It would seem more weight would be faster for downhill...

machinebike

It would seem.
A 25-30 lb bike
vs
A 40-45 lb bike....

Who would reach the bottom of the hill sooner?

Briggo

Few other variables to add into the equation.

Add them in and we may be able to comment.

fat daddy

Depends on the hill ... one with lots of changes in direction and I would take the lighter bike as it will change direction easier and require less bleeding of speed before the bends

Long downhill, straight with flat sections and poor surface, I'll go heavy to maintain speed.

Also how big is the rider ... a lightweight won't be able to manhandle a heavy bike down a course as quickly as he would a light bike

Why is the Heavy bike heavy ?? ... frame, fatter tyres, more suspension ?

cooldad

The one with the best rider.

cooldad

In reality, as the combined weight including rider would be relatively smaller, the lighter bike any time, all other things being equal.
Unless you consider riding over a cliff DH, although once terminal velocity is reached, all bets are off.

Giraffoto

All other things being equal - and they never are, but what the hey - the lighter bike will be more manouevrable and the lower unsprung weight that it represents will make for better grip and traction. So the lighter bike it is. Downhill bikes are heavy because they're strong - if weight was desirable, DH competitors would ballast them, and I'm pretty sure none of them do that.
Weight doesn't make you go downhill faster. A 20Kg bike has twice the gravitational force pulling it down that a 10Kg bike has, but it has to accelerate twice the mass. Acceleration = Force applied/Mass, so both will experience the same acceleration due to gravity.

The Rookie

Giraffoto wrote:

Acceleration = Force applied/Mass, so both will experience the same acceleration due to gravity.

As the weight doesn't change the aero, then the heavier bike will have a higher terminal velocity as the force increases but the aero drag hasn't.

rockmonkeysc

It depends on the rider and the track.
Strong rider on a short, steep track then heavy would probably not be slower but add in rider fatigue on longer tracks due to cornering and getting back up to speed then weight makes a difference. Also consider that most tracks have flat sections where light weight helps. A light bike is easier to Corner quickly and get airborne over rough ground.
It does get to a point where a really light bike feels less stable over rough ground.
If I was riding Redbull Hardline or Rampage I'd take a 45lb Brooklyn Racelink. At Fort William I'd go for a 32lb Mondraker Summum carbon.