LT Intolerant

I'm having a friendly debate w a few friends and curious as to opinions of BFers.

There is a 4.1 mile TT w an average grade of 3-4% and one short section as steep as 7%.

Some feel that TT bars/bike/helmet/shoe covers et al are a must.

Others argue that in a TT that short w a grade of 3-4% the aero advantage isn't a big deal and that weight savings are more important.

The average TT bike is roughly 17-18 lbs while the average road bike for those on either side of the debate is closer to 14-15 lbs.

Any thoughts on who has it right and why?

Thx in advance for any insight

gene r

dgearhart

Depends on how fast you'd ride it on the TT bike. If you are not going at least 18 - 20 mph+, then aero will do little to help. Pro riders would go aero, but they ride insanely fast.

dmotoguy

we had a race similar to that last year, the fast guys were on time trial bikes if that means anything...

StanSeven

I would say most people are faster with a TT setup as long as they stay seated and are reasonable smooth. A 3-4% grade will keep you seated. The 7% means standing, at least for me, and that's where you loose the aero benefit.

tanhalt

Does the overall average include the 7% section? Any downhill/flat sections?

The real question is: Can you stay in your aero bars on the 3-4% grade or are you going to be going on to the cow horns? If you can stay in the aero bars...then use the TT bike. Once you get out of the aero bars and sit up though, you might as well be on your road bike and take advantage of the lighter weight. Either way, you should still use all of your other "aero goodies" (such as skinsuit, helmet, shoe covers, etc.) including a deep front wheel and a disc rear (or VERY deep rear).

Perhaps some shortie clip-ons for the road bike if portions are less steep and you think you'd want to get more aero?

umd

Is this about SNSR? OC Roadie asked about this a few weeks ago.

botto 

if you've got it, use it.

bas2205

4.1 miles means all out and I vote TT-equipment. Going lightweight on hills usually means that you are either trying to save yourself for the "rest" or it's REAL climbing.... 3-4% is nothing in a TT this short, I'd count that as a "false flat". Assuming the 7% section is short, say clearly less than half a mile, just stay in the big ring and power over it... Sounds a bit like the Stuttgart World Cup course from 2007!

LT Intolerant

This is seemed to me to be a fair compromise so thanks Tom.

Yes Steve, it is for the SNSR.

Thanks all for weighing in.

curiouskid55

If you are talking about San Dimas virtually everyone will be using their best climbing setup, no TT rigs, no aero helmets.

DrWJODonnell

If you will average more than 15mph, I would say aero will trump weight. Usually a 3-4% grade can be hit much faster.

LT Intolerant

I'd always heardread that 20 mph was where the aero advantage kicked in.

In terms of hitting a 3-4% "much faster" maybe for genetic mutants like YOU that's the case!

grolby

There is no point at which it "kicks in," being more aerodynamic is ALWAYS faster. The question is the tipping point where it becomes more effective to focus on being light instead of aero, or where the benefit is more than negligible. 15 mph is certainly fast enough to be worth it, and I might even take the aero gear if I thought I would be as slow as 12 or 13 mph. Aero gear gets you the most benefit at lower speeds. For a TT this short, definitely aero.

DrWJODonnell

I was bored:

Comparison between a rider + bike of 80 kg with a CdA of .300 and switching to a 2kg heavier setup which drops CdA to .250. Take 3 scenarios where the wattage the rider can expect to put out will be 250, 300, or 350 watts.

Assumptions
80kg rider and bike (176 lbs)
CdA (upright) of .300
250 watts output
3% grade = 7m/s=15.7mph=15.67min
4% grade = 6.06m/s=13.6mph=18.09min
5%grade = 5.27 m/s = 11.8 mph=20.85min
82kg rider and bike (176 lbs)
CdA (upright) of .250
250 watts output
3% grade = 7.12 m/s=15.9mph=15.47min
4% grade = 6.09 m/s=13.6 mph=18.08min
5%grade = 5.25 m/s = 11.7 mph=21.02min
Aero breakpoint = 4% grade Answer=Maybe Aero depends on grade

80kg rider and bike (176 lbs)
CdA (upright) of .300
300 watts output
3% grade = 7.88m/s=17.6mph=13.98min
4% grade = 6.92m/s=15.5 mph=15.87min
5%grade = 6.10m/s = 13.6 mph=18.09min
82kg rider and bike (176 lbs)
CdA (upright) of .250
300 watts output
3% grade = 8.05 m/s=18.0 mph=13.67min
4% grade = 7.00 m/s=15.7 mph=15.67min
5%grade = 6.11 m/s = 13.7 mph=17.96min
Aero breakpoint >5% Answer = Always aero


80kg rider and bike (176 lbs)
CdA (upright) of .300
350 watts output
3% grade = 8.65m/s=19.3mph=12.74min
4% grade = 7.70m/s=17.2mph=14.30min
5%grade = 6.86 m/s = 15.3 mph=16.08min
82kg rider and bike (176 lbs)
CdA (upright) of .250
350 watts output
3% grade = 8.88 m/s=19.9mph=12.36min
4% grade = 7.82 m/s=17.5 mph=14.06min
5%grade = 6.90 m/s = 15.4 mph=15.97min
Aero Breakpoint >5% Answer = Always aero


So, it will of course depend on much more than this, but assuming our rider is an 'average weight' rider and can putout some of these 'average powers' while maintaining 'average positions' it seems quite likely that aero will be the way to go.

tanhalt

Good stuff, Doc...BUT, I think your .300 m^2 assumption might be a bit high for someone riding a road bike but still with all the other aero goodies including wheels. That's going to lower the "road" CdA and the "breakpoints" a bit.

Also, from having done these sorts of calculations for a similar type TT (2006 Conquer the Canyons - which actually had some flat and downhill sections) I learned that all of the drag calcs go out the window as soon as you sit up onto the cowhorns of the TT bike. I'm willing to bet that most folks won't be able to stay in their bars at grades up to 4%...mostly because they aren't geared low enough.

Also, it can be helpful to think of the course in the terms of "time" at each grade instead of "distance"...you'll travel faster over the shallower grades and so the aero advantage doesn't add up as much as the weight advantage adds up for the steeper sections, since you spend a longer time on the steeper grades, proportionally speaking. It's along the same line of thinking of why it makes sense to put out incrementally more power when you're going slower...

The only person I've heard to effectively "rock" a full-aero TT setup on the SDSR TT that LT is asking about was Eric Wohlberg...and we all know what sort of freak he is

I still say a light road bike with deep wheels, possibly shorty clip-ons, and skinsuit, booties, and aero helmet is the call for that course...

cslone

My vote in the other thread was road bike with deepest wheels possible, and to wear everything you would wear in a TT(aero helmet, skinsuit, etc.).

umd

LT is asking about the SNSR TT not the SDSR TT. It's on Saturday in Nevada.

Here is OC Roadie's thread
https://www.bikeforums.net/showthread.php?t=508321

tanhalt

Aaah...I was off by a letter (D vs. N)...sorry 'bout that

See my post in that other thread...at least I'm consistent