[Closed] Nearly cracked 50mph today...

Bloody hell, I'm still on a quest to break 50mph, and I've ridden most of the big roads in Scotland 🙁

So what are the factors here, body weight? Tyre pressure? How can folk hit 55mph on MTBs when I'm tucked with my belly on the nose of the saddle coming down the Kenmore road and still just managing 47mph?

Swear I would have broken 50 on the Lechtd road if the crosswind hadn't been trying to lift my front wheel off the road...

edit: weird double post there.

Compacts are for women only arent they?

I think body weight makes a huge difference. You only get a small increase in frontal area thus drag but a large increase in the force pulling you down the hill for the extra lard some of us carry. The tandem is noticeably quicker for this reason

A good tuck helps as well- head low but arse in the air gives good results.

A tailwind makes a huge differnce - even a few mph as at speed almost all the drag is aero

51mph down the Graine Road from Bury to Blackburn.

First ride out on my new (to me) road bike, with 16 year old alloy forks 😀

Used to love doing this as a kid, but the above just scared me.

Bacup Rd on the MTB sounds interesting.
I take it that the straight drop to Bacup is faster than the curvy descent to Tod?

48.8 is the highest my speedo's ever read, somewhere on the Dartmoor Classic- no doubt if you knew the road you could easily get >50 but I didn't arrive at the right speed to do it.

On that particular bit of road it wasn't scary but I don't think there are many places in the UK I'd want to go much over that, and I'd probably be terrified at 60 even if I could get up to it.

I think body weight makes a huge difference. You only get a small increase in frontal area thus drag but a large increase in the force pulling you down the hill for the extra lard some of us carry. The tandem is noticeably quicker for this reason

That sport of speed must be absolutely terrifying for the stoker!

Hmm, about to prove myself completely ignorant of the laws of force and acceleration here, but I thought your acceleration due to gravity remained the same regardless of your weight (i.e. 9.8m/s2) and that heavier folk would merely accelerate slower down the hill to the same top speed? Didn't Galileo prove this with an orange and a cannonball?

Anyway, apologies, I've dragged a good thread into physics and engineering...

edit: dammit, contradicted myself, you can't 'accelerate slower'... Now I'm really confused.

I once hit 55 on the road from Lochranza to Sannox, on Arran.

Thats my fastest ever location too. Don't think I managed 55mph though, was catching my mate fairly quickly who claimed at top speed of 45mph so call it 48mph.

Above 30mph on a bike feels fast, above 40 feels bonkers. Maintaining 50mph + on Alpine descents must take balls of steel

13th - yes the acceleration due to gravity in a vacuum is constant and the same regardless of the weight of the object.

Its when you add in wind resistance that it changes. We are also talking abot terminal velocity not acceleration.

A fat bastard has a much larger force pushing him down the hill - but only a bit larger wind resistance so the acceleration is the same until wind resistance becomes a factor and the terminal velocity will be higher.

think parachute v lump of lead

Hmm, about to prove myself completely ignorant of the laws of force and acceleration here, but I thought your acceleration due to gravity remained the same regardless of your weight (i.e. 9.8m/s2) and that heavier folk would merely accelerate slower down the hill to the same top speed? Didn't Galileo prove this with an orange and a cannonball?

While this law is true, it only holds in a vacuum or if we ignore the effects of air resistance. Think of a big feather weighing 10grams and a ball bearing weighing the same, which will fall faster?

Galileo used different sized cannonballs I think, so whether he intended to or not, he was cancelling out the effect of air resistance by using objects with different mass but the same density and shape

I hit 49.9 mph going down Old Wyche Road in Malvern on an mtb, I think it's a bit too lumpy to do it quickly on a road bike...

I've hit 51.8 on rollers does that count? 😀

Hit 51mph going down Butser hill offroad (between Portsmouth and Peterfield literally E of the A3) a couple of years ago. I had the advantage of being about 17st kitted up plus a tailwind. not scary at all for some reason!
My previous max was 49.5mph on my old Marin coming down the Radar station rd from the top of the Pennines to Knock (E of Penrith) that was scary,

Right, right, I've figured it out. 😀

I'm ignoring stuff about wind resistance, assuming thats the same for everyone.

The cannonball analogy doesn't work here because that is objects in freefall, and the force applied to them is the force DUE to the acceleration of gravity (i.e. they both accelerate at 9.8, however the heavier object experiences a greater force to achieve this).

The difference with bikes is the reaction force from the road. your acceleration due to gravity pushes you into the road. The heavier you are, the more you push into the road. The road pushes back at an angle perpendicular to the road surface. On any road that slopes downhill, a proportion of this 'push back' will in fact be 'push along' and THAT is the acceleration you feel on the downhills.

Difficult to explain without diagrams, high school physics though... 😳

13th - try wiki on [url= http://en.wikipedia.org/wiki/Terminal_velocity ]terminal velocity[/url] teh only difference for us is that on a slope you only get a % of the downward force but everything else remains the same as a freefalling object in air

I'm ignoring stuff about wind resistance

once went down a hill (with howling tailwind) on the drops, 52x13 on 700c wheels, REALLY going for it, max was 49.9mph.
properly gutted that i hadnt broken 50.
fast forward a decade (to the late 90s)
went over the top of a hill on the A689 alston to bishop auckland road, on an aging mtb with tioga psycho tyres, panniers, (with 2 dozen eggs in em!)
no front brake and the rear mech screwed into the middle of the block due to a snapped cable.
i knew i was really shifting, big tailwind too, eyes streaming because of the wind, got further down and as the road levelled i glanced down to find i was still doing 55.
max recorded was 62mph.
i am still slightly gutted that i didnt do 100kph, but tbh, i doubt i will go that fast on a bike again.

If so you're missing the whole point

No I'm not, on my particular little tangent I'm trying to figure out why, for two otherwise identical riders, the heavier would be faster on a downhill, which is why I'm ignoring wind resistance.

Basically, its correct to say that the heavier rider has a bigger advantage due to gravity, but its not actually gravity thats pulling him down the hill, its the road 'pushing' him down the hill 8)

Edit: I'm descending into needless pedantry here, step away from the thread!

"Can recommend the road from Dunoon to Loch Striven for high speed thrills. Though the front end coming off the ground over some crests is a bit interesting !"

StuCol - Agreed, I was brought up living on that road. There are some pretty quick bits!

13th - nope 'tis the wind resistance that makes the difference in the terminal velocity - read the wiki link

The difference with bikes is the reaction force from the road. your acceleration due to gravity pushes you into the road. The heavier you are, the more you push into the road. The road pushes back at an angle perpendicular to the road surface. On any road that slopes downhill, a proportion of this 'push back' will in fact be 'push along' and THAT is the acceleration you feel on the downhills.

Nope. Sorry wrong I'm afraid, if you ignore wind resistance then 18stone rider will accelerate at the same rate as a 9stone rider, the physics works the same as in freefall

Having ridden the same road a number of times and getting various different top speeds (50-61mph) I think the things that make the difference is wind direction and how much speed you create on the bike before you run out of ability to pedal. eg on the same hill when my legs are knackered I've never got a high speed, but with fresh legs you can spin out quicker ie more towards the top of the hill so you have already given gravity a big helping hand.

Definately having a big gear helps. I've only ever got over 60 on a 53 - 12

Yep, but I'm not arguing terminal velocities, as they would be equal for two riders who were identical in every respect bar weight. Hence the cannonball/orange experiment.

I did 54.9mph down this (51°39'37.84"N 3°26'23.53"W) got speed wobble and sh*t my pants somewhat, speed camera wend off at the bottom with me doing about 48mph.

I'll be take it easy from then on, coming off at that speed just doesn't bear thinking about.

*waves white flag* I've argued myself into a circle... 😕

I see where you're coming from TJ, top speeds ~ terminal velocities, as you're at the point where you can't usefully propel yourself any further.

Still not convinced about the effect of rider weight...

"I'll be take it easy from then on, coming off at that speed just doesn't bear thinking about."

Why? Its more likely to mean less broken bones than if you fall of at 10mph. Of course there would be no skin left on your body...

Have hit around 50 on the bit of road between Abinger and Westcott on the road bike. Didn't feel too bad as the road is smooth like a snooker table. 40 off the top of Ranmore and I was not happy at all; very nervous in fact.

You mean Hollow Lane? From Abinger Common down to the A25 near the Wootton Hatch? I pinch flatted coming down there in the pouring rain on the road, by the houses at the bottom. A pinch flat at 40 is a lot scarier than not pinch flatting at 60!

I've regularly done 50-something coming down from Coldharbour to Dorking. Was noticeably quicker with some 50mm deeps. That they had a penchant for rolling fully inflated tyres was something I tried to forget!

IMO it's the gradient as much as the length that's key for good top speed. Did c53 in a 40 (setting off a speed camera) on the A283 on a fairly innocuous looking hill. PB was in Lanzarote, perfect road really, absolutely dead straight, wide, perfect surface and a good gradient. Forget the exact figure, 60+. Felt less scary than some of our roads at 50 though! That was in a very tight pace line with some nutter triathletes. Annoyingly we got to the bottom, round the roundabout and came back up again, 2 miles of utter torture!

Not really sure about folk thinking it's harder/more scary on an MTB though. When your hands are right by the stem, and your nose is on the bars it's fairly irrelvant what shape your frame is!

41mph down Mastiles Lane descent earlier this year. Yes, that's off road for those that don't know.

Okay 13thfloormonk I'll try and explain.

We will use freefall as an example.

So you have an 80kg sky diver gravity acts on him with a force of 800N (with some rounding) He jumps out of the plane and initially and accelerates at 800 / 80 = 10ms2 (accelaration being equal to force divided by mass). However as his speed builds so does air resistance. It builds until the point where it equals the force of gravity on him, so he stops accelerating, he has reached terminal velocity.

No imagine we stick 20kg of lead on his back and he is falling in the same position, Gravity is now exerting a force of 1000N on him therefore the air resistance requred to stop him accelerating is also 1000N, in order for the air resistance to equal 1000N he needs to be falling a bit faster so his terminal velocity will be higher.

Same basic principle holds true for blokes on bikes freewheeling down a hill

[url= http://www.sportsci.org/jour/9804/dps.html ]Cycling specific explanation of the physics behind climbing and descending[/url]

Man, road bikes are exciting.

Eh... really? Gravity is not a force, its an acceleration. Acceleration due to gravity is 9.81 m/s2.

In your example the 80kg sky diver will experience a force (due to the accleration of gravity) equal to 80*9.81=785N. The 100kg skydiver will experience a force of 981N. The net result however is that the two skydivers will both accelerate at 9.81 m/s2.

What you are suggesting (heavier falls faster) is in direct disagreement with established principles (or at least, what I have assumed are established principles from memory).

13th - add in the effects of wind resistance than given the same frontal area a heavier object will reach a higher terminal velocity

TJ, I agree, when descending on a road. However, this would not happen in freefall, as has been proven repeatedly (experiments on the moon with a hammer and a feather, air resistance non-existent, both objects land at the same time).

I wasn't actually disagreeing with the fact that heavier riders could achieve higher speeds on descents, I was just trying to establish how, and, for fear of turning this into a thread that never ends, still believe that my intial argument is correct.

Try the cycling specific paper above ^^^

The thing that stops me, is my confidence in the brakes (excuse the pun).

DH bike - hydraulic brakes, 9" front rotor, 8" rear rotor, 6-pot calipers (plus nice rigid dual crown forks.. + 20mm axle..)

Road bike - mechanical calipers + rim brakes!!!!

I've had great fun bombing down some hills up in the Lakes with the DH bike - knowing I can stop pretty quickly, even with just one finger... but on a road bike?? No chance!!

Sorry KCR, I did, meant to refer to it in my last post.

It confirms that yes, heavier riders will go faster downhill. But it just says 'because of their greater mass to area ratio'. Suggesting that ultimately, being heavier helps. Doesn't explain HOW.

I'm aware I'm totally killing this thread though so I'm going to drop the subject. 8)

For what its worth though, my top speed is a mere 47mph. Crosswinds on the Lechtd, traffic on Ben Lawyers and heavy rain and mist on the Bealach Na Ba have all conspired to prevent me breaking that record! 😀

teh how is they have a greater force acting on them to push them downhill but similar wind resistance to the skinny guys - so have a higher terminal velocity

Eh... really? Gravity is [s]not[/s] a force, [s]its an acceleration[/s]. Acceleration due to[i] the force of[/i] gravity [i]on Earth[/i] is 9.81 m/s2

What you are suggesting (heavier falls faster) is in direct disagreement with established principles (or at least, what I have assumed are established principles from memory).

Do i need to explain big G and small g as well?

Do i need to explain big G and small g as well?

My clumsy/wrong definitions aside, will everyone PLEASE drop the subject of wind resistance, I keep stating, or trying to state, I am considering the hypothetical situation of two otherwise identical riders of different mass. Therefore wind resistance should not be a factor, they experience it equally.

The established principles I refer to do not only hold true for a vacuum, they also hold true in the instance of two otherwise identical [s]riders[/s] objects of different mass.

The point I have been trying to make is this:

If we agree that in freefall, two aerodynamically identical objects off different masses will still fall at the same speed, then why should it be that two aerodynamically identical riders of different masses will achieve different speeds on a downhill?

If we agree that in freefall, two aerodynamically identical objects off different masses will still fall at the same speed,

ignore

Shit, its finally twigged. Apologies all!

edit: I was going to say 'the ball had finally dropped' but sensed that wouldn't help.

To be fair you've got the basics right its just one aspect that you got completeley backwards

If we agree that in freefall, two aerodynamically identical objects off different masses will still fall at the same speed,

...yeah... thats why I edited my last post, I'm still having trouble with that aspect.. 😳 🙄

I'm going to drop it now, without some heavy duty research (or maybe just a children's encyclopedia) I doubt I'll get to the bottom of it.

13thfloormonk

Without going too far into it, it's fair to say the biggest factor affecting an individual's top speed down a road is wind resistance, or aerodynamic drag. No matter the person's mass, the coefficient of friction of the road is the same, and gravity is the same. The variable is a person's mass, and more importantly, their mass/frontal area ratio.

Now effectively, the aerodynamic drag is an acceleration force acting to slow you down, where gravity is the acceleration force speeding you up. Both are independent of a person's mass, but whilst gravity is a constant, aerodynamic drag is affected heavily by frontal area.

So given that F=Ma, we can all agree that a fat person will have more force acting upon them from above than a skinny person. But fat people generally don't have significantly higher frontal areas (bigger yes, but not usually proportionately so), so whilst there is more force acting to slow them down, it is not acting to slow them down as much as a skinny person... If that makes sense?

Terminal velocity occurs when forces due to gravity equal forces acting up due to aerodynamic drag. Increase the frontal area, the object will slow down. Increase the mass for the same frontal area, the object will speed up.

In the case of a guy riding a bike down a steep hill, take a 10st man versus a 15st man. The heavier guy has 50% more mass acting as a force downwards (you then need to work out its effect as a result of the gradient of the slope using trigonometry) than the skinny guy. But the fat guy will probably only have 20% more frontal area (hence experience only 20% more drag)... The result...?

Fatties go faster down hills!