PhilO Gearing makes a huge difference. If I was to go down the same hill without peddling like mad on the top section I doubt I would get above 45mph.

I can see that it might make a difference on some hills – I assume that your favoured hill has a gentle run-in and a steep but short section towards the end? IME most good hills allow you to reach terminal velocity before you run out of road, no matter what your launch speed (see my experience of the SS equalling the road bike on my local hill). YMMV, however… 8)

Certainly weight has a big impact on initial acceleration when you first point downhill. At 15-15.5st I really notice this, especially when skiing. I can easily out accelerate far better skiiers from start, and carry momentum easier / further across flat sections (if I get right down on the skis).

Type of bike and air resistance will be more important for sustained speed, surely?

Weight makes no difference to downhill speed (or a very very small amount of difference). Type of bike makes a big difference just because you can get more aero on a road bike.

I'm 8.5 stone. Even on my road bike that is about 3 sizes too big for me, where I can put my mouth on the stem very easily, I still get left for dead on descents. Long, straight descents are where its most obvious, I'll pop it into top gear, thrash it until I spin out, then get super aero, and everyone else comes past me freewheeling, on the hoods. Its annoying, but its the laws of physics.

Getting more aero on a road bike? Not necessarily, as mountain bikes usually have a lower top tube, so when you sit on it (under the saddle), you can probably get as, or even more, aero then a road bike.

Basically, if you sit someone on a road bike, and send them off down a hill, their speed will increase if they tuck in, but not as much as if they suddenly ate an entire greggs.

On my roadbike downhill 40mph scares the death out of me when cars pull out 😀

i think these suggestions that weight affects acceleration and speed down hills go against a fundamental law of physics. obviously assuming that things like bearings resistance and rolling resistance impact negligibly on different rider weights.

The hammer and the feather

Potential energy = mgh. Two riders at the top of a hill, the fat one has more potential energy.

They start going down the hill, the potential energy gets turned into kinetic energy. mgh = 0.5mv^2

The feather hammer thing works because the masses cancel, to give gh = 0.5v^2

But you have to take in air resistance.

Gravity provides 9.8N acceleration per kg. ma = F

The fat rider will have a bigger F then the slim rider. The air resistance difference will be very small.

So someone 100kg will have 908N pushing him down the hill. Someone 50kg will have half that, 454N. Take away maybe 30N for the slimmer rider from air resistance, and 40N from the fatter rider, then divide by their masses.

Fat rider has acceleration of 8.68ms^-2

Slim rider has acceleration of 8.48ms^-2

you've confused me with science, there. 😆

now i want to believe you because you have used letters in place of words, which always looks convincing. but i don't know what to think. 😕

I realised I hadn't really explained myself properly, I've edited, have another look.

This is based on real world experience, and I then proved it using physics to myself.

60mph on a road near Hawes on me HT, (combination of long hill and cat'n'mouse drafting with chums). Low 50's mph offroad on a track in Plymbridge, (combination of big rings and youthful exuberance).

There's also a road near Lopwell Dam where if can hit 25mph on the first corner and hold it, you'll freewheel to 50+ without trying. Huge run off as well.

The force pushing you down the hill is due to your weight and the incline. Without wind resistance your size wouldn't matter.

However wind resistance is proportional to speed.

Terminal velocity is when you are going fast enough that the wind resistance is equal to the force produced by your weight

if you double the weight of a person you do not double their wind resistance – hence they reach a higher terminal velocity.

theflatboy – if you need convicing turn the argument around.

Who goes uphill quicker – heavy rider or light rider??

If I have to carry laptop and / or files on my commute, the climbs are significantly more strenuous, but the bike wants to run away more on the downs

ETA, which of course only reflects the energy required to get the heavier mass to an elevated posn – ie higher potential energy position, but ignores the air resistance effects encountered going DH, as very clearly explained by TJ…. which also explains a tandem's rapidity of descending!!

Who goes uphill quicker – heavy rider or light rider??

Yeah, but that's about your power to weight ratio, where weight is what's holding you back.

Downhill its your weight to wind resistance ratio, where its the wind resistance holding you back.

TJ explained it nicely, better then me.

TJ explained it nicely, better then me.

Indeed,explained very well by TJ.

I edited to cover my ommision of air resistance. But, the climbing analogy is not just down to the power:weight discrepency.

As you have explained, a more massive body at a given elevation will have more potential energy than a lighter body at the same elevation – hence more energy required to get it there in the first place…

As you have explained, a more massive body at a given elevation will have more potential energy than a lighter body at the same elevation – hence more energy required to get it there in the first place…

Power is defined as joules per second. Its near enough the same thing.

Mentioning tandems, there's one thing I never quite understood. Why aren't tandems quick up hill? They have twice as much power as one rider, but not twice as much weight (I'm assuming a tandem is lighter then two bikes), and much less air resistance then two riders.

RealMan – I cant see how your argument is any different to mine…

Yeah, but that's about your power to weight ratio, where weight is what's holding you back

Power is defined as joules per second. Its near enough the same thing

So, as I am saying, it takes more energy to get a heavy object from 10m to 100m. And conversely, descending, a heavy object has more potential energy to convert to kinetic energy.

Regarding power:weight ratios, presumably that's down to the biomechanics of the human form.

IF the riders had the same power:weight then you might expect the hill climb time to be the same (but the heavier rider would still expend more energy). Human physiology being what it is, the heavier rider would normally have lower power:weight than a lighter rider, hence the longer climb

Real man – there is no real weightsaving with a tandem – ours is around 50 lbs. Wind resistance is negligible at low speeds.

So why are they so much slower?

59mph on road ,dont know offroad as I don't use a computer

ridden by fat biffers?

Back when I rode a mountain bike with (relatively) skinny tyres, a big ring and long barends I had a wired computer that was pretty accurate, average speed compared with the time taken to cover known distances, and maximum speed corresponded to other vehicles, I hit 42mph twice on long downhills, but never managed higher, and once nearly had an aneurism after pedalling like a nutter down the slight hill I live on and overtook a Fiesta at the bottom just before the road flattened out. Saw a rear passenger do a comedy double-take out of the corner of my eye as I went past. Had to slow and turn off just after which was good 'cos I was getting flashing lights in front of my eyes and a serious case of hyperventilating. The same computer showed 32mph, and the car had to have been doing over 25mph, so my speed is plausible. The bike was a Cannondale SuperVee 3000 with Judy's on and those incredibly long barends that Cannondale fitted, and the bike weighed 28lb.

So lowey overtook me because he is fat …seems plausible to me.
Re accuraccy no real idea but the bit we went down was long steep downhill that was bowling green smooth and had nothing there[stationary or moving] to tell relative speed. We all realised when we tried to brake that we were really shifting and fair shredded our brakes. Think two of us had to change pads iirc.I assume the GPS is accurate and even a 20% error gets 40 mph [or 60mph!]

My maximum speed this year is 48mph (grid ref ST183321) and my average maximum speed for the 78 rides I've done this year is 39.8mph. Living at the bottom of a three and a half mile (17% in places) climb to the top of the Quantocks means that most of my rides (and my commute) finish with a long fast road descent.
There is another road descent off the Quantocks (dropping down into Crowcombe grid ref ST144368) where the GPS tells me I've done 63mph in the past (more than once).
The best I've managed offroad is 38.8mph (Triscombe Combe grid ref ST157358).

About 45mph-ish on a mtb. Down quite a long and steepish road, pedalling all the way.

the zoofighter would pwn you all, you know. On dh tyres and everything.

Oh, 35mph or something lame like that for me!

54mph on a skinny slicked hard tail on the road
51mph on road bike
43mph on a forestry commission trail.

being around 15st, I do find in general, my terminal velocity is higher than other riders in the groups.

Interesting calculator to work out your speed. Enter negative numbers for gradient and headwind to get descents and tailwinds.

Managed about 51mph down the Devils Elbow with a tailwind on loaded touring bike.

Never seem to get more than about 30mph offroad on the mtb before I wimp out.

Above about 50mph on the road, on a decent descent it starts to become less efficient to pedal than just to tuck and freewheel, as the turbulence created by your moving legs slows you down more than they are putting in

Far less than that – I'd suggest by 40mph you're definitely better off tucking for any normal mortal. It's not the legs going round either, but that fact you can't pedal in a proper extreme tuck.

Why aren't tandems quick up hill? They have twice as much power as one rider, but not twice as much weight (I'm assuming a tandem is lighter then two bikes), and much less air resistance then two riders.

It's a bit of a myth that tandems are slow up hill IMO. Given two evenly matched and compatible riders they should go up the hill just as fast on a tandem. It just seems like it's slower because the same two riders will go faster on the flat than they would on solos, hence the difference between flat and climb is more marked. Of course there are some inefficiencies involved in riders fighting each other, but this is pretty minor. Certainly when I ride the tandem with mrs aracer we go slower up hills than I do on my own, but faster than she would on her own. On the flat we do a very similar speed to what I would on my own, but a lot faster than mrs aracer would.

88 km/h offroad (quad track/fire road) in poland.

<random info>
I did a multisport race a few years back, the Goldrush, and the tandem category was indeed that – you rode a tandem on the road sections, paddled a double kayak but MTB separate bikes.

Day 3 started with a 50k roadie paceline of 9 tandems, **** me, awesome. We smashed the roadies on their TT bikes, i think our average speed was somewhere high 40's. It was on that ride i saw 102 on the way down to the Clyde dam (massive long steep downhill on a nice smooth road). Climbing the roadies would sit in behind us and the strong ones would give us a bit of a tow, near the top they would just pull out and tuck in behind us wibbling when we got over 90kmh. Great fun.

46mph is the fastest I know I have done through a speed trap, thats offroad during a DH race.The fastest guy (Josh Bryceland) that day hit 57mph through the trap.
Don't have a speedo but I have probably been faster.

On the MTB the fastest I know of was 48mph on the road down from the top of the marin trail.

42.5mph on the road down from lugnaquilla into glenmalure, although I reckon it was a bit slower as it used to overegg my 15mile commute by a mile.

About 42mph on the mountain bike on road. The local fast road downhills are quicker on the mounain bike than the racer as the surface is so bad.

My maximum speed this year is 48mph (grid ref ST183321) and my average maximum speed for the 78 rides I've done this year is 39.8mph. Living at the bottom of a three and a half mile (17% in places) climb to the top of the Quantocks means that most of my rides (and my commute) finish with a long fast road descent.
There is another road descent off the Quantocks (dropping down into Crowcombe grid ref ST144368) where the GPS tells me I've done 63mph in the past (more than once).
The best I've managed offroad is 38.8mph (Triscombe Combe grid ref ST157358).

I know your hill you must be well fit riding up that all the time.I always find time spent in The Carew Arms affects my ability to ride up it well

49.6mph on a road bike, tucked in, going down a long, steep, straight hill with a good run out at the bottom (A53 from the Roaches into Leek, if anyone knows it).

I'm pretty confident that I would never be able to ride my hardtail off-road anywhere near as fast as that.

Here's what 60mph looks like
http://www.youtube.com/watch?v=0tFpNsZXWgc&feature=related

53mph on roadbike with 25mph tailwind down a gert big hill cranking it to the mAX, It's fookin scary! can only do 35-40mph without the tailwind on same hill.