Author Topic: Ideal bike weight as a percentage of body weight?  (Read 4776 times)

Karla

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    • Lost Byway - a Pacific bike ride
Re: Ideal bike weight as a percentage of body weight?
« Reply #25 on: October 02, 2017, 12:24:51 pm »
In theory it's pretty simple, for a given input of power, speed will be directly proportional to all up weight (rider+bike). Assuming that friction (air and road) remains constant between the two. So if you are putting out 200W, and you plus bike == 100kg, then you will have a speed of x, if you plus bike =80kg and you're putting out 200w, then your speed will be proportionally greater than x.

Where this becomes more complex tho is that aero drag represents a far bigger factor about your speed than bike weight. I often see Dutch bikes with clip on aero bars. They may weigh 25kg, with a 80kg Dutchman on, but in the Omnidirectional headwind, those bars make a significant difference. It's also worth noting that aero drag effects things based on a cube root or some such (no doubt Kim will be long shortly to correct me), so an improvement in your frontal area by x, will have a much more significant impact as a proportion, than a similar change in weight...

Weight (okay, mass, but we'll assume constant gravity ;) ) has several effects:

1) It slows you down going uphill
2) Rolling resistance increases in direct proportion to weight
3) There's the noted effect on acceleration
4) If your pedalling style involves throwing the bike about, the weight of the bike acts as a damper on this.

The power required to ascend against gravity is proportional to your rate of vertical ascent (plus a constant for not rolling down the hill).
The power required to overcome rolling resistance is proportional to speed (at least for the region that bike tyres are designed to be used in).
The power required to overcome air resistance is proportional to the cube of airspeed.

What the latter point means is that when you're riding up any significant gradient, your air drag falls to virtually nothing and you're basically fighting gravity plus a bit of rolling resistance.  I heard a talk once from a bigwig in BC who said that Chris Froome is 95% fighting gravity on any alpine climb.  When I re-ran the calculation for my friend riding up a pass on holiday last year, it was more like 99% - because we were going at half of Froome's speed so air drag was even lower.


There are several ways in which bike weight will have a bigger effect on a club rider than on Chris Froome:

1) Being 99% rather than 95% of their resistance going uphill means that there's a proportional gain there
2a) Because they are travelling slowly and air resistance is less, rolling resistance will be a larger portion of their overall resistance, so the proportional increase due to greater weight will have a greater effect on them.
2b) The club rider is likely to be running heavier tyres than Chris, so the proportional increase in rolling resistance will again have a greater effect on them.

Samuel D

Re: Ideal bike weight as a percentage of body weight?
« Reply #26 on: October 02, 2017, 12:56:35 pm »
(Again, it's possible for lighter riders to be more efficient and more powerful than heavier riders, etc).

An astonishing example of that is reported here, about Dan Martin.

These discussions can be interesting and pedagogical to the uninitiated, but they usually give the wrong impression to people who lack a nice judgement of scale and proportion. That’s most of us.

For practical purposes, it’s better to simply say that bicycle weight doesn’t matter to performance. The range is usually a matter of 2 kg, the bicycle spends most of its life on approximately flat terrain, the cyclist is an order of magnitude heavier than the bicycle, and there are usually much more significant gains curiously left untaken (e.g. the billowing jersey, the chain last oiled three weeks ago, tyres that defeat Dunlop’s original intention, a lack of sleep, the dreadful British diet, poor riding position (made worse by indoor training with its concentration on power and lack of aerodynamic drag), and the typical weekend cyclist’s amazing inability to read the wind and draft efficiently).

The marginal-gains mantra says we should care about bicycle weight despite all of this, since the burden of weight is in addition to all of the above, but the scale of the effect is tiny. That way lies counterproductive schemes such as Igus pedal bushings. It’s ironic but indicative of the industry that Garmin and PowerTap power-meter pedals use plain rather than ball bearings, needlessly throwing away some of the very power they’re measuring. But hey, they’re lighter!

I heard a talk once from a bigwig in BC who said that Chris Froome is 95% fighting gravity on any alpine climb.  When I re-ran the calculation for my friend riding up a pass on holiday last year, it was more like 99% - because we were going at half of Froome's speed so air drag was even lower.

What pass was that and do you recall the particulars? 99% is an extraordinary result, particularly since most cyclists I encounter use tyres with much greater rolling resistance than Chris Froome’s. When I did a similar calculation for an Alpine climb (Col du Frêne) I did, my work against gravity was only 89%. Bicycle weight over a reasonable range would not have made much difference even on that climb.

Karla

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    • Lost Byway - a Pacific bike ride
Re: Ideal bike weight as a percentage of body weight?
« Reply #27 on: October 02, 2017, 01:02:36 pm »
What pass was that and do you recall the particulars? 99% is an extraordinary result, particularly since most cyclists I encounter use tyres with much greater rolling resistance than Chris Froome’s. When I did a similar calculation for an Alpine climb (Col du Frêne) I did, my work against gravity was only 89%. Bicycle weight over a reasonable range would not have made much difference even on that climb.

Probably the Joux Plane.  This was on proper road tyres.

On any hill above a few percent gradient, adding 1 kg to a 100kg all-up weight will slow you down 1%.  It's really not a particularly hard calculation.

Re: Ideal bike weight as a percentage of body weight?
« Reply #28 on: October 02, 2017, 01:04:45 pm »
2kg is a very narrow weight range. What makes you pick that figure? None of my bikes is within 2kg of Marcel Kittlel's for sure!
The earth is vast and beautiful and contains many miraculous places. (Chekhov)

Samuel D

Re: Ideal bike weight as a percentage of body weight?
« Reply #29 on: October 02, 2017, 01:19:00 pm »
Probably the Joux Plane.  This was on proper road tyres.

You won’t get close to 99% of work done against gravity on that climb. A bicycle’s chain drive alone is not that efficient. Show your assumptions if you dare.

I don’t dispute your obvious point that climbing speed is dominated by power-to-weight ratio.

2kg is a very narrow weight range. What makes you pick that figure? None of my bikes is within 2kg of Marcel Kittlel's for sure!

It’s a number that felt about right for the range of bicycles we’d consider at any given purchase. If you want a steel bicycle with disc brakes and rack mounts, you’re not looking at 6 kg Pinarellos. If you want a carbon-fibre racing machine, you’re not looking at Bob Jacksons. Bicycles may weigh from 5 kg to 25 kg, but when choosing one, we only consider models with somewhat similar characteristics and design intentions.

Re: Ideal bike weight as a percentage of body weight?
« Reply #30 on: October 02, 2017, 01:25:15 pm »
That certainly makes sense at the more specific uses and the lighter weights. But there's a wide range of bikes being used for many purposes. Have a look at an audax for example and you'll see everything from CF roadies to Bob Jacksons and beyond. There's someone here who famously audaxes on a Pashley, and I would be amazed if no one's ever used a Spec Tarmac. Or stand at the side a main road at commuting times.
The earth is vast and beautiful and contains many miraculous places. (Chekhov)

citoyen

  • Cat 6 Racer
Re: Ideal bike weight as a percentage of body weight?
« Reply #31 on: October 02, 2017, 01:31:41 pm »
Sounds about right if you're talking about typical mid-market carbon or aluminium "road bikes" (anything from around £1k to £3.5k) - the weight range for these bikes seems to be roughly 7.5kg-9.5kg. Steel bikes start at around 10kg but there are some that are closer to 9kg.

Of course, that's before you add pedals, Garmin, a couple of full bidons, saddlepack stuffed with spare tube, pump/CO2 and multitool, plus any other accessories. Maybe even a set of mudguards.

Audaxers are another kettle of fish entirely.

Karla

  • car(e) free
    • Lost Byway - a Pacific bike ride
Re: Ideal bike weight as a percentage of body weight?
« Reply #32 on: October 02, 2017, 01:36:10 pm »
Probably the Joux Plane.  This was on proper road tyres.

You won’t get close to 99% of work done against gravity on that climb. A bicycle’s chain drive alone is not that efficient. Show your assumptions if you dare.


Oh feck off.  If you really want to split hairs about power at the pedals vs power delivered to the back wheel, how about you stop making gross generalisations like "weight doesn't matter".

What are you going to do, attack me with a bicycle pump?

Re: Ideal bike weight as a percentage of body weight?
« Reply #33 on: October 02, 2017, 01:38:32 pm »
Audaxers are another kettle of fish entirely.
Yeah, they range from 22kg* to 122kg.  :D

*A six year old.
The earth is vast and beautiful and contains many miraculous places. (Chekhov)

Samuel D

Re: Ideal bike weight as a percentage of body weight?
« Reply #34 on: October 02, 2017, 02:33:39 pm »
Have a look at an audax for example and you'll see everything from CF roadies to Bob Jacksons and beyond. There's someone here who famously audaxes on a Pashley, and I would be amazed if no one's ever used a Spec Tarmac. Or stand at the side a main road at commuting times.

True enough. And from one weight extreme to another, there is certainly a practical performance difference. If you could convince the Pashley rider to use a Tarmac instead, they’d be noticeably faster.

Oh feck off.  If you really want to split hairs about power at the pedals vs power delivered to the back wheel, how about you stop making gross generalisations like "weight doesn't matter".

I only brought up the pedals because they vividly illustrate that weight doesn’t matter. Any super-light pedal you care to mention is notionally bought to go faster but ends up slowing down the cyclist, because weight was saved by eliminating ball bearings. That shows how irrational the weight argument has become, egged on by screeds of scientifically illiterate writing in the cycling press.

It reminds me of the Drillium craze that once hindered top cyclists even as they thought it helped. Now we know better, so we remove ball bearings instead. What a time to be alive!

Secondly, although weight doesn’t matter is a “gross generalisation”, it’s a useful one for most cyclists – certainly more useful than the myths and half-understandings and – especially – the lack of sense of proportion that instead clouds the groupthink. It’s the best sort of generalisation: one that would result in better outcomes for most people if they applied it.

What are you going to do, attack me with a bicycle pump?

You wouldn’t want whacked by the pump I haul around in defiance of the weight weenies, but you’re safe if you show the assumptions that led to your 99% claim. What are you shy about? It was fun until you became so indignant.

Karla

  • car(e) free
    • Lost Byway - a Pacific bike ride
Re: Ideal bike weight as a percentage of body weight?
« Reply #35 on: October 02, 2017, 07:11:16 pm »

Oh feck off.  If you really want to split hairs about power at the pedals vs power delivered to the back wheel, how about you stop making gross generalisations like "weight doesn't matter".

I only brought up the pedals because they vividly illustrate that weight doesn’t matter. Any super-light pedal you care to mention is notionally bought to go faster but ends up slowing down the cyclist, because weight was saved by eliminating ball bearings. That shows how irrational the weight argument has become, egged on by screeds of scientifically illiterate writing in the cycling press.

It reminds me of the Drillium craze that once hindered top cyclists even as they thought it helped. Now we know better, so we remove ball bearings instead. What a time to be alive!

Superlight pedals?  I never mentioned 'superlight pedals'.  I mentioned power at the pedals vs power at the rear wheel.  You'd just been talking about drivetrain loss in the previous post.  Geddit yet?

Re: Ideal bike weight as a percentage of body weight?
« Reply #36 on: October 02, 2017, 10:58:39 pm »
wouldn't it be nice if the bike with all the accessories (lights, mudguards, toolkit, gps, water bottle) still weighed 7-8kg?.. i can ride without all that on a short summer's ride, but most other times the bike gains a couple of kgs that dampen the agility somewhat.

Re: Ideal bike weight as a percentage of body weight?
« Reply #37 on: October 11, 2017, 08:47:37 am »
The other day I saw a photo of a friend's hill climb bike: 5.8kg. One of the extreme weight saving measures he's taken is removing the bar tape. I suspect the purpose of this is more psychological than gram reduction: "I didn't take the *&^% bar tape off to give up now!"
The earth is vast and beautiful and contains many miraculous places. (Chekhov)

Re: Ideal bike weight as a percentage of body weight?
« Reply #38 on: October 16, 2017, 07:34:06 pm »
... there are usually much more significant gains curiously left untaken (e.g. the billowing jersey, the chain last oiled three weeks ago, tyres that defeat Dunlop’s original intention, a lack of sleep, the dreadful British diet, poor riding position (made worse by indoor training with its concentration on power and lack of aerodynamic drag), and the typical weekend cyclist’s amazing inability to read the wind and draft efficiently).
One problem with all that is the non-cycling sacrifices involved. A jersey even tighter than the roadie norm (you suggested wearing one 2 sizes smaller in an earlier post) is probably going to be uncomfortable and is certainly going to make you look a bigger dork than lycra does anyway, getting more sleep means curtailing evening activities ie having less fun or doing less work, eating healthily involves changing your diet relative to family and friends, not eating cake etc, and losing the many excess kilos off your body before the excess grams off your bike, well... Obviously if you're really dedicated to riding speed, you'll do all these things and more, but for the average leisure rider, cycling is only one part of life. So it's easier to concentrate on bike weight even if it's not easier gains.
The earth is vast and beautiful and contains many miraculous places. (Chekhov)

Re: Ideal bike weight as a percentage of body weight?
« Reply #39 on: November 19, 2017, 12:20:02 pm »
There is a rule of thumb re rider weight vs tyre pressure, which goes pressure in bars = 1/10 rider weight in kilos, to the limit of the manufacturer's spec.
15% deflection is a much better rule of thumb, if a tad difficult to measure.

https://bikesportbicycles.com/wp-content/uploads/docs/TireDrop-OptimizingTirePressure.pdf

http://www.dorkypantsr.us/bike-tire-pressure-calculator.html

Blodwyn Pig

  • what a nice chap
Re: Ideal bike weight as a percentage of body weight?
« Reply #40 on: November 19, 2017, 01:13:00 pm »
3 rd post does actually say  ''all up weight 36lbs''  so can one assume this is bike, lights, saddlebag, water bottle, mudguards, heavier grade tyres, tools etc???? That's prob what Olive is, 2 racks, saddlebag, tools, tube,  schmidt, discs, lights,leather Spa saddle, pump!! but then I'm prob over 200lbs, but not fat, 6'2". Sometimes it feels sluggish,  as it would  do, other times, it inexplicably flies along (wind direction taken into account). The only other variable is ME, some  good days some less than good.  ::-)  I have a chum who has a Giant something or  other feather weight thing,  and I can outride him up hill  easily,  so its not all about the bike.

dim

Re: Ideal bike weight as a percentage of body weight?
« Reply #41 on: November 19, 2017, 07:54:57 pm »
Differences in seatposts have not made any difference in comfort to me, although I only weigh 66 kg. I’ve tried narrow and fat, carbon and aluminium, all with no perceptible difference (different forks or cranks, on the other hand, have made a clear difference). Anyway, how can you change the seatpost diameter without buying a new frame?

Cyclists believe extraordinary things about bicycle weight. Like you, I’ve been told I’m hampering myself by riding a heavy bicycle – except in my case, mine was about 10 kg and theirs was about 8 kg! (But theirs were carbon, with all the alleged magical properties of that material.) This was shortly before a long climb out of a café where I put in an effort to make a point, dropping three-quarters of the commentators, all of whom were racers.

For steep climbs, the maths is simple: speed equals power divided by all-up weight. In your case, the all-up weight was around 220 lb. Therefore each pound lost (around that weight) produces a speed increase of less than half of one percent. It doesn’t matter where the pound is lost from: belly, frame, or wheel rim. On most hills, where aerodynamic drag and rolling resistance still play a role, the difference is even less than that. And on the flat, the difference is for all practical purposes exactly nil.

thats a load of toss ....

imagine if you rode a lighweight carbon bike 2 1/2 kilos lighter than your 10KG? bike

does it make a difference? ....

YUP ....big time .... thats why people spend a lot of money making their bikes lighter .... I've had many bikes and I know from experience

as for Carbon, anyone who says 'steel is real' has not ridden a decent carbon bike


“No great mind has ever existed without a touch of madness.” - Aristotle

LittleWheelsandBig

  • Whimsy Rider
Re: Ideal bike weight as a percentage of body weight?
« Reply #42 on: November 19, 2017, 07:57:42 pm »
Does a 3% weight reduction in the weight of bike-and-rider make a difference? Yes, nearly 3% quicker on steep climbs and close to 0% quicker on flat ground. The rest comes from heads and legs...
Wheel meet again, don't know where, don't know when...

Re: Ideal bike weight as a percentage of body weight?
« Reply #43 on: November 19, 2017, 08:47:11 pm »
Does a 3% weight reduction in the weight of bike-and-rider make a difference? Yes, nearly 3% quicker on steep climbs and close to 0% quicker on flat ground. The rest comes from heads and legs...

So the question is: can you shave that 3% off just the bike?

LittleWheelsandBig

  • Whimsy Rider
Re: Ideal bike weight as a percentage of body weight?
« Reply #44 on: November 19, 2017, 09:08:10 pm »
Of course but it'll take a fair bit of dosh. I can take more than 5% off my fat arse and do it for a lot less money. That process is very likely to make me faster on flat ground too, unlike making the bike lighter.
Wheel meet again, don't know where, don't know when...

Re: Ideal bike weight as a percentage of body weight?
« Reply #45 on: November 19, 2017, 09:32:38 pm »
Of course but it'll take a fair bit of dosh. I can take more than 5% off my fat arse and do it for a lot less money. That process is very likely to make me faster on flat ground too, unlike making the bike lighter.

Those of us in that happy position may even save money by spending less of food, beer etc;)

I tipped the scales today at 179lbs and a suggested 21.4% body fat. I always take the latter statistic with a large pinch of salt, but losing 5% or even 10% isn't entirely unreasonable. That would be consistent with my 20s running self.