Proof that lowracers are real fast

[Ryan Cousineau]

In article <[email protected]>,
[email protected] wrote:

> Robert Chung <[email protected]> wrote:
> : completely different story. I don't think any of the tested bikes were lowracers. In any event,
> : I'm not sure why putting a rider closer to the ground would affect CdA. If you took exactly the
> : same bike and held it up on a stick in the air, do you think the CdA would change?
>
> Low seat on lowracers puts the wheels in the body shadow, maybe that is what makes the difference
> between M5 Low Racer and Shock Proof: http://www.m5-ligfietsen.com/english/snelheid2.htm
>
> Also winds are supposed to be weaker close to the ground.
>
> Trikes are a different chapter altogether. I think if your trike only has about 5 cm of ground
> clearance, maybe that means there can't be very large swirls of air below you to slow you down.
> Why do you think F1 cars are so low? :-/

F1 cars are so low because they're trying to lower the centre of gravity for better handling. They
also would love to pick up some ground effects with a shaped undertray, as in CART, but the current
plank and undertray height regulations make ground effects minimal.

Lowracers definitely do NOT want ground effects of the same type as race cars use, since they induce
drag and slow the vehicle down (straight-line speed losses are more than paid back by corner speed
gains, though).

If lowracers are faster than standard bents, (and the chart you reference above shows about a 2%
difference between the unfaired M5 lowracer and the more upright variant) I suspect the difference
comes down to frontal area and slightly better drag coefficient for some reason. But that test
suggests the effect is very slight.

--
Ryan Cousineau, [email protected] http://www.sfu.ca/~rcousine President, Fabrizio Mazzoleni Fan Club

 

[Risto Varanka]

Ryan Cousineau <[email protected]> wrote:
: Lowracers definitely do NOT want ground effects of the same type as race cars use, since they
: induce drag and slow the vehicle down

Ok, how do you get or avoid ground effects?

: If lowracers are faster than standard bents, (and the chart you reference above shows about a 2%
: difference between the unfaired M5 lowracer and the more upright variant) I suspect the difference
: comes down to frontal area and slightly better drag coefficient for some reason. But that test
: suggests the effect is very slight.

The Shock Proof is quite like a low racer, AFAIK it only has a seat that is located higher and full
suspension. Seat angle is probably very similar. Suspension could screw up the test results though
if they didn't take it into account as a rogue variable

--
Risto Varanka | http://www.helsinki.fi/~rvaranka/hpv/hpv.html varis at no spam please iki fi

 

[Robert Chung]

"Ryan Cousineau" <[email protected]> wrote in message
news:[email protected]...
>
> If lowracers are faster than standard bents [...] I suspect the difference comes down to frontal
> area and slightly better drag coefficient for some reason.

Well, I agree with your suspicion about the frontal area. I'm less sure about the drag coeff.

 

[Tom Sherman]

[email protected] wrote:
>
> ... Also winds are supposed to be weaker close to the ground....

In my experience this difference is very significant - so much so that only lowracers and low trikes
make sense in flat windy areas. [1]

[1] To reverse Jon Isaacs' [2] argument that recumbents do not make sense in areas with significant
geographical relief.
[2] Not only have I observed others riding upright bicycles, but I have also actually ridden several
of them extensively.

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

 

[Ryan Cousineau]

In article <[email protected]>,
[email protected] wrote:

> Ryan Cousineau <[email protected]> wrote:
> : Lowracers definitely do NOT want ground effects of the same type as race cars use, since they
> : induce drag and slow the vehicle down
>
> Ok, how do you get or avoid ground effects?

Well, "ground effects" is a pretty broad term in aerodynamics, and I don't want to abuse it too
egregiously.

When you want to increase the downforce on a vehicle by using ground effects, you shape any vehicle
surfaces that are close to the ground, like the undercarriage (I know, duh). I think the current
state of the art is to put a big concave tunnel of some sort under the car, with the goal of
inducing a vacuum or other magic. The reason for doing this is that ground effects tend to have a
better downforce to drag ratio than ordinary wings (for race cars, downforce is generally good, and
drag is generally bad, but most race cars will sacrifice a lot of the latter to get the former).

One classic solution was on the Chaparral race car: have plastic skirts on the side of the car, and
a big sucking fan evacuating air from under the car. Think hovercraft in reverse. The only reason
this isn't universal today is because it was universally outlawed.

A lowracer has virtually no use for ground effects, because lowracers don't have horsepower to
spare. So streamliner speed-record vehicles generally avoid downforce-creating shapes. Lift is also
bad, though, because it reduces traction and makes the vehicle unstable. Really fast land-speed
machines (like the Thrust SSC, but also lesser classes) have to worry about having some downforce
simply because their speeds are so great that almost any shape threatens to become airborne.

An empirical guess is that if you want to see what a downforce-avoiding shape looks like, look at
current streamliners.

> : If lowracers are faster than standard bents, (and the chart you reference above shows about a 2%
> : difference between the unfaired M5 lowracer and the more upright variant) I suspect the
> : difference comes down to frontal area and slightly better drag coefficient for some reason. But
> : that test suggests the effect is very slight.
>
> The Shock Proof is quite like a low racer, AFAIK it only has a seat that is located higher and
> full suspension. Seat angle is probably very similar. Suspension could screw up the test results
> though if they didn't take it into account as a rogue variable

I think the most important aerodynamic difference between the Shock Proof and the M5 is that the M5
seems to have less frontal area. Since the wheels have to touch the ground, and the easy way to
reduce the frontal area of something like the Shock Proof is to somehow put the rider in the
"shadow" of the wheels, the rider has to get lower to the ground. There is also an aerodynamic
tendency towards long, slender shapes, though how much that gets screwed up by using that theory on
a totally non-aero human and hpv is a question for the wind tunnel.

horses for courses,
--
Ryan Cousineau, [email protected] http://www.sfu.ca/~rcousine President, Fabrizio Mazzoleni Fan Club

 

[Risto Varanka]

Ryan Cousineau <[email protected]> wrote:
: A lowracer has virtually no use for ground effects, because lowracers don't have horsepower to
: spare. So streamliner speed-record vehicles generally avoid downforce-creating shapes. Lift is
: also bad, though, because it reduces traction and makes the vehicle unstable. Really fast
: land-speed machines (like the Thrust SSC, but also lesser classes) have to worry about having some
: downforce simply because their speeds are so great that almost any shape threatens to become
: airborne.

Thanks for all the explanation. Maybe some extra downforce would make sense on criterium-optimized
fully faired trikes, but I don't know if anybody even started building those yet

: I think the most important aerodynamic difference between the Shock Proof and the M5 is that the
: M5 seems to have less frontal area. Since

Well the Shock Proof and the Low Racer both are bikes made by M5. )

--
Risto Varanka | http://www.helsinki.fi/~rvaranka/hpv/hpv.html varis at no spam please iki fi