = Less Air resistance = Higher terminal velocity.

Sure man. It has an impact. My point was that other factor (e.g body position as mentioned above) have a much larger impact on your terminal velocity than the air density. I'll factor it in the next version though as a lot of people seem to want to see it.

[kraffft]

looks pretty good.

seems to miscalculate at 1% grade.

not too familiar with spreadsheets, but the metric to standard conversions didn't seem to be working either.

I've been wondering about how close bearing technology is to the limits of physics.. more of a pressing question at low speeds and flatter ground..

be interesting in general to take some of these theoretical numbers and compare them to carefully measured hills/runs.

[IRJohnnyRotten]

looks pretty good.

seems to miscalculate at 1% grade.

not too familiar with spreadsheets, but the metric to standard conversions didn't seem to be working either.

I've been wondering about how close bearing technology is to the limits of physics.. more of a pressing question at low speeds and flatter ground..

be interesting in general to take some of these theoretical numbers and compare them to carefully measured hills/runs.

This is because 1% slope is a drop of 1ft over 100ft. Negligible. Since the model has friction built in, the lateral component of the force required to get you moving is not sufficient to overcome the friction. Hence it gives you a terminal velocity of 0 i.e. you will not move.

With regard to bearings, Lush has quite a good atricle:

Workshop | ABEC ratings explained | ABEC ratings and what they actually mean

It suggests that even ABEC 5 bearings are good enough for most. Pay more attention to cleaning and maintaining them!

[SeanML]

. What will be the result of using these higher grade bearings in Skating Applications ?
The most noticeable result is that you will end up with less money in your wallet and the people that sold you the bearings will be eating out at restaurants at your expense for a few days. Under the following conditions, you may notice an improved performance.

(a) You spend a lot of money (i.e. thousands of dollars) to have your equipment (wheels and board) redesigned and manufactured to suit these high precision bearings. You will need to use some type of shock absorber that allows for absolutely no vibration.

(b) You will have to be prepared to skate on a perfectly smooth surface and make no attempts to use your feet to propel yourself. (Doing so would cause shock loads to the bearings and any extra precision would be lost very quickly).

(c) After you work out how you are going to achieve the above two criteria, you may (and that is only "may") experience a better result than using ABEC 1 bearings after you attain a speed of about 390 KPH (240 MPH) with 65mm wheels and grease lubrication.

Similar calculations for different wheel sizes and lubrication methods
52mm Wheels Grease Lubricated Bearings 310 kph (195 mph)
52mm Wheels Oil Lubricated Bearings 370 kph (230 mph)
65mm Wheels Grease Lubricated Bearings 390 kph (240 mph)
65mm Wheels Oil Lubricated Bearings 460 kph (290 mph)
70mm Wheels Grease Lubricated Bearings 420 kph (260 mph)
70mm Wheels Oil Lubricated Bearings 500 kph (310 mph)

This made me LOL.

[Dimm]

Air resistance is HUGELY affected by different altitudes. I've skated roads at 12,500ft and roads at 12.5 ft, and trust me, there is a difference. For instance, I have gone nearly 60mph on a 3% grade, however that was at over 11,000ft. At that kind of altitude, one guy can be standing up airbraking while another tucks with no difference in speed. Read up on the physics of our atmosphere. Same reason cars are less powerful at altitude: Less Air. Less Air = Less Air resistance = Higher terminal velocity.

Considering the human body can't even cope with a 1% change in atmospheric oxygen content the chances that you've gone high enough to experience any real change in drag (without an oxygen tank) is minimal.

[Bookworm]

A- I think this is worthy of a bump so I will.

Considering the human body can't even cope with a 1% change in atmospheric oxygen content the chances that you've gone high enough to experience any real change in drag (without an oxygen tank) is minimal.

B- I doubt this, have you ever been trapped in a small room with people? I bet that is a decreased oxygen situation. what about hotboxing a room/car? We can definitely survive on some varied concentrations of Oxygen. also, 11,000 feet sounds unbelievably high (I don't even know where you need to GO to skate at 11k feet)

[Bomb303]

11,000 feet sounds unbelievably high (I don't even know where you need to GO to skate at 11k feet)

Colorado...

[Bookworm]

Colorado...

is colorado really that tall? I just figured you'd need to go way up the slopes to get that high and I wouldn't expect to see a 3% grade there.

edit:

http://www.2-stroke-porting.com/altiden.htm

U.S. Units Standard Atmosphere
This table give density in slugs per cubic foot because it uses the American system of altitude in feet, pressure in inches of mercury and temperature in degrees Fahrenheit. While people often use pounds per cubic foot as a measure of density in the U.S., pounds are really a measure of force, not mass. Slugs are the correct measure of mass. You can multiply slugs by 32.2 for a rough value in pounds.(Pounds per cubic foot have been added to Chart)

Altitude Pressure Temp. Density - Weight -
(ft) (in. Hg) (F.) slugs/cubic ft. Lbs/cubic ft.
0,000 29.92 59.0 0.002378 0.07657
1,000 28.86 55.4 0.002309 0.073435
2,000 27.82 51.9 0.002242 0.07219
3,000 26.82 48.3 0.002176 0.07006
4,000 25.84 44.7 0.002112 0.068006
5,000 24.89 41.2 0.002049 0.065977
6,000 23.98 37.6 0.001988 0.0640136
7,000 23.09 34.0 0.001928 0.0620816
8,000 22.22 30.5 0.001869 0.0601818
9,000 21.38 26.9 0.001812 0.0583464
10,000 20.57 23.3 0.001756 0.0565432
11,000 19.79 19.8 0.001701 0.0547722
12,000 19.02 16.2 0.001648 0.0530656
(Chart shortened for snowmobile elevations.)

0.054772 / 0.07657 = 0.715319316
at 11k feet air IS almost 30% thinner, and I'm sure that effects wind resistance GREATLY.

[Bomb303]

is colorado really that tall? I just figured you'd need to go way up the slopes to get that high and I wouldn't expect to see a 3% grade there.

Denver is roughly a mile above sea level, Golden/Boulder foothills are around 7-8K feet. I can think of a few runs off the top of my head that are around 10-11k feet, as you go further into the mountains.

[tupaz]

well i determine my velocity speed by this simple chart, moving is 1-10 wind in hair 10-20 getting excited 20-30 stoked 40-50 adrenaline and everything- around- me- is- a- blur lightspeed

[rva]

to the OP, how do I fill in the value in field # 3? the one wherein you key in your frontal surface area? I dont know how to measure what my frontal surface area is?