FRAME STIFFNESS

[roubaixtuesday]

It is quite interesting to stick race wheels into a touring bike and go for a ride like that.

Hmmm. I've now got a Kinesis 4S as my touring set up, which is most of the way to a road bike anyway.

But I might stick a 48 spoke tandem wheel on the front of my road bike and see how that feels - I think it would fit. That should give as big a contrast as possible!

[JakobW]

Yes, somebody did. (Don't know why the front part doesn't load)

Is that actual test data, though? Looks like a finite element computer model to me.

I suppose I was thinking that strain gauging should be cheap enough for someone like Jan Heine to instrument a couple of bikes and try and quantify the differences between 'planing' and 'non-planing' frames.

[scottg]

I thought Bates with CantiFlex tubes solved the problem of frame "whip" years ago.

I Ti Bates with steel Diandrant fork would be great, a Carbon Gablier would fun too.

[Keezx]

Yes, somebody did. (Don't know why the front part doesn't load)

Is that actual test data, though? Looks like a finite element computer model to me.

I suppose I was thinking that strain gauging should be cheap enough for someone like Jan Heine to instrument a couple of bikes and try and quantify the differences between 'planing' and 'non-planing' frames.

I don't remember where I found it, just stored it years on the PC. Shows nice where the critical area's are.
Cracks will most likely be found in the red zone's......

[Valbrona]

... there is not much lateral bracing that you can do...

Design of bicycle frames, as of course Colin knows, does have a number of limiting factors. For example, cranksets have to be able to spin without chainrings or crank arms hitting chainstays, chainstays have to spread out wide enough to accommodate wheels with OLD 130/135, and there sometimes has to be enough clearance built in for fat tyres and mudguards. All these things can limit the size and shape of tubes, and especially in the case of metal tubes.

If I was designing a frame I would by default design it around the PF86 bottom bracket standard as you have got a lot more real estate in the bottom bracket area. Something like PF86 allows chainstays to be pushed further apart (because in the case of PF86 you have got a significantly wider BB shell), and you can achieve significant increases in frame stiffness this way. With a wider BB shell, joining frame tubes can also be of larger diameter.

[531colin]

..........
When riding normally, the bending load in the BB is effectively resisted by the seat tube alone; there is little or no load on the handlebars (you can usually pedal 'the same' with just fingertips or no hands) , and the remaining parts of the frame are not able to transfer load efficiently to the saddle, which is where the force is reacted laterally. ...........

Errr....run that by me again?
Just tapping along, I push the pedal down. Because the pedal is off to one side of the frame, there is a resultant sideways force on the bottom bracket.
It seems to me that sideways force is resisted by the rear wheel contact patch with the road, and to a lesser extent the front wheel. The saddle has no contact with anything apart from my enormous inertia.
For the seat tube to resist the bending load on the bottom bracket, doesn't the seat tube have to flex? and doesn't flex in the seat tube mean that the chainstays and downtube must flex, as they are all joined?

[Mr Evil]

In discussions of frame stiffness, it is worth bearing in mind that differences are most likely not actually detectable if you were to do a blind test.

... (Don't know why the front part doesn't load)

its there, but your browser is as crappy as mine and doesn't show it...

It's not your browser, it's the forum, which doesn't limit image size. Big images can be wider than the space available, and the excess is cut off. If anyone with the appropriate authority is reading, all you need is a rule like this in your CSS:

Code: Select all

.content img { max-width: 100%; }

[531colin]

In discussions of frame stiffness, it is worth bearing in mind that differences are most likely not actually detectable if you were to do a blind test........

As earlier, please go to Spa, ride the prototype and report back.

[Brucey]

..........
When riding normally, the bending load in the BB is effectively resisted by the seat tube alone; there is little or no load on the handlebars (you can usually pedal 'the same' with just fingertips or no hands) , and the remaining parts of the frame are not able to transfer load efficiently to the saddle, which is where the force is reacted laterally. ...........

Errr....run that by me again?
Just tapping along, I push the pedal down. Because the pedal is off to one side of the frame, there is a resultant sideways force on the bottom bracket.
It seems to me that sideways force is resisted by the rear wheel contact patch with the road, and to a lesser extent the front wheel. The saddle has no contact with anything apart from my enormous inertia.
For the seat tube to resist the bending load on the bottom bracket, doesn't the seat tube have to flex? and doesn't flex in the seat tube mean that the chainstays and downtube must flex, as they are all joined?

you could be doing gymnastics on the bike, trying to tear the frame apart etc but I can think of only three conditions that allow you to generate a lateral reaction load on the tyres;

1) you lean the bike over, or
2) you accelerate your CoG sideways, or
3) you make some kind of active steering input

I'm betting that since when tapping along you are essentially doing none of these things, (and if it were otherwise you wouldn't be able to ride 'no hands' in a straight line...) that there normally isn't any lateral reaction load at the wheels with each pedal stroke.

It is a bit clearer what is going on if you ride no (-ish) hands and try to bear more weight on one pedal at the bottom of the stroke; this loads the BB shell in a similar way to pedalling, but there is no chain tension of course. When you do this you ought soon feel the saddle pushing against your thigh on the same side. I think this is in essence how the reaction forces of 'normal pedalling' are generated, (and why it is that you benefit from having a saddle nose).

If the seat tube bends, the included angle between the stays changes slightly on both sides. The seat stays are pretty bendy really; they may as well be pin-jointed for all the stiffness they have in that plane, (as in fact they were in many older frames).

cheers

[Mr Evil]

In discussions of frame stiffness, it is worth bearing in mind that differences are most likely not actually detectable if you were to do a blind test........

As earlier, please go to Spa, ride the prototype and report back.

I have ridden lots of frames and noticed huge differences between some of them. But could I do the same in a blind test? I doubt it.

[old_windbag]

please go to Spa, ride the prototype

Colin will you be posting a full image of the prototype? I guess its in the public domain if you have people riding them at Spa's shop. Also as this one is in titanium which will be pricey, even though your pricing is much lower than enigma etc. Do you envisage creating the same frame, if a success, in steel tubing, perhaps triple butted to save some weight. I'm thinking it sounds an interesting frame+fork if in the £3-500 ball park, clearly the Ti can't be.

[greyingbeard]

Im nowhere near the spa shop. You're welcome to post one to me. I promise to give it back when Ive finished with it

[531colin]

please go to Spa, ride the prototype

Colin will you be posting a full image of the prototype? I guess its in the public domain if you have people riding them at Spa's shop. Also as this one is in titanium which will be pricey, even though your pricing is much lower than enigma etc. Do you envisage creating the same frame, if a success, in steel tubing, perhaps triple butted to save some weight. I'm thinking it sounds an interesting frame+fork if in the £3-500 ball park, clearly the Ti can't be.

If you go here http://wheel-easy.org.uk/ride-calendar.html and download "bike set up" there are some photos of me on the 54cm prototype on my mate's trainer in his garage. This run of prototypes have carbon forks with alloy steerer, we will go to full carbon forks where you can't use a big spacer stack, so the frame will get a high front end and more top tube slope to keep similar standover height.
I don't know about yet another frame, not that its up to me, but I'm trying to get the idea accepted that the different frames should be clearly different in function. We already have a steel tourer with rim brakes, and an identical tourer except in titanium, and the same again with existing rim braked Audax bikes, in both materials.
As well as this titanium disc bike, we are trying to develop a steel tourer with discs, and a steel winter/audax bike with discs, as well as taking over the Sabbath bikes, so I think any more new frames will have to wait a while.

[gloomyandy]

Nice looking bike Colin. Will the front fork be capable of taking a low rider rack?

[old_windbag]

Thanks for that. It looks good, Ti always does and for ease of keeping clean and not fretting about stone chips or corrosion it's great. To some extent theres an answer to my question in that you have disc tourer and disc audax style bikes in steel as possible products too. You'll have to try and make the disc audax have clearance for 32mm tyres plus guards, then we can use reohn2's world renown vittoria voyager hypers . The Spa Audax I have may just fit gp4000s2's in 28mm with guards but I suspect I may have to file the slots in the guards to get 5mm closer to brake. But that's a future test, manufacturers quoted sizes are quite variable. To accomodate 32mm tyres with guards opens up interesting options without going into CX/touring style tyres.