Climbing and tire rolling resistance

Discuss light weight issues concerning road bikes & parts.
Alexandrumarian
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Location: Romania

by Alexandrumarian

I've tried searching but I guess the terms are too generic. Anyway, it is pretty clear, on the flat, say cruising around 30-32kph, there is a roughly 15w difference between a pair of very fast tires with latex compared to an average tire with butyl. This is in line with the brr tests and my own.
What I am curious about and unfortunately too far away from any hill to test myself is how the two pairs would compare on a hill (say 8-10%) at an average Joe wattage, say 220-250.
More exactly, when climbing at 7-10kph, those 15w turn to what. 5...10...?

Thanks!

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

by TobinHatesYou

Alexandrumarian wrote:
Sun Sep 01, 2019 9:57 am
I've tried searching but I guess the terms are too generic. Anyway, it is pretty clear, on the flat, say cruising around 30-32kph, there is a roughly 15w difference between a pair of very fast tires with latex compared to an average tire with butyl. This is in line with the brr tests and my own.
What I am curious about and unfortunately too far away from any hill to test myself is how the two pairs would compare on a hill (say 8-10%) at an average Joe wattage, say 220-250.
More exactly, when climbing at 7-10kph, those 15w turn to what. 5...10...?

Thanks!

Rolling resistance scales fairly linearly with speed, so slowing from 30km/h to 10km/h would result in roughly 1/3rd the watts saved. It’s a bit more complex than that, but considering margin of error, it’s a good enough estimate.

by Weenie


Marin
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Location: Vienna Austria

by Marin

Image

Stueys
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by Stueys

Great chart, not seen something like that before

Gary71
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Location: Brisbane Australia

by Gary71

I agree with Stueys - great chart. But i do have a question for smart geeks out there.

Wouldn't the bearing / drivetrain friction increase as the gradient gets steeper due to the likelihood of the cadence slowing and torque increasing, thereby putting more pressure on the bearings?

TobinHatesYou
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by TobinHatesYou

Gary71 wrote:
Thu Sep 05, 2019 10:04 am
I agree with Stueys - great chart. But i do have a question for smart geeks out there.

Wouldn't the bearing / drivetrain friction increase as the gradient gets steeper due to the likelihood of the cadence slowing and torque increasing, thereby putting more pressure on the bearings?

No.

When power is constant, so is angular momentum.

AJS914
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by AJS914

Alexandrumarian wrote:
Sun Sep 01, 2019 9:57 am
there is a roughly 15w difference between a pair of very fast tires with latex compared to an average tire with butyl. This is in line with the brr tests and my own.

How did you get 15 watts? BRR shows less than one watt between latex and light butyl and 1.9 watts between latex and standard butyl.

https://www.bicyclerollingresistance.co ... s-clincher

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Calnago
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by Calnago

Marin wrote:Image
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. And that’s for a 75kg bike+rider combo. Wonder how that would change for a 100kg bike+rider combo. And that chart even assumes a 300w average power. I wish I could sustain 300watts up a super long hill, but since I can’t my actual speed would be even lower, and hence the effect of aero goes down as well.
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AJS914
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by AJS914

Unless you are doing a hill climb ITT, then any aero advantage is going to help you before, during, and after the climb.

You talk about a tipping point but I'm not sure the chart reflects time saved. Anecdotally, my club races up some 3-4% climbs on our Saturday group ride. If I get in the drops or low on the hoods I see my speed go up by 1mph which is a large percentage gain in performance. I've run weight calculations on those climbs on Analytical Cycling's site and if I drop say 10 pounds the net improvement in time is a handful of seconds on a 20 minute climb.

Alexandrumarian
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Joined: Mon Mar 28, 2016 6:34 pm
Location: Romania

by Alexandrumarian

AJS914 wrote:
Thu Sep 05, 2019 4:18 pm
Alexandrumarian wrote:
Sun Sep 01, 2019 9:57 am
there is a roughly 15w difference between a pair of very fast tires with latex compared to an average tire with butyl. This is in line with the brr tests and my own.

How did you get 15 watts? BRR shows less than one watt between latex and light butyl and 1.9 watts between latex and standard butyl.

https://www.bicyclerollingresistance.co ... s-clincher
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.

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Calnago
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by Calnago

AJS914 wrote: ...You talk about a tipping point but I'm not sure the chart reflects time saved...
The chart purports to report the energy required to overcome the various forces of resistance at various gradients. In the example with the 75kg bike and rider that point at which the energy required to overcome gravity vs aero is the same at a 2% grade (43% of total energy being spent equally to overcome aero vs gravity). Beyond that it takes more energy to overcome gravity than air resistance associated with 300w in the example, which is more watts (for me) than I would be producing on a long sustained climb. In other words, the tipping point would be an even lower grade if your bike+rider weight is greater than 75kg. In any case, it’s just an interesting observation and one that seems to reflect what I feel in reality.
At least that’s how I’m reading it.
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TobinHatesYou
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by TobinHatesYou

That chart is only showing what is affecting your speed at each % grade given a constant weight and a constant CdA. You can rest assured that the tipping point for a 75kg system weight at 300W is about 6%.

In a steady state up a 6% grade, a 75.5kg rider+bike putting out 302W is faster than a 75kg rider+bike putting out 300W. Basically 2W in aero gains trumps 500g in weight savings. For a pro, the tipping point is an even steeper grade.

Hexsense
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by Hexsense

How about rider weight vs bike weight ratio
compare to
rider aero drag vs bike aero drag ratio though?

6.8kg light bike + 60kg rider vs 8.5 kg heavy aero bike + 60 kg rider is just 2.5% lighter in total weight of 66.8 vs 68.5 kg. Even less in percentage if you are heavier and have extra load (like water bottles).
Would aero bike weight 8.5kg save more total aero drag than 2.5% ? I would think so by a wide margin.

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Calnago
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by Calnago

I think the chart is pretty clear. It’s saying that it takes more energy to overcome gravity than aero for a 75k rider above 2% gradients. And that’s consistent with what I feel on climbs as well. I’ve never been climbing a 6% grade thinking “geez, I wish my bike was more aero right now”. What I am thinking is “Geez, when will this hill end”. Especially at the speeds I’m climbing, being more aero doesn’t even enter my thoughts. Anyway, that’s just my observations, which are consistent with that chart. Discuss further if you like. I’m out.
Colnago C64 - The Naked Build; Colnago C60 - PR99; Trek Koppenberg - Where Emonda and Domane Meet;
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by Weenie


Hexsense
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by Hexsense

Calnago wrote:
Thu Sep 05, 2019 7:20 pm
I think the chart is pretty clear. It’s saying that it takes more energy to overcome gravity than aero for a 75k rider above 2% gradients. And that’s consistent with what I feel on climbs as well. I’ve never been climbing a 6% grade thinking “geez, I wish my bike was more aero right now”. What I am thinking is “Geez, when will this hill end”. Anyway, that’s just my observations, which are consistent with that chart. Discuss further if you like. I’m out.
Let's get 6% as example.
80% of the drag is gravity, which is directly related to weight.
10% of drag is aero drag.

Let say you save 1kg from 75kg of total weight in the example. The weight and gravity force is reduced by 74/75 => 1.35%,
that is around 1.08% of total drag (from multiplying to 0.8 since 80% of total drag is gravity drag).
To match that from aero drag alone, you have to save 1.08% of total drag out of 10% aero drag.

So, the aero combo must create 11% less aero drag vs non aero combo to match it. since 0.11*10% is roughly 1.1%
Would 1kg extra aero stuff grant you 11% more aero? It should.

In summary, 1kg difference is just 1.35% weight saving.
On 6% grade, since weight matter more than aero, you have to save 11% of total aero drag to match just 1.35% of weight saving.
The question is how much as percentage of total aero drag of a bike that weight 1kg extra (1.35% total weight) will save you... Is it more or less than 11%?
Last edited by Hexsense on Thu Sep 05, 2019 7:46 pm, edited 4 times in total.

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