TT'ers - Apart from speed, does a rear disc wheel give more stability?

wheels
#1

Hi,

I do TT’s with a front Tri-Spoke and up until now, I have only had an 80mm deep section rear wheel.

I have found that I tend to get buffeted about in moderate winds and also when some traffic passes, when doing TT’s. Nothing too severe, but severe enough to be a bit troublesome.

A few days ago I borrowed a rear disc wheel and did a TT. I seemed to go about 30 seconds faster on the 40k TT than my average times, but the biggest thing I noticed was less buffeting around at the front. (It was still quite a breezy day 10mph to 12mph winds).

Am I imagining this improved stability or has anyone else noticed this?

What are your thoughts because I can’t explain it to myself why there may be an overall stability improvement with a rear disc wheel and my trispoke front.

Thanks.

#2

Yes. I’ve heard it described as moving the center of force rearwards. Essentially, a greater proportion of the lateral force on the bike is under your center of gravity and on a fixed-position wheel, making it easier to handle.

I haven’t ridden a disc enough to personally attest to it, but I have definitely heard people describe what you are describing.

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#3

I have my disc cover on my Tri bike all the time (since it’s a pain to take off and on) and I live on the flat Canadian Prairie where the wind blows All. The. Time. and I certainly don’t notice a negative affect from it more than when I ride my roadie (I ride my Tri bike most often though). Others often comment “You were riding with a disc?!?”, but I’ve read that it actually helps stabilize things more as well, which seems consistent with what I’ve noticed.

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#4

Thanks for the replies.

I’m wondering whether it acts like a keel on a boat in some way, perhaps helping to keep things moving in a straight line.

I would love to hear more about other people’s experiences with a disc wheel, especially if you noticed the difference between using one and not using one.

I will try to borrow it again for another ride.

I’m hoping it is true before I decide to spend the $$$'s or $,$$$'s.

#5

Depends how you describe stability. For me it feels less nimble, so the handling/acceleration is less good. That could be seen as the disc being more stable. Equally pop a bit of wind in to that equation and suddenly the bike with a disc is less stable.

It’s faster which is the main reason to get one, but it does compromise handling a little just not as big an effect as a deeper section front wheel does, well that’s my opinion of it anyway.

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#6

Apart from the potential aero impact (sounds like a weathervane effect, no clue if that is true or not), the higher moment of inertia will give more resistance against angular inputs - in other words, at the same wheel rpm (thus bike speed), the wheel will resist roll (tipping over to the side) and yaw (turning left/right) more than a lighter, lower moment of inertia wheel. Having that extra inertia in the back as opposed to the front makes the bike easier to handle; placing an equivalent moment of inertia in the front would create more resistance to steering inputs.

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#7

You could look at www.wheelbuilder.com to get a cover to try on the cheap before dropping more money on an actual disc–not sure if they can be used in your TT’s though?

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#8

That’s a good idea. Yes you can use these covers, but they must be installed properly etc.

I can borrow the rear disc wheel again for a TT and I can see how it feels.

I vaguely recall something on the net that a rear wheel can help balance out buffeting in the wind better than a general deep section rear wheel, when also using a front aero wheel.

Would still love to hear others opinions on this. I must say I would guess that at least 60% of people ride rear disc wheels in the TT’s I ride, but they probably do it to gain seconds and may not have experienced riding a front aero wheel without the rear disc.

Thanks.

#9

I’m no scientist so I may well be completely wrong. But can only go with my experience, surely more rotating weight which generally comes with deeper section wheels, let alone a disc, will make a bike harder to handle.

This is my real world experience, I love riding my CAAD10 with 30mm alloy clinchers as it handles fantastically well, goes where I want it. My 58mm carbon clinchers will often have a mind of their own. And if there’s some wind about; hold on! Unless you’re going in a straightline. Which aligns with the ‘deeper = greater stability’ theory. But I don’t think this is true in crosswind…

Recently riding a friend of mine was almost blown off the road on his full TT rig, I was far more contolled with my 58mm wheels. Make of that what you will.

#10

You are right, there are two effects - mechanical and aerodynamic. The mechanical one results from higher moment of inertia, and makes the bike want to stay upright and go straight - just see a bike as two spinning tops with horizontal axes, the heavier the tops, the higher the precession moments that result from any angular momentum on the axis. The aero effect is twofold: lower drag (the reason for using a disc in the first place), but also higher lateral forces resulting from cross-wind components, affecting handling (and drag too, by the way).

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