Cycling UK Forum

pwa wrote:I freewheel downhill faster than other members of my family when we cycle together but I've always thought that was mostly due to me being heavier

I'm the biggest in our group, always fastest down but slowest back up

NEvans wrote:

pwa wrote:I freewheel downhill faster than other members of my family when we cycle together but I've always thought that was mostly due to me being heavier

I'm the biggest in our group, always fastest down but slowest back up

Yes, that's the way it goes. Those of us who carry a touch of excess weight can partly compensate by acquiring descending skills to make the most of what gravity does for us when we point the bike downhill.

Mick F wrote:My guess is better tyres, but it's only a guess based on what I see.

I would have thought that Aerodynamics would be the first order term, followed a long way out by rolling resistance, which would be tyres and hubs. As you are renown for having a shiney clean chain on your bike I would surmise that your hubs may be better serviced and spin better than your club mates.
?

whenever you go downhill, weight is the most important thing; basically you have a gravity engine and the power is pro-rata with weight. Next comes aerodynamics (moreso at speed), then rolling resistance.

Things like bearings need to be very knackered or badly adjusted indeed before they make an appreciable difference.

cheers

Brucey wrote:whenever you go downhill, weight is the most important thing; basically you have a gravity engine and the power is pro-rata with weight. Next comes aerodynamics (moreso at speed), then rolling resistance.

Things like bearings need to be very knackered or badly adjusted indeed before they make an appreciable difference.

cheers

Coming off Fleet Moss heading towards Hawes on 28mm tyres some time back I baulked below terminal velocity (my Garmin record says 48.5mph), & started to check my speed a bit with a little braking, because of the bouncing around I was getting from the poor road surface.

We're going back that way in the summer (staying near Kettlewell) & I may well try going that way again to see how the Kona with 40mm Hypers does.

I accept my theoretical maximum velocity may be a little lower due to the slightly worse aerodynamics of the wider tyres. But my actual speed achieved could end up being higher due to the smaller effect of speed over the rough surface on bike control. That's always assuming I get similar conditions, which is always a problem for single tests on real roads.

Brucey wrote:whenever you go downhill, weight is the most important thing; basically you have a gravity engine and the power is pro-rata with weight. Next comes aerodynamics (moreso at speed), then rolling resistance.

Not sure I agree. Terminal velocity is when Weight = Drag, Drag is proportional to Frontal Area and Velocity^2. I would expect riders & bikes to vary in mass by about 30% (except for extreme circumstances) which you should be able to match by changing your position on your bike. ... though if you are chatting on a descent your position will probably not vary by 30%...

belgiangoth wrote:

Brucey wrote:whenever you go downhill, weight is the most important thing; basically you have a gravity engine and the power is pro-rata with weight. Next comes aerodynamics (moreso at speed), then rolling resistance.

Not sure I agree. should be able to match by changing your position on your bike.

I am a heavy rider and my decent speed is clearly visible compared with lighter riders. Variation of position does then make noticeable differences in downhill speed, but I agree with Brucey in weight is the more important factor.

Difference is when you watch the professionals on TV, there I think it all comes down to technique and ability (and guts!).

Brucey wrote:whenever you go downhill, weight is the most important thing; basically you have a gravity engine and the power is pro-rata with weight. Next comes aerodynamics (moreso at speed), then rolling resistance.

Things like bearings need to be very knackered or badly adjusted indeed before they make an appreciable difference.

cheers

Is it, explain how? If I'm sat up with a billowy jacket, someone 20kg lighter than me with a smaller frontal area and decently snug outer garment plus aero position will absolutely go past me on a downhill, I'm at odds with your theory, please post your findings as to how you came to such a discovery.

Your velocity will increase until your weight is equal to the drag force. Drag force is proportional to the square of the velocity and proportional to the area. So if I weight 10% more than the person next to me (including the bike) but take up 10% more area we should descend at the same speed.
I would hazard that if two people are descending together they will have roughly the same position on the bike, but from there it gets trickier as more massive people are not necessarily larger in cross sectional area.
You also have to consider aerodynamics and how air flows, but I would again assume that there would not be that much in it when out on a club run.

As the tyre manufactures will say:"Let confusion rain long and hard on this topic, And may opinions never meet."

belgiangoth wrote:[So if I weight 10% more than the person next to me (including the bike) but take up 10% more area we should descend at the same speed

But you are unlikely to take up 10% more area because the weight is cubed but the surface area is only squared, so generally heavier riders will be proportionally less affected by wind resistance than lighter riders.

I use 32mm up to 38mm on my bike but mostly 32mm of late. I run them at about 80psi. I found it was uncomfortable on a rough canal path so I lowered the pressure, turned out to be 65psi when I pumped them up again and read off the pump's gauge. The increase in comfort was clear to me. My speed wasn't much different. I guess there's other factors limiting that.

BTW if changing your tyre width and pressure is noticeable then does that mean you're a slow cyclist? I mean I thought rolling resistance is significant up to 15mph but above that it's aerodynamics that has the biggest effect. If true then I'd use good 32mm width tyres and play around to get the best pressure for your weight. I reckon I'd pump them up towards the higher end then ride it. By reducing the pressure a little at a time and retesting it with a ride you'll sooner or later hit on the highest pressure that gives you good comfort. Read this off and keep your tyres at this pressure.

The utility cyclist wrote:

Brucey wrote:whenever you go downhill, weight is the most important thing; basically you have a gravity engine and the power is pro-rata with weight. Next comes aerodynamics (moreso at speed), then rolling resistance.

Things like bearings need to be very knackered or badly adjusted indeed before they make an appreciable difference.

cheers

Is it, explain how? If I'm sat up with a billowy jacket, someone 20kg lighter than me with a smaller frontal area and decently snug outer garment plus aero position will absolutely go past me on a downhill, I'm at odds with your theory, please post your findings as to how you came to such a discovery.

In theory if you have two objects of the same shape and volume but different mass, inertia will slow down the acceleration of the heavier object, so it will not have that much of an advantage on descending initially, but once it gets going it will carry the speed further as the slope eases because of the advantage that mass gives via momentum. I find that when I ride with lighter family members I have to sit up on descents to avoid leaving them behind.

gravity doesn't work like that; the force is in proportion to the weight and the acceleration is constant.

cheers

Brucey wrote:gravity doesn't work like that; the force is in proportion to the weight and the acceleration is constant.

cheers

Yes, to be clear what I am saying is that while we might expect a heavier object to accelerate faster, inertia means it doesn't, it accelerates at the same rate as a lighter object of the same volume and shape. But terminal velocity is higher for the heavier object. Drop a ping pong ball off a skyscraper and when it hits the ground it will be travelling slower than a ball bearing of the same size.

We have a few very long descents around here where I top out at about 40mph, when wind resistance stops me going faster. My Missus tops out a little slower than that.