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Rear Tire width and Aerodynamics
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First time poster, sorry if this is a stupid question.

I recently flatted @ the ARK state TT, running a Conti GP TT rear Tire on a HED Jet C2 disc (puncture from small sharp rock)

Emailed HED for a suggestion, and per their advice have switched to the Force 24mm rear tire....Reason being my weight (195 lbs)

My question....what kind of aero penalty if any am I paying by running a wider tire ,wider than the width of the rim (23 v 24), in the rear?

Does CRR trump drag when talking about a rear tire?

The bike is a SHIV TT if that matters
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Re: Rear Tire width and Aerodynamics [BeefSupreme] [ In reply to ]
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Pretty minimal difference on a rear wheel and at your size might be offset by CRR. CRR doesn't trump drag, but when talking about a rear wheel and tires that are already good aerodynamically in both sizes, it's less of an issue.

Plus, a super fast TT tire didn't save you any time if you get a flat. The heavier you are, the easier it is to flat. Running higher pressures and higher load on the contact patch makes the tire more firm, and easier the cut. Lower pressures, it's more flexible and harder to cut. Those TT tires without puncture strips are probably best suited for super high quality, swept roads and riders under 170lbs... or even 160lbs.


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Re: Rear Tire width and Aerodynamics [BeefSupreme] [ In reply to ]
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Realistically, the aero difference from 23-24 on a rear is even hard to see in the wind tunnel on a wide rim like your C2 if the tires are at equivalent pressures. You will also find that running higher tire pressures makes all tires larger, so assuming the tires caliper at 23 and 24 respectively at 90psi, the 23 will measure 24 at 110psi or thereabout, so somewhere you'll find that you can run the 24 with greater comfort due to the lower pressure and likely have near-zero penalty to the 23 at higher pressure.

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Re: Rear Tire width and Aerodynamics [joshatsilca] [ In reply to ]
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Thanks for the advice. I really like the look and ride of the Force. That settles it for me. One less thing to worry about.
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Re: Rear Tire width and Aerodynamics [joshatsilca] [ In reply to ]
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When you say near-zero penalty - are you speaking about both the aero profile and Crr? I thought larger tyres only roll faster if pumped up at same PSI as smaller tyres - which is often the misconception amongst consumers that bigger = comfort. Only comfortable if you run less PSI, but then you sacrifice some Crr... right?

Thx in advance Josh.
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Re: Rear Tire width and Aerodynamics [slowshortround] [ In reply to ]
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This is such a good question.
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Re: Rear Tire width and Aerodynamics [joshatsilca] [ In reply to ]
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joshatsilca wrote:
Realistically, the aero difference from 23-24 on a rear is even hard to see in the wind tunnel on a wide rim like your C2 if the tires are at equivalent pressures. You will also find that running higher tire pressures makes all tires larger, so assuming the tires caliper at 23 and 24 respectively at 90psi, the 23 will measure 24 at 110psi or thereabout, so somewhere you'll find that you can run the 24 with greater comfort due to the lower pressure and likely have near-zero penalty to the 23 at higher pressure.

The Force measures around 26mm at 100psi on my Jet 6 C2, which should be the same rim. Perhaps a bit smaller when brand new, but definitely more than 24mm. I think the GP TT is almost as big, though. I would assume this would not be that good aerodynamically (the brake track is 23mm wide).

I would personally look into the Attack instead, should measure around 24mm at 100psi on that rim.
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Re: Rear Tire width and Aerodynamics [BeefSupreme] [ In reply to ]
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I recently flatted @ the ARK state TT, running a Conti GP TT rear Tire on a HED Jet C2 disc (puncture from small sharp rock). Emailed HED for a suggestion, and per their advice have switched to the Force 24mm rear tire....Reason being my weight (195 lbs)

If a sharp rock punctured your tire it must have been the sidewall. I don't see how a wider tire would help.

The side grooves on the Force likely make it more aero than the GP TT. But a smaller tire like the Attack should be better than either.



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Re: Rear Tire width and Aerodynamics [rruff] [ In reply to ]
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rruff wrote:
I recently flatted @ the ARK state TT, running a Conti GP TT rear Tire on a HED Jet C2 disc (puncture from small sharp rock). Emailed HED for a suggestion, and per their advice have switched to the Force 24mm rear tire....Reason being my weight (195 lbs)

If a sharp rock punctured your tire it must have been the sidewall. I don't see how a wider tire would help.

The side grooves on the Force likely make it more aero than the GP TT. But a smaller tire like the Attack should be better than either.



How did you deduce that?

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Re: Rear Tire width and Aerodynamics [ZackCapets] [ In reply to ]
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I run over zillions of pieces of broken glass and never once have had one (or a rock) puncture the tread of a Conti tire. The sidewalls puncture relatively easily.
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Re: Rear Tire width and Aerodynamics [rruff] [ In reply to ]
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A small very sharp rock went right through the middle of the tire. Think "the worlds smallest arrowhead" discovered on Arkansas highway. Just bad luck, i understand that, but according the the Continental diagrams, the tread on the TT is much thinner than the attack, force, GP 4000. Maybe one of these tires would have survived, maybe not. I considered running the Attack on the rear but was advised against it, because I am heavier, by HED in an email. I run the Attack up front on a FLO 90 and it looks like a bullet on the wide rim. I suppose I could spit the difference and run a 23 4000s but I like the idea running lower pressure with the Force so long as the extra drag isnt too much. Given my size I am probably reading to much into a 1-2mm tire width difference......especially on the rear.
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Re: Rear Tire width and Aerodynamics [rruff] [ In reply to ]
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rruff wrote:
I run over zillions of pieces of broken glass and never once have had one (or a rock) puncture the tread of a Conti tire. The sidewalls puncture relatively easily.

I have little bits of rocks cause punctures a few times, but usually not instantly. It gets in there, then works its way into the tube after a while.

This is why it is import to check tires for cracks and pick out any rock shards before a big race.



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Re: Rear Tire width and Aerodynamics [jackmott] [ In reply to ]
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I think i probably picked it up riding from the parking lot to the start line. We had to park @ a truck stop and ride through a major intersection to the starting line. It was interesting
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Re: Rear Tire width and Aerodynamics [BeefSupreme] [ In reply to ]
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I rode in AR once... many years ago. Didn't know you had rocks that sharp!

I have about the same weight front and rear on my TT bike. If you run lower pressure, consider that it may also increase Crr depending on the road surface.
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Re: Rear Tire width and Aerodynamics [slowshortround] [ In reply to ]
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slowshortround wrote:
When you say near-zero penalty - are you speaking about both the aero profile and Crr? I thought larger tyres only roll faster if pumped up at same PSI as smaller tyres - which is often the misconception amongst consumers that bigger = comfort. Only comfortable if you run less PSI, but then you sacrifice some Crr... right?

Thx in advance Josh.
This Conti chart illustrates that you should be able to run wider tires with 10-15 less PSI for the added comfort and still have similar or less Crr as next narrower size tire.


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Re: Rear Tire width and Aerodynamics [rruff] [ In reply to ]
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So next up for me is Louisiana State TT in Aug.....supposed to be smooth, fast and flat.

Keep the Force, Run the Attack... @ 115psi?, or shut up about it and run 4000s?
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Re: Rear Tire width and Aerodynamics [BeefSupreme] [ In reply to ]
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Don't ask me! If it's smooth I'd run 20mm Supersonics... but if fat tires ease your mind, then run what you have.

Looking at the Conti graph above, I wonder if we aren't missing something when we test on small rollers. I get much less Crr delta vs tire size than they show.
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Re: Rear Tire width and Aerodynamics [rruff] [ In reply to ]
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rruff wrote:
Don't ask me! If it's smooth I'd run 20mm Supersonics... but if fat tires ease your mind, then run what you have.

Looking at the Conti graph above, I wonder if we aren't missing something when we test on small rollers. I get much less Crr delta vs tire size than they show.

What psi do you run the Supersonics at? I'm 165 lb and was running 130, but I went to 115 the last race as an experiment and I think I was faster, given I had a good av speed even though my power was a bit lower than normal.
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Re: Rear Tire width and Aerodynamics [ridenfish39] [ In reply to ]
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I am 180 lbs and running Supersonics on my Edge 45's (ENVE) and I run 100 on the front and 110 on the back. Both are 23mm
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Re: Rear Tire width and Aerodynamics [ridenfish39] [ In reply to ]
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130 psi on a smooth road and less if it isn't.
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Re: Rear Tire width and Aerodynamics [rruff] [ In reply to ]
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rruff wrote:
130 psi on a smooth road and less if it isn't.

There are no smooth roads here in NJ or Pa :( I'm amazed these things have held up so far. I probably just jinxed myself for the State tts coming up LOL
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Re: Rear Tire width and Aerodynamics [rruff] [ In reply to ]
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rruff wrote:
Don't ask me! If it's smooth I'd run 20mm Supersonics... but if fat tires ease your mind, then run what you have.

Looking at the Conti graph above, I wonder if we aren't missing something when we test on small rollers. I get much less Crr delta vs tire size than they show.

I wonder about roller applicability to the real world as well. Wheel Energy tests with "bumpy" rollers that simulate road vibration at different frequencies. There's a chance that roller testing doesn't do enough to test the suspension characteristics of a tire at a given pressure and the effect this has on rolling resistance in the real world.
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Re: Rear Tire width and Aerodynamics [GreenPlease] [ In reply to ]
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Andy Coggan has done field tests that broadly support the findings of roller tests.
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Re: Rear Tire width and Aerodynamics [Old Albion] [ In reply to ]
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Haven't seen any that sufficiently answer that question...

The contact patch will be roughly the size of the weight on the tire x psi. The big difference between the roller and road is that the radius of the surface you are rolling on is quite small for the roller vs flat for the road, and the shape of the contact patch is very different. If you are comparing tires of similar size it doesn't matter, but it wouldn't surprise me if there was a size dependent variable that gave larger differences between sizes when a larger roller is used.
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Re: Rear Tire width and Aerodynamics [rruff] [ In reply to ]
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rruff wrote:
Haven't seen any that sufficiently answer that question...

The contact patch will be roughly the size of the weight on the tire x psi. The big difference between the roller and road is that the radius of the surface you are rolling on is quite small for the roller vs flat for the road, and the shape of the contact patch is very different. If you are comparing tires of similar size it doesn't matter, but it wouldn't surprise me if there was a size dependent variable that gave larger differences between sizes when a larger roller is used.
Doesn't Conti use a very large roller? I thought I saw that posted on here once before.

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