It's a little bit about the bike...

neeb

Because most of us are understandably cynical when it comes to the BS from manufacturers about faster, stiffer bikes & wheels etc, it can come as a surprise to discover that the bike can actually make a bit of a difference..

Most of my training in the winter is indoors on a taxc fortius. I have three bikes - a Ti Enigma, carbon Scuro RS and an old Alu cross bike. Previous winters I've had the Ti bike on the trainer with Campagnolo Eurus rear wheel, but this year I initially used the cross bike, with a low-end alex a-class wheel. Exactly same setup otherwise, same tyre, same roller tension etc. I was finding that I just couldn't get my power output and finishing times up to what I was managing last year. I know what figures I should be able to get when I'm doing several sessions a week, but it just wasn't happening, and it was beginning to seriously affect my confidence and motivation. So I tried putting the carbon bike on the trainer instead, with the same Eurus wheel (the Ti bike is elsewhere at the mo.). I was instantly able to generate the sustained power I am used to.

The difference seems to be nearly 15W in a 40 minutes max effort session. As it happens this about the same as the difference I can expect between being moderately untrained (say if I haven't been doing anything for 3 or 4 weeks) and being in reasonable shape.

I guess the lost power must be either due to frame/crank flex or wheel flex of some sort. Bike setup (contact points) are exactly the same. I must admit it came as a bit of a shock to discover that the bike does actually make such a significant difference!

Chiggy

What's 15 W in percentage terms of what you expected?

Check the chain for stiffness / lubrication.
Check the pedal bearings.

neeb

Chiggy wrote:

What's 15 W in percentage terms of what you expected?

Check the chain for stiffness / lubrication.
Check the pedal bearings.

I was getting about 255-260W instead of 270-275W, so a bit more than 5%. Pedals and chain were fine. The only other factor I can think of is that the cross bike had a 9sp cassette and the carbon bike 11sp, so it was a bit more difficult to find the best gear when doing simulated climbs.

Tom Butcher

Isn't this more likely to be due to a slightly different clamping tension of the wheel on the roller ? Tyre size and tyre pressure may be slightly different. There may be some slight differences in drivetrain efficiency and drag from the hubs but I can't imagine it's anything like15W.

neeb

There may be some slight differences in drivetrain efficiency and drag from the hubs but I can't imagine it's anything like15W.

Yeah, I wouldn't have thought so either. I'm pretty pedantic about tyre pressure and clamping force though, because I know it makes a difference. I was using exactly the same blue tacx tyre which I always inflate to 105psi before each session. The roller clamp is always set at 3.5 turns (i.e. tightening it 3.5 turns after it first makes contact with the tyre).

The tacx system has a calibration option that tests the braking force of the roller (given the setup) and compensates. I don't generally use that because it varies so much according to whether the tyre is warmed up or not. However, with my standard setup I always get a recommenced calibration of around zero immediately after finishing a climbing route, as long as I inflate to 105psi and use 3.5 turns. I did notice with the cross bike that the recommended calibration was a bit off, and perhaps if I had "accepted" the calibration it would have wholly or partially compensated for the 15w difference. But that 15W being compensated for still must be being lost through the bike & wheel combo.

The alex wheel is perhaps not quite as true as the Eurus, and might be slight "out of round". Maybe that's making a difference to the dynamic braking force of the roller. I need to try using the Eurus wheel with the cross bike and see if it's the wheel or the bike that's making the difference.

Pokerface

I can get a good 50w more* out of my race bike. Party due to fit and comfort - but probably mostly due to oval chainrings vs round ones. Oval chainrings give me more power. Or at least higher power readings. :oops:




* top-end sprint power, not FTP

Chiggy

Pokerface wrote:

I can get a good 50w more* out of my race bike. Party due to fit and comfort - but probably mostly due to oval chainrings vs round ones. Oval chainrings give me more power. Or at least higher power readings. :oops:




* top-end sprint power, not FTP

Is that Peak Power, or the average due to a less fluctuating application of torque?

Pokerface

Chiggy wrote:

Pokerface wrote:

I can get a good 50w more* out of my race bike. Party due to fit and comfort - but probably mostly due to oval chainrings vs round ones. Oval chainrings give me more power. Or at least higher power readings. :oops:




* top-end sprint power, not FTP

Is that Peak Power, or the average due to a less fluctuating application of torque?

Mostly peak power, but that will also affect average power. My oval rings just help get the most power out of my particular pedal stroke - although I did read an article in one of the cycling mags a while back that indicated that they do tend to add a fair bit of wattage to peak sprinting power - more so than to FTP.

rozzer32

Mostly peak power, but that will also affect average power. My oval rings just help get the most power out of my particular pedal stroke - although I did read an article in one of the cycling mags a while back that indicated that they do tend to add a fair bit of wattage to peak sprinting power - more so than to FTP

Didn't help Thor against Cav :P

Pokerface

rozzer32 wrote:

Mostly peak power, but that will also affect average power. My oval rings just help get the most power out of my particular pedal stroke - although I did read an article in one of the cycling mags a while back that indicated that they do tend to add a fair bit of wattage to peak sprinting power - more so than to FTP

Didn't help Thor against Cav :P

Yeah - pretty sure THor doesn't actually use the Rotor Q-Rings on his bike - just the Rotor crankset. (Some Cervelo riders have opted for regular round rings).


But I get your point.

rozzer32

Pokerface wrote:

rozzer32 wrote:

Mostly peak power, but that will also affect average power. My oval rings just help get the most power out of my particular pedal stroke - although I did read an article in one of the cycling mags a while back that indicated that they do tend to add a fair bit of wattage to peak sprinting power - more so than to FTP

Didn't help Thor against Cav :P

Yeah - pretty sure THor doesn't actually use the Rotor Q-Rings on his bike - just the Rotor crankset. (Some Cervelo riders have opted for regular round rings).


But I get your point.

Well in that case, that is where he is going wrong!!

jonmack

http://velonews.competitor.com/2010/12/ ... ike_152040

Specifically,



On the Cav vs Thor subject, how did Cav do in the world championships compared to Thor...

Pokerface

jonmack wrote:

http://velonews.competitor.com/2010/12/gallery/a-look-at-thor-hushovds-2011-garmin-cervelo-race-bike_152040

Specifically,

So - pretty much just backing up exactly what I just said about him not having Q-Rings on his bike?

scapaslow

How are you measuring power? If you are using Fortius power then my experience is that it is generally inaccurate and can vary from one session to the next which could easily explain your discrepancy.

If you want an answer to your problem try and get hold of a Powertap or similar if you can and repeat the exercise.

I had a recent period of use with the Fortius, despite being careful about calibration etc... where i was completing previously done routes with greater power than before but taking longer?
I've also experienced a lot of variance in the calibration results. For instance with several days between two Fortius sessions and the bike not being touched in between - the calibration value on Fortius after the same warmup time as before sometimes gives a higher value than the previous session?

I hired a PT a few months back and mostly don't bother calibrating at all now. I'm getting best results when i use a much lower tyre pressure (approx 80psi, yellow conti tyre) and if i do calibrate then i'll aim for +1 to +1,5 after a quick warmup on catalyst.

jonmack

Pokerface wrote:

jonmack wrote:

http://velonews.competitor.com/2010/12/gallery/a-look-at-thor-hushovds-2011-garmin-cervelo-race-bike_152040

Specifically,

So - pretty much just backing up exactly what I just said about him not having Q-Rings on his bike?

Haha yeah sorry, it was more aimed at rozzer32.

neeb

scapaslow wrote:

How are you measuring power? If you are using Fortius power then my experience is that it is generally inaccurate and can vary from one session to the next which could easily explain your discrepancy.

If you want an answer to your problem try and get hold of a Powertap or similar if you can and repeat the exercise.

I had a recent period of use with the Fortius, despite being careful about calibration etc... where i was completing previously done routes with greater power than before but taking longer?
I've also experienced a lot of variance in the calibration results. For instance with several days between two Fortius sessions and the bike not being touched in between - the calibration value on Fortius after the same warmup time as before sometimes gives a higher value than the previous session?

I hired a PT a few months back and mostly don't bother calibrating at all now. I'm getting best results when i use a much lower tyre pressure (approx 80psi, yellow conti tyre) and if i do calibrate then i'll aim for +1 to +1,5 after a quick warmup on catalyst.

After 3 years of using the fortius I have some theories about accuracy discrepancies and the best way to deal with them to get consistent results on individual routes. I apologise in advance for this necessarily long & nerdy rant!

The problem with the system is that there is quite a lot of variation in the power readings you get for a given effort (and hence the times for a route) depending on how well warmed up the roller and tyre are and also the type of the route, e.g. how much climbing etc. So the calibration value you get will vary wildly depending on when you take it. My experience is that calibrating for each ride introduces far more inconsistency than it solves. I find that if I am very careful with the tyre pressure and roller resistance I can get pretty consistent results without calibration. Rather than setting calibration to compensate for variations in setup, I have tweaked the setup so that I get a recommended calibration value as near to zero as possible when it is tested at the end of a long climbing route. I often test the calibration at the end of a session and it is nearly always within the range -0.5 => 0.5, but I always keep it set at zero. I'm sure if I tested it nearer the beginning of a session or after a short warmup it would vary a lot more, just because there is more potential error in the system before it has completely warmed up.

Basically, the paradox is that the best time to calibrate is right at the end of a session, but then of course you can't calibrate for that session... The way around that as I see it is to be really pedantic with the setup and to monitor the calibration at the end of each session on a big climbing route just to check that it is remaining at around zero (or whatever constant value you have set it to) without actually changing it from session to session.

So anyway, with the cross bike I used exactly the same setup - tyre pressure, roller resistance etc, but was consistently getting power readings about 15W lower for 20 - 40 minute climbs. I also consistently got recommended calibration values at the end of sessions that were a little "out of range" compared to the other bikes. Perhaps if I had set this recommended calibration value for this bike it would have dealt with the 15W variation, but it would still be the case that the 15W power loss must be coming from the bike, as everything else is the same...

I had a recent period of use with the Fortius, despite being careful about calibration etc... where i was completing previously done routes with greater power than before but taking longer?

That shouldn't happen - however accurate or inaccurate the power readings, as far as I know there is a completely consistent relationship between measured power and time for a route - the software basically uses measured power and gradient to calculate speed at any given time in a completely predictable way. Obviously there isn't a completely consistent relationship between average power over a route and time taken, as it will depend on how the power is distributed, e.g. whether you put more effort into climbs or descents etc (you could get a higher average power by putting lots of effort into descents without increasing your average speed as much as you would if you put the effort into climbs).

Mr Dog

Enough words and more sexy pictures of bikes! 8)

Chiggy

Its a good job cyclists don't have to be emissions tested

scapaslow

That shouldn't happen - however accurate or inaccurate the power readings, as far as I know there is a completely consistent relationship between measured power and time for a route - the software basically uses measured power and gradient to calculate speed at any given time in a completely predictable way. Obviously there isn't a completely consistent relationship between average power over a route and time taken, as it will depend on how the power is distributed, e.g. whether you put more effort into climbs or descents etc (you could get a higher average power by putting lots of effort into descents without increasing your average speed as much as you would if you put the effort into climbs).

I know it shouldn't happen but it did! This was on a sustained climb - Col Du Tourmalet (so no slacking!) I offered the files to Tacx for consideration but funnily enough they never took me up on it... I'm not the only one this has happened to.

I think calibration is only important to ensure that you have enough pressure on the tyre to avoid slippage on climbs. I find that +1,5 or so is about right.

As you suggest getting accuracy with a Fortius is a challenging task. At least with the PT i've been using i can now see the inaccuracies. I'm sure that your 15W can be put down to general inaccuracy of the machine.

A couple of things i've noticed comparing PT values with Fortius over a period of time are:
-when the unit is cold it tends to under read the power values and once warmed up it then over reads
-it over reads power on any grade over 1% and can over read by 20 to 50W on a climb
- on the flat it's generally pretty accurate

I should also point out that the PT is not error free either with the Fortius. On several occasions after a downhill section on an RLV the PT readings have dropped and then stayed well below where they should be once on the flat or uphill again. I don't know why this happens?

neeb

I know it shouldn't happen but it did! This was on a sustained climb - Col Du Tourmalet (so no slacking!) I offered the files to Tacx for consideration but funnily enough they never took me up on it... I'm not the only one this has happened to.

Could the rider weight or scale factor have been set differently? I've never had this particular type of inconsistency.

As you suggest getting accuracy with a Fortius is a challenging task. At least with the PT i've been using i can now see the inaccuracies. I'm sure that your 15W can be put down to general inaccuracy of the machine.

A couple of things i've noticed comparing PT values with Fortius over a period of time are:
-when the unit is cold it tends to under read the power values and once warmed up it then over reads
-it over reads power on any grade over 1% and can over read by 20 to 50W on a climb
- on the flat it's generally pretty accurate

Yup, I've noticed these inaccuracies too, but I've always had the impression that they are reasonably consistently inaccurate. All that matters to me is that I can compare the time/wattage I get on one ride on a particular route to the next ride on that same route. If you always set the unit up exactly the same and always do the same ride from cold (plus the same warm-up if appropriate) I find the readings are remarkably consistent. For a given route with the same slope profile and starting from cold, the inaccuracies should be duplicated each time. You can't compare values for two different routes or for rides on the same route where other conditions are different (tyre & roller pressure, different length of warmup from cold before starting etc), but otherwise I seem to be able to get figures for each ride on a given route that appear only to vary significantly with my fitness or perceived effort. Maybe I am just lucky with the particular unit I have!

I know that the inaccuracies of the fortius compared with real power as measured by a power meter will easily be +/- 15W or more, but I don't think the variation from session to session on a given route with the same setup has to be nearly that much. If it was, I wouldn't be able to use it at all to measure fitness improvement / effort, as the variations I get due to these factors are about that level of magnitude at +/- lactate threshold. But it does seem to work for me - my times on a one hour route might vary by 2 or 3 minutes (or 15W average power) in a way that makes sense given my fitness / perceived effort.

freehub

Surely 15w is just a variation in performance? Some days I'm averaging 300-310w, another day I could be averaging like 290-300w.

neeb

freehub wrote:

Surely 15w is just a variation in performance? Some days I'm averaging 300-310w, another day I could be averaging like 290-300w.

Yes, I'm the same. But if you suddenly find that all of your numbers are 15W less due to some other cause, you definitely notice, especially over a few sessions. And especially if they are all 15W more again when the problem is sorted! But yes, initially I had put it down to performance / fitness, and it was starting to affect my motivation. As soon as I swapped the bike I was starting to get "near best" times again, which had been impossible with the other bike even when I thought I was feeling really strong.

amaferanga

If you've got an accurate power measuring system then 15 Watts is significant. I'd certainly notice that on the turbo.

Btw neeb - is your turbo in your house or in a garage? Presumably the temperature will effect things if its in an unheated garage.

neeb

amaferanga wrote:

Btw neeb - is your turbo in your house or in a garage? Presumably the temperature will effect things if its in an unheated garage.

It's in the living room (with a very big fan ). I sometimes have the window open but the temp shouldn't vary by more than a couple of degrees.

Chiggy

The good results were with the Campag wheel. The reduced results were with a low-end Alex wheel.
The power is detected at the roller on the Fortius.

The low-end Alex wheel with its wider moment of inertia and radius of gyration; and the difference in turbulence created by its spokes absorbs the missing power before that power gets to the roller.

How much lighter is the Campag wheel over the Alex wheel?

Discuss.

SBezza

Do both wheels have different tyres, that could quite easily cause a difference. Sorry if I have missed it, but is the difference in the average watts for the complete session, or feel at a particular HR/effort.

scapaslow

amaferanga wrote:

If you've got an accurate power measuring system then 15 Watts is significant. I'd certainly notice that on the turbo.

+1

Especially at threshold. Might be insignificant if you can put out a lot of power, but I'm pretty limited in the power i can put out- so for me a 30min interval at 230W (PT watts) is hard work but achievable. To do that at 245W is something i've not yet managed.

I can certainly feel the difference in that 15W.

Chiggy

15W is five Sturmey dynohubs. A LOT to lose just changing a wheel.

Its 4%, which is a lot of transmission losses.

58585

I'll throw in another useless opinion
Just be aware that you are not measuring your power output directly;
If you were using an SRM and regularly "won" 15W by switching bikes I would accept that your position on each bike allowed you to produce differeing levels of power - there would also be an affect of drive train losses (tiny) and air resistance (huge) on "what you got" for the given power output on each bike in the real world.

If you were using a powertap and saw the same difference when changing bikes I would say pretty much the same thing; that the difference would mainly be due to position, but the different drive train losses might be playing a small part.

You are measuring the power not at the crank, nor at the wheel but at the trainer - you may think you have eliminated all the variables by calibrating and checking tyre pressure etc, but nevertheless you are not measuring your power output directly so you can't claim that the bike is making any difference to your power output.

If you performed a test on both bikes (on the road) with an SRM fitted and identical riding positions you would get the same power output, with a power tap there might be a slight difference due to drive train losses but this would be statistically impossible to measure (unless you had a chain that was rusted solid on one bike).

Certainly for the same power output on different bikes/wheels your speed will be different but there is no way a well maintained bike is losing 15W thorugh drive train losses or frame flex!

Chiggy

58585 wrote:

I'll throw in another useless opinion
Just be aware that you are not measuring your power output directly;
If you were using an SRM and regularly "won" 15W by switching bikes I would accept that your position on each bike allowed you to produce differeing levels of power - there would also be an affect of drive train losses (tiny) and air resistance (huge) on "what you got" for the given power output on each bike in the real world.

If you were using a powertap and saw the same difference when changing bikes I would say pretty much the same thing; that the difference would mainly be due to position, but the different drive train losses might be playing a small part.

You are measuring the power not at the crank, nor at the wheel but at the trainer - you may think you have eliminated all the variables by calibrating and checking tyre pressure etc, but nevertheless you are not measuring your power output directly so you can't claim that the bike is making any difference to your power output.

If you performed a test on both bikes (on the road) with an SRM fitted and identical riding positions you would get the same power output, with a power tap there might be a slight difference due to drive train losses but this would be statistically impossible to measure (unless you had a chain that was rusted solid on one bike).

Certainly for the same power output on different bikes/wheels your speed will be different but there is no way a well maintained bike is losing 15W thorugh drive train losses or frame flex!

This is the difference between Brake Horse Power or Cheval Vapeur or Pferdestärkes at the crank; and Roller Power on a chassis dynamometer.

neeb

SBezza wrote:

Do both wheels have different tyres, that could quite easily cause a difference. Sorry if I have missed it, but is the difference in the average watts for the complete session, or feel at a particular HR/effort.

Same tyre (I mean exactly the same blue tacx tyre). 15W difference is average for a particular section of the session that is all uphill (about 7% simulated slope) for 35 mins or so where I am generally trying to keep at threshold / 85-90% of HRmax.

Certainly for the same power output on different bikes/wheels your speed will be different but there is no way a well maintained bike is losing 15W thorugh drive train losses or frame flex!

I'm beginning to think it must be some weird interaction between the trainer and the bike/wheel. As I mentioned, the alex wheel is a little out of round. Or maybe frame flex, while not significantly reducing power through the drive train, causes the tyre to move sideways more when in contact with the roller.

I realise this all seems a bit pointless and speculative - all I know is that there is a significant difference to the measured performance I get with different bikes on the same trainer, and it's enough to disrupt the measurements I use to gauge fitness / training benefit.