Is watts per kg or just watts most important in a TT?

Eddie2593

I was always told that pure watts on a flat TT is most important, thats why powerful riders such as cancellara are so good at it.But i was watching the TT in the tour de suisse yesterday and i heard one of the commentators say Leipheimer is about as good as cancellara because he has a high watts per kg and has less frontal area to catch the wind.
Now im conufsed... :P

Zachariah

It is watts/kg but over a specific time - many non-cyclists, for instance, can get on a bike and produce a respectable W/kg number but only for a few seconds.

And clearly Leipheimer, while very good at TT-ing, is not as good as Cancellara, or he'd have a World TT Victory already. His top W/kg is probably not quite as high as Fabian's, but Levi can keep it up for longer, which is why Levi has several GC titles and Cancellara wins the TTs.

AidanR

On a flat TT it's watts per unit drag, which will be very closely linked to frontal area. This is probably correlated with rider weight, as heavier riders tend to be the larger riders, but then it depends on the riders' build and position on the bike too.

danowat

Weight has a lesser effect on TT's, mainly because the majority of TT's are on flat(ish) courses (at least here in the UK).

Drag is the key, but even larger riders (see Chris Boardman) can "shrug" down into a decent enough tuck

GeorgeShaw

Speed is power versus resistance. On big hills resistance = gravity, so weight is most important. For TTs, resistance = drag, so frontal area is most important.

SteppenHerring

If you assume (big assumption) that power is correlated with mass, then bigger riders are better off. This is because frontal area doesn't increase as fast as mass.

If you assume a spherical rider (me) the volume (mass) increases with the cube of the radius but the cross-sectional area only increases with the square of the radius. Same applies to any solid object.

So, in theory, drag only increases with the 2/3rds power of ... power. In theory.

StageWinner

The TdS TT was a little hilly in places, so I would think the W/kg figure came into play more so than if it had been purely flat. Which is why they were talking about it.


But as others say - on a flat TT, the pure wattage is generally more important than W/kg

SteveR_100Milers

In reality, even "flat" courses are not flat at all, so from my own personal and other clubmates / friends who were large and now not so have all found that losing weight has had a bigger benefit than any disc wheel or pointy hat. Not only is it easier to maintain your speed up those imperceptible drags, but you can probably get lower and more aero with less compromise on power output. Plus your glutes and lower back don't have to support so much body mass, so can effectively oppose bigger forces from the quad (didnt realise how important this was until I had a Bike Science bike fit). Excess weight is a big handicap whatever course or bike you are riding. My PBs all came after losing a stone last year, and I know that not having lost another stone is holding me back this season.

SBezza

Steve it isn't the weight so much though, if you lose weight, you lose bulk, and hence are more aero. Weight plays a little part, but most courses weight doesn't play a huge part, unless you really are very overweight. Being overweight might help on the hill on the R25/3 though

You might have found that you have got alot fitter as well as losing weight, and that played more of a part in the faster times as well. Unless you can compare 2 rides in similar conditions and with a power meter it is really hard to say.

For UK TT's it is more about Watts/Cda, being lighter means less bulk and likely to be more aero.

incog24

Definitely watt/cda. But if you've got a stone to lose, that may be symptomatic of general fitness issues that are holding you back anyway...

Pokerface

My TT times have dropped quite a bit in the last year. As has my weight. But I'm fitter overall - so it;s hard to say that one thing benefitted me more than another, etc.

But certainly on the non-flat courses, less body weight has made a huge difference.

Simon Notley

As stated above, on a hill it's watts per unit mass (i.e. W/kg), on the flat it's watts per unit frontal area (W/m^2).

Now, it is highly likely that a bigger rider will have both a greater mass and a greater frontal area than a smaller rider. However because mass is proportional to volume, but frontal area is (obviously) an area, the heavier rider will have an advantage.

Imagine two riders A and B with the same W/kg, if rider A weighs 50% more than rider B, the chances are that his frontal area is perhaps only 30-40% more*, meaning that - all else being equal, the heavier rider is better on the flat.

*If you assume that riders are spherical (!) then mass is proportional to r^3 and area to r^2, meaning that area is proportional to mass^(2/3). In this case a 50% increase in mass requires only a 31%

EDIT - just noticed that this has already been said and I missed it on my skim read! Apologies to first poster!

SteveR_100Milers

SBezza wrote:

Steve it isn't the weight so much though, if you lose weight, you lose bulk, and hence are more aero. Weight plays a little part, but most courses weight doesn't play a huge part, unless you really are very overweight. Being overweight might help on the hill on the R25/3 though

You might have found that you have got alot fitter as well as losing weight, and that played more of a part in the faster times as well. Unless you can compare 2 rides in similar conditions and with a power meter it is really hard to say.

For UK TT's it is more about Watts/Cda, being lighter means less bulk and likely to be more aero.

Technically I totally agree, however, personal experience / observation is suggesting something slightly different. On a pan flat course, I'm reasonably competitive, anything hilly and I'm dead. However, take the R10/17 near abergavenny, its not pan flat, but one of the fastest 10 courses in SW DC. When I lost about 3-4 kgs last summer without changing bike position and wth only about 4 weeks of additional training, I knocked just over a miunute and a half of my course PB. Looking at my Garmin traces, it was because I was 3-4 mph faster up the slight outward "uphill" drag and similarly after the turn. I was same speed / no more than 1 mph slower on the corresponding downhill sections. Maybe for other riders its a different story, but if you are a large rider (and I don't mean overweight), then the gains in speed on anything uphill no matter how slight the gradient outweigh the downhill speed losses significantly. One of my clubmates who is a fair bit older than me has lost several kgs in the last couple of months, he has just smashed out a 56 and a short 22; PBs by a large margin. OK I am sure additional training has helped, but alone cannot explain a sharp step up in performance such a short time. It's not everything, of course power to frontal area is probably much more influential, but weight loss might just be better than any disc or pointy helmet!

SBezza

Steve

I am not heavy and I struggle on hillier courses in comparison to riders I am close to on flat courses. Some people can just climb hills quicker, and it may not be a weight issue. Last year some of the road racers that weighed the same as me absolutely killed me going up the hills, as they are used to actually racing full bore up them. On a normal TT I will beat them fairly easily.

I could say my improvements this year are due to losing 1 or 2 kgs, but I know that this is not the reason, it is just fitness and putting out more power.

Most standard distance TT courses are not that hilly to be honest, they will have drags in them, but even then alot of the time differences will be pure power. As you lose weight you will get fitter to be honest as Pokerface said, I would imagine it is more fitness than weight that has caused the times to come down, though obviously the lack of weigth will always help to a degree, it is less mass to accelerate for one.

If you have a hilly TT then yes, it is going to be W/KG, but for the norm in UK TT-ing, it will be W/Cda.