Wheel Base and Downhill Speed

Jim Reilly said:

Many years ago (OK. 20 or more) the thought with downhill skis was: "longer skis are faster than
shorter ones". Could the same be said about wheel base?

It *could*. Whether it contains any truth is another matter entirely. If no corners are involved,
speed is mostly due to:

1. aerodynamics, and
2. the bravery / stupidity / lack of imagination of the rider

If it's a twisting descent, factor two becomes the more important...

Dave Larrington - legslarry.crosswinds.netlegslarry.crosswinds.netOpen ↗
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Editor - British Human Power Club Newsletter
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Yes, and with similar accuracy.

There is an aerodynamic relationship with increasing length and drag. Hydrodynamically it is
expressed as hull speed = 1.34 x WL^1/2.

"stratrider" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Many years ago (OK. 20 or more) the thought with downhill skis was: "longer skis are faster than
shorter ones". Could the same be said about wheel base?

Jim Reilly Reading, PA

Years ago when I thought about physics and did some math, I was under the impression that, for a
given mass on SKIS, youR increased your speed by increasing aerodynamics and reducing friction twixt
snow and ski. (longer ski meaning more surface area, meaning less pressure per area, yielding less
friction, as I remember)

For a given slope, mass, and aerodynamic setup, then, would not your speed increase be only due to
ski length. I'm not sure if this was ever true, it was just a set of physics problems to demonstrate
competence with the week's lessons.

Even if this IS true then it isn't applicable to wheelbase, as that's a completely different set of
phenomena.

As Rich P. has pointed out here many times, above 15mph or so, virtually all speed increase is
gained by lessening wind resistance... (assuming all have already thrown the brain aside and are
free-falling)

In article <[email hidden]>, [email hidden] says...

Quoted message said:

As Rich P. has pointed out here many times, above 15mph or so, virtually all speed increase is
gained by lessening wind resistance... (assuming all have already thrown the brain aside and are
free-falling)

Kinda gives a new meaning to the term 'Terminal Velocity'.

--

Cletus D. Lee Bacchetta Giro Lightning Voyager clee.orgclee.orgOpen ↗
- Bellaire, TX USA -

"Doug Huffman" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

Yes, and with similar accuracy.

There is an aerodynamic relationship with increasing length and drag. Hydrodynamically it is
expressed as hull speed = 1.34 x WL^1/2.

Sorry, this is related to wavemaking drag and has no relationship to aerodynamics.

Len

Another vote for aerodynamics being more important than wheel base. There is hill that I go down a
lot. It is a curvy bike trail through trees (close) and then it bottoms out down low along a
beautiful river.

I have found that (on my Haluzak) if (1)I hit the top hard and then coast or (2)hammer all the way
down my maximum speed only varies by 2 mph between the two methods. I hit 25-27 mph and all the
pedaling in the world does not increase that. So my now preferred method of doing that downhill is
to hammer the top and then just glide (coast) effortlessly through the curves on my land luge. Don

I think Steve Delaire recently posted that one thing they found in coast downs and fastness tests
was that the longer bikes were faster but they didn't know why. bill "yet another erroneous
factoid" g

Don said:

Another vote for aerodynamics being more important than wheel base. There is hill that I go down a
lot. It is a curvy bike trail through trees (close) and then it bottoms out down low along a
beautiful river.

I have found that (on my Haluzak) if (1)I hit the top hard and then coast or (2)hammer all the way
down my maximum speed only varies by 2 mph between the two methods. I hit 25-27 mph and all the
pedaling in the world does not increase that. So my now preferred method of doing that downhill is
to hammer the top and then just glide (coast) effortlessly through the curves on my land luge. Don

a&b said:

I think Steve Delaire recently posted that one thing they found in coast downs and fastness tests
was that the longer bikes were faster but they didn't know why.

All else being equal, a lower bicycle will be faster downhill than a taller bicycle since the
distance between the center of mass of the bicycle/rider combination and the center of mass of the
earth will be less, and of course gravitational attraction varies inversely with the square of the
distance between the masses in question.

Tom Sherman - Various HPV's Quad Cities USA (Illinois side)

Proof, with references please ;-) bill "and then a miracle happened" g

Tom Sherman said:

All else being equal, a lower bicycle will be faster downhill than a taller bicycle since the
distance between the center of mass of the bicycle/rider combination and the center of mass of the
earth will be less, and of course gravitational attraction varies inversely with the square of the
distance between the masses in question.

Tom Sherman - Various HPV's Quad Cities USA (Illinois side)

a&b said:

Proof, with references please ;-) bill "and then a miracle happened" g

The relevant reference is "Philosophiae Naturalis Principia Mathematica"; Newton, Sir Isaac; 1687.

Tom Sherman - Various HPV's Quad Cities USA (Illinois side)

I think that speed down hills has more to do with overall handling characteristics than with any
one factor.

FWIW, my Wishbone Classic is a lot more secure-feeling at really high speeds (40+) than my
Pharobike.

The Wishbone has a much longer wheelbase and a much higher center of gravity than the itty-bitty
Pharobike.

Maybe it's just altered perception from being so low, but once I get the Pharobike up to the high
30s I don't really *want* to go much faster. I'm having enough fun. I think I hit 41 or 42 once.

One time I had the Wishbone up to 52 mph.

Gearing might also be a factor... I have the Pharobike rigged with a top end gear of something like
110-115 inches (depends on the tire I'm using). So at 35 mph I'm pedaling pretty fast, which upsets
the bike. The Wishbone has a top end of 125 inches or so, which means that I don't start really
flailing until a much higher speed.

Tom Sherman <[email hidden]> wrote in :"]news:[email hidden]:

Quoted message said:

a&b said:

I think Steve Delaire recently posted that one thing they found in coast downs and fastness tests
was that the longer bikes were faster but they didn't know why.

All else being equal, a lower bicycle will be faster downhill than a taller bicycle since the
distance between the center of mass of the bicycle/rider combination and the center of mass of the
earth will be less, and of course gravitational attraction varies inversely with the square of the
distance between the masses in question.

Tom Sherman - Various HPV's Quad Cities USA (Illinois side)

Technically true, but in practice it's meaningless. A human being would not be able to tell the
difference without _very_ precise measuring systems.

Perhaps the longer wheelbase has less induced rotation due to irregularities in the road surface
(cracks, holes, etc). A short wheelbase will rotate more about its cross axis due to these
irregularities. This increased rotation means that more load is placed on the contact surfaces due
to torque, so the tires will deform more. Deformed tires mean more kinetic energy lost until the
tires go back to their proper shape.

Or, to look at it differently. You have a long board with a fixed hinge at one end, a weight in the
middle, and you holding it toward the other end. The farther out you go, the less force you have to
exert to raise your end of the board one inch because you get more torque and the distance you raise
the weight is less. On a bicycle, the hinge is the rear tire contact point, the weight is you, and
the person lifting is the front tire contact point when it hits a ridge in the road. The farther
forward the front tire is, the less force is necessary to raise the front of the bike over the
obstacle. All of that force is applied through the tire, deforming it. The process of deforming and
returning to its correct shape absorbs kinetic energy from the bicycle, slowing it down.

That, at least, is my best guess.

-Bill

Tom Sherman <[email hidden]> wrote in message news:<[email hidden]>...

Quoted message said:

a&b said:

I think Steve Delaire recently posted that one thing they found in coast downs and fastness
tests was that the longer bikes were faster but they didn't know why.

All else being equal, a lower bicycle will be faster downhill than a taller bicycle since the
distance between the center of mass of the bicycle/rider combination and the center of mass of the
earth will be less, and of course gravitational attraction varies inversely with the square of the
distance between the masses in question.

Tom Sherman - Various HPV's Quad Cities USA (Illinois side)

Tom: Seems like maybe you know the math. Could you take a hypothetical situation, like a Haluzak vs.
your lowracer, and calculate the speed difference on a given hill--irrespective of wind resistance?
That is, based purely on the gavitational attraction business, how much faster IS a lowracer than
some taller bikes, all other things being equal? It would be interesting to know, but this has got
to be a pretty small number.

Come to think of it, if I recall this correctly, if I, on my "high racer" and you on your lowracer
zoomed down a hill at the same speed, wouldn't I age LESS by the time we finished? Or have I got the
"twins' paradox" reversed? Anyway, seems like we could make some sort of physics argument for one
type of bike over the other based on relative ageing! 🙂

Scott Haluzak high! Greenspeed low...

Scott

Tom Sherman said:

a&b said:

Proof, with references please ;-) bill "and then a miracle happened" g

The relevant reference is "Philosophiae Naturalis Principia Mathematica"; Newton, Sir Isaac; 1687.

Tom Sherman - Various HPV's Quad Cities USA (Illinois side)

I'm pretty sure nobody is serious about this, but lowering the CG by 2 feet increases gravitational
pull by about .000019%, but you're also running through denser low-altitude air. If the center of
pressure drops only one foot, air density is about .0036% greater, more than offsetting the huge
gravitational gain.

Dave Lehnen

I can't wait to see the answer to this one. The number will be so small that I'll bet a single fly
in the chest on the way down could compensate for the "speed increase"

Tom was jest funnin' with us. You're talking about a difference of inches between two numbers
measured in thousands of miles.

Bill Hamilton said:

Perhaps the longer wheelbase has less induced rotation due to irregularities in the road surface
(cracks, holes, etc). A short wheelbase will rotate more about its cross axis due to these
irregularities. This increased rotation means that more load is placed on the contact surfaces due
to torque, so the tires will deform more. Deformed tires mean more kinetic energy lost until the
tires go back to their proper shape.

Or, to look at it differently. You have a long board with a fixed hinge at one end, a weight in
the middle, and you holding it toward the other end. The farther out you go, the less force you
have to exert to raise your end of the board one inch because you get more torque and the
distance you raise the weight is less. On a bicycle, the hinge is the rear tire contact point,
the weight is you, and the person lifting is the front tire contact point when it hits a ridge in
the road. The farther forward the front tire is, the less force is necessary to raise the front
of the bike over the obstacle. All of that force is applied through the tire, deforming it. The
process of deforming and returning to its correct shape absorbs kinetic energy from the bicycle,
slowing it down.

That, at least, is my best guess.

-Bill

I think there are some remarks to be made here: first of all: how do you ensure that the two bikes
compared _only_ differ in length, i.e. how sure are you the frontal area is exactly the same, the
shape factor is exactly the same, the same tires used on the same pressure (exactly!) thus exactly
the same rolling resistance. And most importantly in this case, the total weight is the same. With
all the rest truely the same (takes some effort ;-) a bit extra weight increases the gravitational
pulling power and consequently the equilibrium speed (speed where all resistances summed make up for
the gravitation). And guess which will be (slightly) heavier: the shorter or the longer bike....
Indeed. Unfortunately, you'll get punished for this little gain on the other side of the mountain

Kees, Hurricane on Stelvio's as fast uphill as my DF, but o so much faster down, van Malssen

Andrew Douglas said:

I think that speed down hills has more to do with overall handling characteristics than with any
one factor.

FWIW, my Wishbone Classic is a lot more secure-feeling at really high speeds (40+) than my
Pharobike.

The Wishbone has a much longer wheelbase and a much higher center of gravity than the itty-bitty
Pharobike.

Maybe it's just altered perception from being so low, but once I get the Pharobike up to the high
30s I don't really *want* to go much faster. I'm having enough fun. I think I hit 41 or 42
once....

My Sunset is very similar in overall configuration to the Pharobike LowFat. I find that the handling
becomes more stable with speed, even when I am pedaling at a cadence of 140+ rpm [1]. I am perfectly
comfortable descending at 50+ mph, so I suspect the issue probably one of steering geometry
differences between the two bikes.

[1] I spin out at about 160 rpm.

Tom Sherman - Various HPV's Quad Cities USA (Illinois side)