Cervelo reckon an aero frame is worth 1m in a 200m sprint. Sounds ok, but in reality it is such a tiny time difference. In the past most sprinters on Cervelo teams have tended to use the R5 style bike. They also reckon frame flex/ stiffness and the effect on sprint output/ efficiency is impossible to quantify it is so small. If this is true you have to wonder why 2 frames of similar geo and build can feel so different and also why manufacturers bother to make (or at least market) a bike that is stiff/ aero/ comfortable/ whatever. Breaking news is that all bike are the same! 😕

I think Aero bikes are becoming the “do it all” bike of the industry. That TREK I looked at last week will be up for testing soon and I’m looking forward to taking it out for a spin..
If I have it for a couple of days I’ll ride it in Hampshire and also take it north for some Yorkshire air too 😀

However in order to have vibration damping you have to have flex

Disagree. Vibration energy can be dissipated without actual flex.

Er, how? The bit you snipped just after the bit you quoted – how does an infinitely stiff frame dissipate energy?

Can we talk proper Engineering and back it all up with pretty pictures instead of cyclic arguments? 😀 If you’ve done some tests or modelling you must have some idea roughly what was done and roughly what was shown and roughly what the assumptions/ limitations were. As above, Cervelo seem to support Aracer’s argument wrt frame stiffness in a blog although I still can’t get my head around it completely as there was any detail to explain why they take the position they do. Are we making the assumption of only considering the vertical direction for example?

You talk about movement of the seattube but what about 6DOF of the rear axle relative to the BB for various realistic loads applied to the BB or axle area? I’m sure we’ve all seen some frames cleary, visually flex and a cheapy carbon Ribble I had was so floppy it almost felt like a full sus MTB on the rough roads! Was it my imagination?

aracer – Member
cynic-al » I think your analyis is oversimplified.
You’ve seen my analysis? Could you forward it on, as I mentioned above I’ve mislaid it.

To say that it’s just about vertical movement at the seat tube is oversimplified, how on earth could I be commenting on anything further(atcha, yawn, etc)

Sorry, I’ve lost the calcs I did – was simply a force diagram for a frame to work out the typical forces in the tubes for a given load, and then using typical dimensions for a steel frame I worked out the flex.

Not your imagination at all – lots of difference in the way frames I own flex around the BB under pedalling loads. However we are actually only interested in the vertical direction if we want to know what happens when you hit a bump (or just rough road surfaces) as that’s the direction the forces act in.

@al – it’s about vertical movement at the saddle – given your weight is acting downwards at that point how could it be about anything else? If you want to remove the influence of the seatpost, then that’s basically vertical movement at the top of the seat tube.

Where else do you think I should be determining movement?

I’m happy to continue this discussion, but to be honest I don’t care at all if you all disagree with me – not when it seems both Mike Burrows and the engineers at Cervelo do agree with me.

I’m happy to continue this discussion, but to be honest I don’t care at all if you all disagree with me – not when it seems both Mike Burrows and the engineers at Cervelo do agree with me.

They find the same things with the same assumptions as you. That isn’t surprising. I’ll need some convincing that forces acting across an entire frame/ fork are in one direction only though and important only at one point. The bike doesn’t sit bolt upright all the time and just by virtue of being able to feel a BB twist under load tells you there are forces acting in other directions…. how about sprinting with the bike being tossed from side to side, cornering, even normal pedaling will have some degree of movement in other directions which influences efficiency, handling and comfort 😕 The truth is probably that it is too difficult to model accurately, rather than other directions being completely unimportant. If you ask a simple question you tend to get simple answers

how does an infinitely stiff frame dissipate energy?

I would have thought that vibrations or sound waves in a solid would dissipate energy despite the material being effectively rigid on a macroscopic scale, which is what your tests would measure.

Rather than talking about perfectly stiff materials, my thought was that a frame doesn’t have to be flexing measurably to still damp vibrations. It’s not suspension – it’s suppressing the higher order harmonics.

aracer – Member

@al – it’s about vertical movement at the saddle – given your weight is acting downwards at that point how could it be about anything else? If you want to remove the influence of the seatpost, then that’s basically vertical movement at the top of the seat tube.

Where else do you think I should be determining movement?

You really didn’t think about seat tube flex? And you claim to know what you are talking about?

Ah, why didn’t you say al instead of beating around the bush? I calculated that separately, and the answer is that it is negligible – less effect on vertical movement at the saddle than the vertical movement at the top of the seattube. Because a normal seattube actually has very high bending stiffness and is also pinned at both ends. A seatpost is different as it is only pinned at one end and also smaller in diameter – bending stiffness is proportional to the 4th power of diameter so even a small difference in diameter has a relatively large effect on stiffness.

I thought it was obvious…so that was on a round tubed steel frame?

Will the flattened top tubes on modern alu frames be the same

<backs out and goes in search of the more harmonious environment of a JHJ thread>

Ah, so you don’t think “vibration energy can be dissipated without actual flex” you’re simply arguing about how much flex there has to be? The question then is, if there’s 5mm of vibration at your higher order harmonic, how exactly is a frame with only 1mm of flex damping that out?

Or to look at it another way, check out the materials typically used for vibration damping purposes. Then have a look at the materials between you and the ground when riding a bicycle – which is most similar to those?

Sorry, it wasn’t as I’d dismissed it way before doing the full frame calcs – apologies for not making that clear. We’re still talking about seat tube flex? I shouldn’t think the shape of the top tube makes a huge amount of difference to that, simply how much material there is in it as that’s mostly just under compressive load (there is some bending, but not very much and that’s pretty much all right at the headtube where it doesn’t affect seat tube bending).

Ah, so you don’t think “vibration energy can be dissipated without actual flex” you’re simply arguing about how much flex there has to be?

Well not quite – we haven’t really defined flex, but I’d take that to mean the tubes bending perpendicular to their length. That’s not necessary to dissipate vibrational energy. I’d guess that the tubes of a frame are vibrating in every which way, longitudinally and laterally, and that this energy is being absorbed differently in different materials, regardless of *lateral* flex which is what you’re talking about.

5mm of vibration

Which there would never be in a real life situation which is what I’m talking about.

Or to look at it another way, check out the materials typically used for vibration damping purposes.

Red herring. Totally different situation. Carbon fibre, steel, titanium and aluminium are never used for vibration damping, but that doesn’t mean that they transmit vibrations perfectly, nor does it mean they all do it the same.

My point is (despite all this bickering) that just because you don’t measure lateral flex in tubes, doesn’t mean vibration energy isn’t being lost.

That does at least show that none of us is missing anything obvious, and I’d hope most people at least trust MB’s knowledge more than they might mine.

The bike doesn’t sit bolt upright all the time and just by virtue of being able to feel a BB twist under load tells you there are forces acting in other directions…. how about sprinting with the bike being tossed from side to side, cornering, even normal pedaling will have some degree of movement in other directions which influences efficiency, handling and comfort

Sure they might, but I don’t think I’ve ever seen anybody suggest “this bike is really comfy when I’m pedalling, but so harsh when I’m coasting”, which tends to suggest that human perception isn’t identifying any effects on comfort due to pedalling. Let me just put the cornering one to bed though – when you’re cornering the force is still straight down through the vertical plane of the frame, if it wasn’t you’d fall off.

Which there would never be in a real life situation which is what I’m talking about.
…
My point is (despite all this bickering) that just because you don’t measure lateral flex in tubes, doesn’t mean vibration energy isn’t being lost.
[/quote]

I’m sure there is vibrational energy being lost. But to come back to the real life situation, if there is only 1mm of vibration being damped in the frame then that’s not enough for any noticeable difference in the comfort of the frame given the other bits between you and the road which will damp a lot more than that. The whole point here is that if you looked at the frames in isolation there might well be significant differences, but frames aren’t used in isolation and any flex or vibration damping they provide is dwarfed by the other bits. In the context of vibration damping, given how much flex you can calculate there is, it’s not unreasonable to model them as being infinitely stiff.

BTW I’m hoping this isn’t all just bickering – I’m doing my best to explain my points and have a proper discussion.

I don’t actually give a shit why it is. I’m still grappling with the notion that a superlight ti frame is no different comfort wise to my old OCLV road bike, which was hideously harsh. A lot of people imagined that, very odd.

if there is only 1mm of vibration being damped in the frame

I’m not sure I’d measure vibration damping in mm.. more like attenuation vs frequency… I don’t think we’re talking about the same thing.

Having a frame flex might absorb *hits* but that’s not the same thing as vibration damping. A van’s suspension absorbs many cm of road bumps but the panels still vibrate and make a lot of noise. To quieten that down you stick dynamat on the inside – the panels still flex when you push them, and the suspension still works the same, but the noise is far less. Vibration damped.

I’m saying that a carbon frame or whatever won’t absorb road hits any differently to an alu one for the reasons you say, but the hits could easily *feel* different….

Have to say I’m with njee…

It’s just different ways of measuring it. The 1mm is the vibration amplitude, not the vibration damping. If you want to measure it as a wave, then that is a pretty valid way to measure P-P for a physical wave – in the same way you’d measure P-P in V for an electrical wave, which would directly translate to dBmW given the electrical impedance (for the vibration in the frame you’d need to know the stiffness of the tube to convert to an equivalent dBmJ figure – in this case the wave has energy rather than power). I’m hoping you’re following me here – you’ve done engineering?

In the case of your van panels we are talking a totally different order of magnitude of flex involved. Not only that, but I suspect you’d find if you attached a saddle to them and sat on it that would do a pretty good job of damping the vibrations.

Yes of course but I was giving examples of how small vibrations behave differently to big hits. In your frame flex experiment, where you statically load a frame, you’re not testing the same thing as the riders are reporting.

You’re basically saying that there’s no difference in comfort between frame materials – and other riders are saying there is?

Back to aero…

http://www.met-helmets.com/index.php?option=com_content&view=article&id=255

http://road.cc/content/news/158211-met-manta-aero-helmet-launched-tour-de-france

Liking the look of these, as good looking as the Evade without the specialness of the Air Attack. Wonder when it’ll be available?

Lolz @ the 10W at 50km/h 😆

Why not? Evade claimed @ 20W over Prevail

Aimed at the 50km/h!

I’ve got a Cervelo S3, because it’s red

Ah I see. I too bemoaned Scott for using 45kph recently.

Looking into it I saw cdas quoted for the various helmets. Folk seemed to forget that non aero ones have a larger frontal area , which must also increase with size.

I’ll just leave this here then

Errr, good resurrection there.