Power / weight ratio and speed

Anything about Power (watts)

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Bernie66
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Power / weight ratio and speed

Post by Bernie66 » Sat May 17, 2008 3:19 pm

Does anyone know why non-drafting riders on the fortius league with similar power / weight ratios can have such big differences in speed. I dont think it's likely to be all down to fine adjustments of roller pressure or type of tyre.

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qcscmh
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Re: Power / weight ratio and speed

Post by qcscmh » Sat May 17, 2008 4:47 pm

Bernie66 wrote:Does anyone know why non-drafting riders on the fortius league with similar power / weight ratios can have such big differences in speed. I dont think it's likely to be all down to fine adjustments of roller pressure or type of tyre.
Biggest diferrence is pacing, what event are you referring to.
Are they the same trainer, give us an example, but your right there are many discrepancies. Tyres make a diferrence as well.

Plus power to weight ratios are more relevant on hills than flat courses.
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Jan Snoek
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Post by Jan Snoek » Sat May 17, 2008 4:50 pm

Bernie,

An explaination can be that you are comparing Fortius-riders with Imagic-riders. Are you?

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Rein
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Post by Rein » Sat May 17, 2008 7:34 pm

Jan Snoek wrote:Bernie,

An explaination can be that you are comparing Fortius-riders with Imagic-riders. Are you?
That's what I was thinking... :wink:

Bernie66
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Post by Bernie66 » Sat May 17, 2008 8:33 pm

OK, thanks for the replies. Right now I can't give an example because the league is changing quite quickly and I've just replaced my unpaced ride with a paced one. I am probably comparing fortius with imagic at times but I hadn't noticed it, I'll watch out and see if I detect a pattern. The reason I asked this question is because I generally produce around 4 - 4.5 w/kg but I'm generally in the bottom quarter of the league, where as I definately see other unpaced riders (or at least that's what the riders delare in their 'notes') much higher up with the same w/kg output as I produce. I use fortius.

I'll post examples as I see them.

By the way, are the fortius trainers known to produce spuriously high power readings ?

Jan Snoek
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Post by Jan Snoek » Sun May 18, 2008 11:03 am

Bernie,

As far as I know is that Imagic is giving higher watt outputs compared to Fortius.

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Rein
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Post by Rein » Sun May 18, 2008 5:02 pm

Bernie,
where are you from? Are you from the USA. It's a known problem that especially downhill the 110V-units have a lower max-speed limit compared to 220V-units. Resulting in slower times while putting out high power. Take a look at the rides done by Bruce Hart in Fortius League 6 and you'll know what I mean...
What is the name you are riding on in the vcf-leagues? Or are you referring to another league. If I know that, maybe I can take a look at your rides.

Also, remember that a heavier rider normally produces more watts compared to a lightweight that goes at the same speed. The lightweight will have a much higher w/kg-ratio.
For example: Jan Snoek is faster than me, even though I have a higher w/kg-ratio.

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Post by Bernie66 » Sun May 18, 2008 9:10 pm

Rein
I ride under the name Mark Roberts. I live in UK.

I don't understand why a lightweight rider gives more w/kg. I would have thought that as we all have to add 8kg for the bike then actually the heavier riders are at an advantage in terms of w/kg (and speed). On a road the heavier rider produces more road resistance but on VCF we all produce the same 'road' resistance because the bike is fixed so the heavier riders real wieght is not transferred to the tyre - so this means that the bigger leg muscles of the heavier riders gets converted into pure power then into speed (whereas on the road the power output of the heavier rider doesn't all get converted into speed becuase the road resistance has to be overcome). Regarding w/kg and the bike I'm 60kg so a 8kg bike is 8/68 almost 12% of my combined weight ie I'm having to power a bike which is about 12% of my weight - so it's quite a task for my legs, whereas a 90kg riders bike is only 8/90 about 9% of the riders body weight so there's more of the riders power is there for propelling the rider.

Do the TACX and Imagic trainers measure power in the same way - how is it done ? Does the software detect the resistance of the roller and number of revolutions / minute of the roller and then use a simple calculation ? I ask because another rider I know with imagic trainer asked me if perhaps my wheel circumference was entered incorrectly but in fact fortius doesn't use wheel circumference measurements (so it must be measuring roller speed and resistance).

It's interesting that Jan Snoek is faster than you even though you have higher w/kg, because it suggests that even though you will both fine tune your equipment (which I imagine you both do) the difference in w/kg is not down to technique (when riding or when setting up your equipment), rather it's down to a difference in the way the ride statistics are handled by the software algorithm used to prepare our ride stats for the league. I suppose the real question is who would be the fastest in real life (you or Jan) on a non-drafting course if you were both using the same bike and riding in the same position - I would have thought it should be the person with the higher w/kg (if you were both the same weight).

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Post by qcscmh » Sun May 18, 2008 11:25 pm

I would hope the software allows 'all' the additional factors of a heavier rider at the diferrent speeds and elevation. But then ....

I have also encountered and heard about software and hardware diferrences that effect the speed/power. Never going to be perfect I am afraid.
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Rein
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Post by Rein » Sun May 18, 2008 11:34 pm

Bernie,
I do not know a lot about this. All I know is that the software uses algorythms to calculate the differences between heavier and lighter riders. How it's done, I don't know. You're right when you say that the percentage of the weight of a 8 kg bike is bigger with a lightweight-rider. I don't know if the software accounts for that. But I assume the software accounts for road-resistance, air-resistance etc.
By the way, on a flat road I believe air-resistance plays a bigger part than road-resistance.

If two riders on the real road weigh exactly the same and use the same equipment etc., the one with the highest w/kg-ratio obviously would go the fastest.

But the fact that Jan is faster than me with me having a higher w/kg-ratio is logical. See it like this: it takes more power (watts) to move a heavier body with a certain speed. So if Jan needs to put out 300 W to reach say 30 km/h, and he would weigh 100 kg his w/kg-ratio would be 300/100 = 3 w/kg.
Because I'm lighter, maybe I only had to put out 200 W to reach 30 km/h, so if I weigh 50 kg my w/kg-ratio would be 200/50 = 4 w/kg. Of course, it's not as simple as I put it here, but I think you get what I mean.

When I compare your figures with mine, I don't think there's anything wrong with them. You put out about the same amount of watts as I do (slightly more) but on the other hand, you're 5 kg lighter than me. So that seems reasonable. As soon as you get the hang on drafting, you'll notice you'll go a lot faster using less power. But to be able to compare your watt-readings with others, try the 10M and 25M TT without pacers-events. This should give you a pretty good idea where you stand compared to others.

The effect of drafting is also calculated by the software.
If it's all very accurate, I don't know. I think the whole competition-thing should not be taken too seriously. It's very hard to compare rides, even if they were both done on Fortius. Why? Because the effect of different tyres, roller-pressure etc. is very big and cause big differences in results. Also, the reported watts are way off, both on Imagic and in a lesser degree on Fortius. The watts are calculated, not measured. So if I turn the roller-pressure up, I'd have to put out more power to reach the same speed, but the reported watts would be the same..
Also, on the road w/kg-ratio has more effect when the road is going uphill. On a flat road air-resistance plays a bigger part. Don't know if the software takes that into account...

But I'm afraid the software doesn't take all the variables into account. Too complicated for Tacx I'm afraid... :wink: But even if they could develop software that could do that all, things still would easily be messed up by using different roller-pressure, tyres etc. I'm convinced particularly roller-pressure plays a very big part in the results riders get. As long as the watts aren't measured but calculated this would be the weakest link.

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