Axs and osp: drivetrain inefficiency vs aero

Discuss light weight issues concerning road bikes & parts.
RocketRacing
Posts: 888
Joined: Thu May 10, 2018 2:43 am

by RocketRacing

I was watching the fastfitnesstips video where he made an argument for oversized pully wheels slowing you down over kona... when aero is factored in (30s over the bike course).

It made me wonder about the criticism of axs losing efficiency due to smaller chainrings/cogs. The smaller cogs should speed things up marginally with aero. But... axs also has larger pully wheels... at least slightly larger. I suspect the main issue for drag is medium and long rear cages.

All marginal gains/losses, but it is an interesting thought experiment.

by Weenie


aeroisnteverything
Posts: 193
Joined: Fri Aug 24, 2018 4:43 pm

by aeroisnteverything

I cannot imagine cogs making any detectable difference in aero. Maybe the chainring can be measured, and even then I doubt a 48 vs 52 is really discernible. The derailleur size - only because it’s just hanging out there- might make some difference, but 30 seconds sounds like too much. All that said, SRAM AXS is probably not great on many counts: 1) worse mechanical efficiency due to smaller cogs vs 11-speed; 2) chunky rear derailleur so worse aero there than Dura Ace; and 3) heavier. This might be offset slightly by a more aero efficient chainrings.

AJS914
Posts: 3488
Joined: Tue Jan 28, 2014 6:52 pm

by AJS914

How fast are the Kona guys? 4-5 hours for the bike? 30 seconds out of a 5 hour ride seems like a pretty small measurement.

philipeleven
Posts: 56
Joined: Mon Sep 23, 2019 2:01 pm

by philipeleven

bigger chainring reduce friction between outer and inner plate of chain, but increase friction between chainring and chain. the sweet point is somewhere between 15-25T, so AXS chainring reduce friction actually.

Stefano
Posts: 251
Joined: Sun Dec 19, 2010 4:24 am
Location: Ann Arbor, Michigan

by Stefano

Most elite pursuit teams have been using ever larger chainrings and cogs- if there’s one source I trust for data, it’s teams who spend weeks looking for tenth of a percent differences. To me it’s obvious that smaller chainrings are slower in a system-wide context. And as another poster said, 30s (if that’s even a real number) isn’t much over 4h, just about 0.2%, whereas the drivetrain efficiency difference per Velonews hovers around 3W, which at 250W is 1.2%, six times as much.

alanyu
Posts: 101
Joined: Thu Jun 06, 2019 1:10 pm

by alanyu

Stefano wrote:
Sat Oct 05, 2019 3:25 pm
Most elite pursuit teams have been using ever larger chainrings and cogs- if there’s one source I trust for data, it’s teams who spend weeks looking for tenth of a percent differences. To me it’s obvious that smaller chainrings are slower in a system-wide context. And as another poster said, 30s (if that’s even a real number) isn’t much over 4h, just about 0.2%, whereas the drivetrain efficiency difference per Velonews hovers around 3W, which at 250W is 1.2%, six times as much.
"the fastfitnesstips video where he made an argument for oversized pully wheels slowing you down over kona... when aero is factored in (30s over the bike course). "

It's overall results with aero and big pully wheels together, resulting in 30s slower over 4h, ~0.2%, which means aero debuff is more than drivetrain efficiency buff.

RocketRacing
Posts: 888
Joined: Thu May 10, 2018 2:43 am

by RocketRacing

AJS914 wrote:
Sat Oct 05, 2019 2:23 pm
How fast are the Kona guys? 4-5 hours for the bike? 30 seconds out of a 5 hour ride seems like a pretty small measurement.

Not sure about kona, but the pros can do 45km/hr plus.

That 30s at kona is what time you are paying big $$$ to lose when you upgrade to a ceramic speed ospw setup. The 30s adds up gains (small drivetrain gains) and losses (aero). It ends up with a net loss at kona. On a pure climbing race you might see slight gains.

RocketRacing
Posts: 888
Joined: Thu May 10, 2018 2:43 am

by RocketRacing

aeroisnteverything wrote:
Sat Oct 05, 2019 12:06 pm
I cannot imagine cogs making any detectable difference in aero. Maybe the chainring can be measured, and even then I doubt a 48 vs 52 is really discernible. The derailleur size - only because it’s just hanging out there- might make some difference, but 30 seconds sounds like too much. All that said, SRAM AXS is probably not great on many counts: 1) worse mechanical efficiency due to smaller cogs vs 11-speed; 2) chunky rear derailleur so worse aero there than Dura Ace; and 3) heavier. This might be offset slightly by a more aero efficient chainrings.
The question is if the aero gains, even if very small, outweigh the losses in drivetrain efficiency. I bet it is about a wash. I bet the derailer slows you down more in the 10/11 tooth than any loss in drivetrain efficiency (taking the entire rider/aero/drivetrain/rolling resistance into account).

1) smaller cogs, larger jockey wheels. The smaller chainring part of axs is not a negative to me. It is about getting the chainring that matches your ability/course that is key. Choices will be lacking for the most powerful riders however. I look for a front chainring that allows me to get up the climbs required, and that puts me in a straight as possible chainline for typical race/cruising gears.... where i will spend 70% of my time. The 10t and say 28 or 32t are for exceptional situations. When i need the 10t it is usually a downhill, and i am saving a bit of energy for the next climb/attack anyway. If i am in the 28t for climbing, it is very steep, or i am losing. P.s. expect a 10t on the next dura ace 12 speed, just like xtr.
2) i should compare dura ace to etap, bit i must say the battery seems a bit aero in design.
3) Weight means verly little on just about every tri/tt course (always exceptions).

TobinHatesYou
Posts: 4196
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

I bet you the eTap FD is a bigger aero pig than the RD. Venturi effect...slightly less turbulent airflow.

Nautiboy
Posts: 13
Joined: Wed Jul 09, 2014 3:54 am

by Nautiboy

RocketRacing wrote:
Sat Oct 05, 2019 11:42 am
I was watching the fastfitnesstips video where he made an argument for oversized pully wheels slowing you down over kona... when aero is factored in (30s over the bike course).

It made me wonder about the criticism of axs losing efficiency due to smaller chainrings/cogs. The smaller cogs should speed things up marginally with aero. But... axs also has larger pully wheels... at least slightly larger. I suspect the main issue for drag is medium and long rear cages.

All marginal gains/losses, but it is an interesting thought experiment.
Hi there, we can research or refer to all available data and video reviews out there but nothing beats doing our own "butt tests without being too overly technical about it" :D . For one, drivetrain efficiency is the most critical aspect for me versus aero-dynamic. Just buy the best aero bike on the market(which i have a few) and once the rider is on it, it almost nullify the advantages from the get-go. And what matters to the cyclist is the power output must be efficiently optimised and translates to tires rolling efficiency. Without an Oversized Pulley, my Garmin's data and body sensors always point towards a less than optimal performance.....so I will never leave home without it : )
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Attachments
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MaxPower
Posts: 33
Joined: Fri Sep 20, 2019 9:30 pm

by MaxPower

Nautiboy wrote:
Sun Oct 06, 2019 4:16 am
RocketRacing wrote:
Sat Oct 05, 2019 11:42 am
I was watching the fastfitnesstips video where he made an argument for oversized pully wheels slowing you down over kona... when aero is factored in (30s over the bike course).

It made me wonder about the criticism of axs losing efficiency due to smaller chainrings/cogs. The smaller cogs should speed things up marginally with aero. But... axs also has larger pully wheels... at least slightly larger. I suspect the main issue for drag is medium and long rear cages.

All marginal gains/losses, but it is an interesting thought experiment.
Hi there, we can research or refer to all available data and video reviews out there but nothing beats doing our own "butt tests without being too overly technical about it" :D . For one, drivetrain efficiency is the most critical aspect for me versus aero-dynamic. Just buy the best aero bike on the market(which i have a few) and once the rider is on it, it almost nullify the advantages from the get-go. And what matters to the cyclist is the power output must be efficiently optimised and translates to tires rolling efficiency. Without an Oversized Pulley, my Garmin's data and body sensors always point towards a less than optimal performance.....so I will never leave home without it : )
BMC1.jpg

BMC2.jpg
Do you have experience with different manufacturers of osp?
I like the look of them and are thinking about it.
Shifting must be like factory setup though and the bearings must be able to take punishment (bad weather).

Karvalo
Posts: 737
Joined: Fri Aug 10, 2018 6:40 pm

by Karvalo

RocketRacing wrote:
Sat Oct 05, 2019 11:42 am
It made me wonder about the criticism of axs losing efficiency due to smaller chainrings/cogs. The smaller cogs should speed things up marginally with aero.
The cassette size difference is minimal, but the AXS derailleur body is huge.

I can't see any possible way it could be argued that AXS is more aero than alternative groups.

Stueys
Posts: 366
Joined: Sat Nov 22, 2014 1:12 pm

by Stueys

I think sram said their flat top chain design was designed to offset the loss of drive efficiency from the smaller cogs. Does beg the question though as to why not have more efficient chain and larger cogs that have less resistance....

RocketRacing
Posts: 888
Joined: Thu May 10, 2018 2:43 am

by RocketRacing

Stueys wrote:
Sun Oct 06, 2019 12:23 pm
I think sram said their flat top chain design was designed to offset the loss of drive efficiency from the smaller cogs. Does beg the question though as to why not have more efficient chain and larger cogs that have less resistance....
Marketing spin. I thought the flat top was more about preserving strength in the thinner 12s chain... but i admit to glossing over most marketing speak.
Last edited by RocketRacing on Sun Oct 06, 2019 12:43 pm, edited 1 time in total.

by Weenie


RocketRacing
Posts: 888
Joined: Thu May 10, 2018 2:43 am

by RocketRacing

Karvalo wrote:
Sun Oct 06, 2019 11:01 am
RocketRacing wrote:
Sat Oct 05, 2019 11:42 am
It made me wonder about the criticism of axs losing efficiency due to smaller chainrings/cogs. The smaller cogs should speed things up marginally with aero.
The cassette size difference is minimal, but the AXS derailleur body is huge.

I can't see any possible way it could be argued that AXS is more aero than alternative groups.
I think if one could run numbers, we would find it to be a slower groupset. Heavier, less mechanical efficiency, less aero on 2x applications (i agree that a ling rd would offset a small chainring) etc. It would only be a worthwhile performance upgrade if it functions better.

I do like the closer spacing between chainrings, and the 12s means you can swap to a higher/lower gear with less cross-chaining. That last part alone may nullify some of the "smaller gear" losses. But again, one should not have smaller gears as long as you choose appropriatly (pros aside, they need more than 50/37).

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