Climbing and tire rolling resistance

Discuss light weight issues concerning road bikes & parts.
TobinHatesYou
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Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

Of course it takes more power to overcome gravity vs air resistance at >2%. That has never been in question. That however, does’t make it the tipping point for when a 75kg/300W system should choose a <500g heavier aero bike. You’re interpreting the chart incorrectly.

AJS914
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by AJS914

Alexandrumarian wrote:
Thu Sep 05, 2019 5:52 pm

I was considering the whole package, both tires, something fast with latex (corsa speed or 5000 for example) compared to a slow tire with butyl. At about 33kph I think it easily adds to 15W when using an old school compound/tubular, 4 season type or cheap tire.
Gotcha. Personally, I always ride a fast tire. I get thousands of flat free miles out of GP4000s so I see no reason to use a truck tire like a Gatorskin. :-) I'll switch to GP5000s when I use up my 4000s.

I did switch back to butyl though so I'll take the 3 watt penalty to not have to pump up tires every day.

shuttlenote
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Joined: Wed Jun 07, 2017 7:17 am

by shuttlenote

This was posted recently as well. Has a small blurb in regards to gradients in the article.
https://www.aero-coach.co.uk/inner-tube ... resistance

bm0p700f
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by bm0p700f

Light people always gloat about they get 1000's of flat free miles from gp4000s tyres. Some of us dont have that luxury.

Karvalo
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by Karvalo

Calnago wrote:
Thu Sep 05, 2019 4:31 pm
That is indeed a great chart and I remember a variation of it quite a while ago but could never find it again until you just posted it here. It kind of flies in the face of a lot of aero claims however, like you have to be riding something like an 8% grade (?) before gravity and weight trumps aero. That’s never been what I’ve seemed to experience in the real world as as soon as things start going up aero goes out the window pretty quickly as I think of ways to lighten the load. The chart above would seem to indicate that the tipping point at which weight and gravity are more important than aero is at about a 2% grade.
You are confusing two seperate things.

1) What is consuming more of your power in total?
2) How much can you reduce those power requirements with equipment choices?

So sure - gravity is consuming more of your power over 2%. That does not mean the tipping point between an aero bike and lightweight bike is 2% - because aero power savings can still be significantly greater than weight power savings at and above those gradients.

TobinHatesYou
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Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

Science!

For the laymen:

As the grade increases and your wattage remains constant, your speed decreases because of gravity. If you remove the portion of your power devoted to overcoming gravity, then the power remaining is what's required to move at that same speed on flat ground.

So that 75kg/300W rider is moving ~40km/h on flat ground. On a 2% grade pushing 300W they are more or less travelling at a speed equivalent to pushing 300-(0.43*300)=171W on flat ground. At 6%, they're moving at a speed equivalent to pushing ~60W on flat ground. You can understand how gaining back even 2W in aerodynamic efficiency is a significant chunk of that "60W."
Last edited by TobinHatesYou on Fri Sep 06, 2019 1:28 am, edited 2 times in total.

alcatraz
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by alcatraz

How about maximizing both aero and weight gains as much as desireable and be happy?

TobinHatesYou
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by TobinHatesYou

alcatraz wrote:
Fri Sep 06, 2019 1:19 am
How about maximizing both aero and weight gains as much as desireable and be happy?

That's not in question, we're just trying to ELI5 why 2% hills are not anywhere close to being the point where you'd choose realistic weight savings over realistic aero gains, even if you're not that strong.
Last edited by TobinHatesYou on Fri Sep 06, 2019 1:30 am, edited 2 times in total.

RedbullFiXX
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Joined: Sat Aug 05, 2017 6:13 am

by RedbullFiXX

alcatraz wrote:
Fri Sep 06, 2019 1:19 am
How about maximizing both aero and weight gains as much as desireable and be happy?
And comfort, need to be able to hold that position to hold the watts steady
Cyclocross, in general, is about riding the wrong bike for the conditions.

TobinHatesYou
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Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

RedbullFiXX wrote:
Fri Sep 06, 2019 1:29 am
alcatraz wrote:
Fri Sep 06, 2019 1:19 am
How about maximizing both aero and weight gains as much as desireable and be happy?
And comfort, need to be able to hold that position to hold the watts steady

Also not a question or variable in this case.

RocketRacing
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by RocketRacing

The stronger the rider (faster), the greater the grade required before weight trumps aero.

The problem with weight is that the bikes weight is such a small portion of the bike + rider system. For me it is 8-9%. And those gains are really only stastically significant at very high average grades.

And for aero, the rider is 70-80% of drag. Small changes there give huge results.

Get the best crr tires you can stomach
Get aero wheels (light if you can)
Get an aero frame (light if you can)
Make the rider as aero as possible (light if you can)

I will keep my light climbing bike, but i have a heavy as hell, but very aero bike on the way (felt ia). It will be fun to see how they compare.

AJS914
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Joined: Tue Jan 28, 2014 6:52 pm

by AJS914

bm0p700f wrote:
Thu Sep 05, 2019 11:18 pm
Light people always gloat about they get 1000's of flat free miles from gp4000s tyres. Some of us dont have that luxury.
I'm not light - 195 pounds. I've been riding GPs in every incarnation for decades - never much of a problem.

TobinHatesYou
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by TobinHatesYou

AJS914 wrote:
Fri Sep 06, 2019 2:48 am

I'm not light - 195 pounds. I've been riding GPs in every incarnation for decades - never much of a problem.

I'm light and I had sidewall tears out the wazoo with GP4Ks and GP5Ks. bm0p700f is barking up the wrong tree... road quality and or riding style play a much bigger role.

Marin
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Location: Vienna Austria

by Marin

There is no general "tipping point".

More aero will always be faster than less aero on any gradient.
Low rolling resistance will always be faster than high rolling resistance on any gradient.
Low drivetrain losses will always be faster than high drivetrain losses on any gradient.
Lighter will always be faster than heavier on any gradient except downhill.


You have to be facing a concrete tradeoff in the form of 500g vs. 0,001 CDA to be able to determine a point where the advatages and disadvantages cancel out.

Lieblingsleguan
Posts: 179
Joined: Mon Mar 23, 2015 9:47 pm

by Lieblingsleguan

TobinHatesYou wrote:
Fri Sep 06, 2019 1:04 am
Science!

For the laymen:

As the grade increases and your wattage remains constant, your speed decreases because of gravity. If you remove the portion of your power devoted to overcoming gravity, then the power remaining is what's required to move at that same speed on flat ground.

So that 75kg/300W rider is moving ~40km/h on flat ground. On a 2% grade pushing 300W they are more or less travelling at a speed equivalent to pushing 300-(0.43*300)=171W on flat ground. At 6%, they're moving at a speed equivalent to pushing ~60W on flat ground. You can understand how gaining back even 2W in aerodynamic efficiency is a significant chunk of that "60W."
But you need to take into account the speed into the savings of "2W in aerodynamic efficiency". The rider from the graph will go about 18kph at 6% gradient (or at 60W at 0% gradient). Saving 2W on aero measures at 18kph equals an aerodynamic saving of 22W at 40kph or 38W at 30mph.

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