Is 2018 the year proper race bikes with discs gain momentum?

Discuss light weight issues concerning road bikes & parts.
pdlpsher1
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by pdlpsher1

cassard wrote:if you follow the real physics principles, at constant speed on flat, it's impossible to feel a 600g difference on the bike. If you do feel a difference, it's in your head.
Part of the weight difference is in rotational weight. Rotors, hubs, extra spokes, extra nipples, and stouter rims. I can easily detect half a pound of rotational weight.


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by Weenie


TobinHatesYou
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by TobinHatesYou

pdlpsher1 wrote:
Thu Apr 05, 2018 2:44 am
cassard wrote:if you follow the real physics principles, at constant speed on flat, it's impossible to feel a 600g difference on the bike. If you do feel a difference, it's in your head.
Part of the weight difference is in rotational weight. Rotors, hubs, extra spokes, extra nipples, and stouter rims. I can easily detect half a pound of rotational weight.

To be fair he said "at constant speed." Rotational weight also carries momentum, so you only feel the difference during acceleration. Also to this day my fastest time up OLH is on 60mm deep 1570g wheels and not my lightest set, so I feel like the consequences of 200g weight/rotational weight are pretty minimal. You feel the extra half pound during accelerations, but not really because it's rotational weight. You'd feel it if it were weight added to the frame just about the same.
Last edited by TobinHatesYou on Thu Apr 05, 2018 2:57 am, edited 2 times in total.

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LeDuke
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by LeDuke

pdlpsher1 wrote:
cassard wrote:if you follow the real physics principles, at constant speed on flat, it's impossible to feel a 600g difference on the bike. If you do feel a difference, it's in your head.
Part of the weight difference is in rotational weight. Rotors, hubs, extra spokes, extra nipples, and stouter rims. I can easily detect half a pound of rotational weight.


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Disc specific rims are generally lighter than rim brake rims.

If you can feel a total of 250g rotating within 3” of the hub, you are far more perceptive than I am.

Just like I doubt you’d notice the ~20-30g difference, per wheel, at the rim.


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cassard
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by cassard

pdlpsher1 wrote:
Thu Apr 05, 2018 2:44 am
cassard wrote:if you follow the real physics principles, at constant speed on flat, it's impossible to feel a 600g difference on the bike. If you do feel a difference, it's in your head.
Part of the weight difference is in rotational weight. Rotors, hubs, extra spokes, extra nipples, and stouter rims. I can easily detect half a pound of rotational weight.


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in an acceleration, if the (114g per wheel, so 227g total is half a pound) is only on the external part of the wheel, let's say the rim, with the same depth. Then you might see it. In any case, we are talking about disc wheels, so disc specific wheels have lighter rims for the same depth as rim brake. The added weight is in the center. To add to that, the center axle is a thru-axle which is stiffer in terms of torsion than a standard quick release. So yes, you might feel the difference as it should accelerate faster and give you more performances.

but at constant speed Even if it was only on the rim because of the force of intertia (heavier but same depth), it's impossible for you to detect the amount of force necessary to produce movement on a straigth line. The role of the weight of the bike is already extremely minimal in terms of participation of the total resistance applied to you, that a mere total of 227g added will add a less test minimal difference.

sorry but either what you say is bs, either it's in your head.
Last edited by cassard on Thu Apr 05, 2018 3:21 am, edited 2 times in total.

spdntrxi
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by spdntrxi

if you disc brake fanboys can feel the modulation.. then pdpsher can feel the rotational weight..... get it ? both are relative so you cant have it both ways.

cassard
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by cassard

Newton's laws aren't relative,

but anyway, talking about the weight of a bike being irrevelent unless you are climbing or accelerating isn't the thing to do on it forum, so I won't add anything.

TobinHatesYou
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by TobinHatesYou

spdntrxi wrote:
Thu Apr 05, 2018 3:19 am
if you disc brake fanboys can feel the modulation.. then pdpsher can feel the rotational weight..... get it ? both are relative so you cant have it both ways.
Rim deflection is a thing, especially with QR based rim-brake wheels. In a perfect world, we wouldn't brake in tight corners, but we feather and modulate front brakes all the time in such situations because, well, we're human. We're not talking about being able to better modulate disc-brakes in perfect straight line conditions with perfectly true wheels. We're talking about the real world where weather affects the behavior of rim-brakes on carbon tracks more than it affects disc-braking. We're talking about pulsing because wheels aren't true or because your brake surface delaminated because of porosity + overheating. These are real situations. Disc-brakes having consistent modulation in extreme conditions is real.

Physics is real.

As I said, pdlpsher1 can probably feel the weight out of the saddle under heavy acceleration, but he can't really distinguish rotational weight from static weight. We're not that precise of a measuring device.

spdntrxi
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by spdntrxi

rotational mass is physics.. so if he can feel it then why not believe him too is all I'm saying.

remember others are throwing terms like constant speed .static this and that... in pdrpushers defense he did not use those terms.

TobinHatesYou
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by TobinHatesYou

spdntrxi wrote:
Thu Apr 05, 2018 3:59 am
rotational mass is physics.. so if he can feel it then why not believe him too is all I'm saying
Because the difference between rotational weight and static weight during acceleration as far as bicycle wheels are concerned is infinitesimal. We're talking about rotating masses that are around 1kg each with differences of about 100-150g each. And as cassard points out, that extra weight is actually confined to a radius much closer to the origin.

So there is a difference, but it's at a magnitude that our squishy bodies and brains can't really distinguish.

pdlpsher1's response, as phrased, is wrong...there's not really any way to get around it. He doesn't really notice the *rotational* weight. He likely just barely notices the weight and only during acceleration. This is the standard belief vs science debate with a bicycle twist.
Last edited by TobinHatesYou on Thu Apr 05, 2018 4:18 am, edited 2 times in total.

spdntrxi
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by spdntrxi

who you calling squishy :)

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LeDuke
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by LeDuke

spdntrxi wrote:rotational mass is physics.. so if he can feel it then why not believe him too is all I'm saying.

remember others are throwing terms like constant speed .static this and that... in pdrpushers defense he did not use those terms.
I’d argue that he needs a recalibration if he thinks that a lighter rim is heavier than a heavier rim.


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MoPho
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by MoPho

spdntrxi wrote:
Thu Apr 05, 2018 3:19 am
if you disc brake fanboys can feel the modulation.. then pdpsher can feel the rotational weight..... get it ? both are relative so you cant have it both ways.


Nice try but your fingertips are the most sensitive part of your body for detecting pressure, your legs not so much.



.

pdlpsher1
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by pdlpsher1

LeDuke wrote:
Thu Apr 05, 2018 4:56 am
spdntrxi wrote:rotational mass is physics.. so if he can feel it then why not believe him too is all I'm saying.

remember others are throwing terms like constant speed .static this and that... in pdrpushers defense he did not use those terms.
I’d argue that he needs a recalibration if he thinks that a lighter rim is heavier than a heavier rim.


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Disk rims are generally wider and heavier. They also need more spokes to transfer the braking force. Rim brake forces are transferred via the rim and not the spokes.

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themidge
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by themidge

MoPho wrote:
Thu Apr 05, 2018 12:13 am
Skip the hamburger!
But if I use my lighter rim brakes then I can eat the hamburger! :wink:

TobinHatesYou
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by TobinHatesYou

pdlpsher1 wrote:
Thu Apr 05, 2018 7:50 am

Disk rims are generally wider and heavier. They also need more spokes to transfer the braking force. Rim brake forces are transferred via the rim and not the spokes.
"Disc rims are sometimes wider." True. "Disc rims are heavier." False. Carbon rim manufacturers have universally stated that disc rims can be made lighter because they don't need to reinforce the structure around a non-existent brake track. Not only is the edge of the rim no longer subject to high pressure from brake caliper actuation, but there doesn't need to be a build-up of material to help absorb heat.

"They also need more spokes to transfer braking force." Campy Bora disc wheels have 21 spokes in the rear, do they not? In the front, yes, the difference is generally around 4 extra spokes, sometimes 6...sometimes 8. But again from a rotational weight standpoint, fairly insignificant. From a total weight standpoint, I believe spokes+brass nipples generally weigh around 5.5g. I very much doubt I'd be able to feel the difference of 22-44g.

Some examples from ENVE since they publish rim weights:
ENVE SES 3.4 tubular rim: 402g front, 409g rear
ENVE SES 3.4 clincher rim: 432g front, 439g rear
ENVE SES 3.4 clincher disc: 390g front, 397g rear

ENVE SES 4.5 tubular rim: 361g front, 399g rear (27mm/25.5mm)
ENVE SES 4.5 clincher rim: 469g front, 489g rear (27mm/25.5mm)
ENVE SES 4.5 AR tubular disc: 388g front, 396g rear (31mm/30.5mm)
ENVE SES 4.5 AR clincher disc: 440g front, 450g rear (31mm/30.5mm)

ENVE SES 7.8 tubular rim: 436g front, 468g rear
ENVE SES 7.8 clincher rim: 541g front, 578g rear
ENVE SES 7.8 clincher disc: 514g front, 549g rear

by Weenie


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