A question about frame design

unixnerd

Whenever you read a review of a new bike they're keen to point out that it's super stiff and that every watt of energy you put in goes to the back wheel because the frame design is so good. But putting aside the hyperbole and donning my engineer's hat (I have one somewhere) I have to try and quantify this (it's an engineer thing, sorry).

So if we take three base bike frames. Let's say one of the better 70's Raleigh steel race frames, my own Cannondale CAAD5 (a few years old but won some Tour stages in it's day) and for the sake of argument a new Pinarello Dogma. And let's assume the test condition is when a fit rider is applying peak power.

Just how much more efficient in terms of power transfer is one frame than the other (assuming the same groupset)? Compared to the Dogma does the Raleigh waste 5% of the rider's energy, presumably by using his/her power to flex the frame?

It'd be interesting to hear some numbers. I'm not a structural engineer but would be interested to hear from one. Can the difference in power transmission be perceived or is it so small as to be a placebo effect only to be measured by the stopwatch?

boggybrn

Interesting question...

I have two bikes; one has a mid '80s Raleigh lightweight division steel frame, and the other is a CAAD9. Where I notice the difference is when climbing. The CAAD9 feels a lot stiffer and is much quicker if I push as hard as I can. However for a long ride on a wet day the Raleigh (with full mudguard) is way more comfortable!

In terms of percentage difference I haven't a clue.

unixnerd

I have a steel tourer and it's more comfortable despite being about 12.5kg. I wonder how much of your perception of quicker is down to the wheels though? I just put a 500g lighter set on my CAAD5 and it feels like a different bike :-)

Wheelspinner

RIDE magazine publishes frame flex data in their bike tests. Basically they load each frame with a set weight and measure the deflection at a couple of points. Comparative data is interesting between framesets, but it's not really a measure of how the bike really rides.

I'm sure it does make a difference though - and doesn't take much to be noticeable either. I had a set of carbon cranks from Fulcrum (Torq R model, equiv to Campag Centaur carbon or thereabouts), and upgraded them to the Campag Super Record 11sp crankset when I changed the groupset over.

I was surprised because I noticed the difference in the stiffness of the cranks alone, with no other changes to the frameset or wheels. And I am no power rider either, just another MAMIL..

unixnerd

I must admit I recently upgraded from Truvativ to Dura Ace cranks and it does feel slightly sharper when I put power down.

Berk Bonebonce

You use the term 'frame design' but talk about materials? A steel frame could easily approach the stiffness of a carbon frame depending on what diameter tubing you use, what shape of tubing you use, and what wall thicknesses; and depending on what type of carbon frame you were comparing it to. I know which one would be heavier.

There are too many variables with bicycle frame design to have very meaningful conversations about stiffness. What point having an incredibly stiff steel frame if that stiffness comes with a significant weight penalty?

Monty Dog

It's not just about absolute stiffness either IME an overly-stiff frame whilst very efficient in the short term becomes tiresome and fatiguing after a couple of hours. Racing on our rough roads often makes it difficult to get the power down when the back wheel is skipping all over the place - I prefer to race on my older titanium bike for that reason as I know I can get the power down sooner and that my back doesn't ache by the time I get to the finish. Wasn't it one of the German bike mags who did a test and found that frame deflection between the stiffest and least stiffest was only a mm or so - barely enough for most riders to perceive?

maddog 2

just set off 5 minutes earlier...

racingcondor

I agree, mostly it's part of the advertising. As Monty points out too much stiffness can be so unforgiving that it's counter productive.

However. I also know a couple of people who used to be terrified descending at 40+ on flexy 80's steel frames and love the added stiffness that comes from their new ones so there is an element of safety to consider as well.

Still, it's more about 'stiff enough' than 'the stiffest frame ever' and that depends on the weight of the rider / fork used / 1,000,000 other small things.

Monty Dog

Rider size, weight, style and use is just as important - my racing weight is just above 60kg so what feels 'stiff' to me would be very different to a 90kg 'sprinter' so quoting absolutes becomes pretty pointless. There's nothing quite like 'railing' downhill corners on a super-stiff bike down ribbons of super-smooth tarmac under the blazing sun - sadly the reality is more like rough, pot-holed roads in the rain...