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TT / tire size question.(8 posts)

TT / tire size question.Mike P
Jan 21, 2004 6:55 PM
I am considering entering a local Time Trial series this year; several 5 Mile flat and 4 mile mountain TT's, alternating flat, mtn each week. I have a set of Am Classic 420s, which from what I can tell would be considered semi-aerodynamic.

Would it be of any benefit to try a 700x19 tire on the front vs a 700x23? Both front and rear 700x19?

The reason I ask is, I notice a substantial difference between the 420's and non-aero wheelsets. Just wondering if trying 19mm tires would also be noticable. Also, I am not really worried about the difference in ride comfort as I would only be riding the 19s for the TT's. How uncomfortable can you get in 5 miles?

Thanks,
Mike
Narrower is better...biknben
Jan 22, 2004 6:08 AM
A narrower tire will reduce the frontal area of the wheel and improve aerodynamics. Understand the difference may be undetectable.

Narrow tires are often reserved for TT where being fast and aero is everything. These narrow tires don't handle as well. A good handling tire isn't neccessary for a simple out n back TT course. The 18 or 19c tire is more than adequate. The 23c tires are more appropriate for road races or crits.

The back wheel is less important because the wind has already been stirred up by the time it gets there. The narrower rear tire may wear faster and add to a more harsh ride. The choice is yours.
Not completely true, maybe...ClydeTri
Jan 22, 2004 8:05 AM
Recent articles I have read state that narrower is not always better. Obviously a 700x19 tire is better than a 77x66 tire to make a point, but, a 700x18 may not be better than a 700x20 in some cases the articles state. Yes, narrower does have less wind drag. But the issue here is rollling friction ( I am doing this from memory so be forgiving). On a extremely flat surface narrower would be have less rolling friction, but as the surface gets rougher, the optimum width increases. Think of a theoretical tire the wide of a needle, can you imagine how it would bump along a road if you rode it. The road is in reality a series of peaks and valleys, miniture mountains and ridges. As the tire width increases it is able to "track" those hills and valleys better, thus reducing rolling friction. Thus there is some optimum width for a particular road you are riding on. This would also be dependent upon the force one is loading the tire with, a lighter person can ride a narrower tire than a heavy person. To sum up as my mind wanders with age, most of the information I have read suggests at least a 700x20 on the fron and a 700x23 on the rear. If you are riding on a pristine track, this could be different as it would be if you are an extremely light rider.
Not completely true, maybe...MShaw
Jan 22, 2004 9:49 AM
I think you're on the right track, but I gotta fine tune some of this.

FWI remember it isn't rolling restistance but aero drag that makes the wheels slow. The best being a tire/rim combo that is exactly the same size. If the tire bulges out past the rim it is a slower combo. Think inverted "U" rather than an ice cream cone where the ice cream overhangs the cone.

Anyone else?

Mike
Not completely true, maybe...ClydeTri
Jan 22, 2004 9:51 AM
That would make sense aerodynamically....so I guess the optimum solution is to minimize both aero and rolling resitance.
I've also heardclimbo
Jan 22, 2004 10:58 AM
that really super serious TT'ers have put silicone in the gap between the tyre and the rim to "improve" airflow over that part of the wheel. Maybe not needed if you have a nice tubular tyre but for clinchers it might save a few seconds?
re: TT / tire size question.JimP
Jan 22, 2004 8:38 AM
Mike,

Another issue may be your weight. You didn't state your size and weight. You may not be able to hold a high enough pressure in the rear tire to overcome pinch flats if the road is rough. I had difficulties with pinch flats on 19mm clincher tires on the rear when I weighed 175 lbs.

Jim
JimP, are we related? 8^)Mike P
Jan 22, 2004 8:59 AM
I may hit 155 when it's raining. I remember reading pinch flats could be of concern for bigger folks riding a smaller tire. I shouldn't have to worry about that though.

Thanks for the reply,
Mike