Road bikes. Why in the main is the flat top tube retained?

[Daveyraveygravey]

upsidedownmark said:

Nope, smaller does not equal stiffer.

Can you expand on why not? If you are using the same tubes and bonding the same way, surely a triangle with 40 cm sides is more stiff than one with 50 cm sides? (Numbers picked at random to illustrate a point)

[idiotgap]

Daveyraveygravey said:

upsidedownmark said:

Nope, smaller does not equal stiffer.

Can you expand on why not? If you are using the same tubes and bonding the same way, surely a triangle with 40 cm sides is more stiff than one with 50 cm sides? (Numbers picked at random to illustrate a point)

I think I get it... If you have a smaller, stiffer, main triangle you end up with a longer floppy seat post levering away the advantage.

[upsidedownmark]

Because the points at which you're flexing it from are 60cm apart, and that doesn't change - the 50cm triangle will have 10cm of extra lever, the 40cm triangle 20cm of extra lever.

Intuitively, yes if you grab a carbon tube 40cm long it will be harder to flex than one 50cm long, but that's kindof a red herring. The loads into the frame come from the wheels, cranks, bars and seat; for a given person the spatial relationship of those points doesn't change no matter how you choose to construct the 'frame' part of the system. Making the triangles smaller just means the 'external' levers are bigger. A large, box section has more rigidity than a smaller, solid beam. Tubes are at their best in compression/tension, probably second best in torsion, and worst in bending, so for rigidity, you want to arrange the loads be be bending as little as possible. The more obtuse the angle (e.g. chainstay-to-seatstay), the better tubes support each other, and so on. Think about the rigidity of a brompton

It also might make more sense if you think in terms of load per degree of deflection, rather than amount of movement. Or just try reductio-ad-absurdum; Reduce the frame to insignificantly small (say 1cm seat tube) bring the seat stays parallel to the chain stay, top tube to the downtube etc, and consider how that works..

For that canyon, I'd hazard a guess you'll get a little more vertical compliance as the rear wheel bump doesn't transfer directly into the top tube.. I'd also expect a little less lateral stiffness at the back as the seatstays are less well placed to resist the twisting of the chainstays - if they were perpendicular (which is impossible) they'd see tension/compression, as they come more in line with the chain stay, they see less of that, and more twisting. But still the overwhelming reason for dropped seat stays is to reduce the projected / frontal area of that region of the bike. Plus the more a tube is angled the more it looks like an elipse (good) to the airflow.

[edit] or just go ride an old skool 'ladies' bike - you'll get it fast enough

Edited by upsidedownmark on Monday 30th March 13:36

[Dr Imran T]

Chaps, let us not forget the power of marketing too. They are always telling us what we must have on a bike etc. I was reading this on the BMC website just the other day.

I was sold on this blurb!

TCC - TCC – Tuned Compliance Concept

It’s not easy to imagine compliance and stiffness working as one, but Tuned Compliance Concept does just this. The most advanced bikes are designed to be dynamic on the road and cutting-edge BMC engineers understand this as an opportunity in functional design.


The ultimate balance between stiffness and vertical compliance

Vertical compliance refers to the level of flexing on a bike frame. Combined with stiffness - it can consume up to 15% of a rider’s energy. Our Tuned Compliance Concept focuses on minimizing this impact by leveraging the technical know how and expertise gathered from years of bike design, to create the ultimate balance between comfort and performance for a ride that is both forgiving and fast .

[Herman Toothrot]

upsidedownmark said:

Because the points at which you're flexing it from are 60cm apart, and that doesn't change - the 50cm triangle will have 10cm of extra lever, the 40cm triangle 20cm of extra lever.

Edited by upsidedownmark on Monday 30th March 13:36

But sloped top tube means same down tube and similar length top tube but shorter seat tube, out of the saddle climb or sprint (where you are exerting most force to notice any difference) your contact points are the same and leverage the same, nothing has changed as you are not sitting down. Horizontal or sloped are equal regarding leverage.. The only change is the junction to seat post and no ones levering the seat post at all, surely this makes the frame with the shorter seat tube (within reason not ladies drop) least flexible?

Trust me I'm not being awkward, I asked the question as I don't understand why the different choices are used.

So far I can see and understand horizontal is more aerodynamic for a cutting edge time trial bike, shorter tucked in seat stays are more aero, why curves maybe added. Regular road bike frame I still can't see a good arguement why horizontals used, you did explain why smaller overall may not be as stiff in all ways, but not specifically that horizontals stronger than sloped overall?

Trek Emonde or Madone = Sloped top tube
Cannondale Super 6 = Horizontal

So fashion or one right the other wrong?

Edited by Herman Toothrot on Monday 30th March 18:41

[gazza285]

Herman Toothrot said:

Try snapping a broom handle - easy, then try and snap the shorter halves left over - really hard. Shorter means less leverage so stronger.

Now apply the broom handle analogy to the 400mm seatpost on a smaller, sloping top tube frame.

[Matt_N]

I'd say it's the other way actually, most frames now have sloping top tubes giving effective top tube length with only a few manufacturers offering traditional geometry.

Colnago are interesting because they offer the C60 in both sloping and horizontal and personally I would go traditional.

[Herman Toothrot]

gazza285 said:

Now apply the broom handle analogy to the 400mm seatpost on a smaller, sloping top tube frame.

And when you are sat down is when you are putting the least power through the system.

[upsidedownmark]

Herman Toothrot said:

But sloped top tube means same down tube and similar length top tube but shorter seat tube, out of the saddle climb or sprint (where you are exerting most force to notice any difference) your contact points are the same and leverage the same, nothing has changed as you are not sitting down. Horizontal or sloped are equal regarding leverage.. The only change is the junction to seat post and no ones levering the seat post at all, surely this makes the frame with the shorter seat tube (within reason not ladies drop) least flexible?

Trust me I'm not being awkward, I asked the question as I don't understand why the different choices are used.

So far I can see and understand horizontal is more aerodynamic for a cutting edge time trial bike, shorter tucked in seat stays are more aero, why curves maybe added. Regular road bike frame I still can't see a good arguement why horizontals used, you did explain why smaller overall may not be as stiff in all ways, but not specifically that horizontals stronger than sloped overall?

Trek Emonde or Madone = Sloped top tube
Cannondale Super 6 = Horizontal

So fashion or one right the other wrong?

Edited by Herman Toothrot on Monday 30th March 18:41

99.9% fashion in my opinion. Given that there are perfectly functional designs with both types, it's fairly obvious that both are perfectly acceptable.

I expect there to be minimal real difference between the dropped top tube, and the horizontal one. That said, if you take it to extremes (as a thought experiment only please!), reduce the seat tube height to nothing. The seatstays merge with the chainstays. The top tube with the down tube. Or, put another way, saw the top tube out of your frame. Now do a standing sprint - what do you expect to happen

Some of the load must travel through top tube. The further up the top tube is attached, the better the leverage it has over the seat tube/seat stays which are transmitting the forces to it, so the less bending force it sees & the less it deflects. Compare stopping a spinning wheel by putting your fingers in the spokes vs grabbing the rim (again thought only please!)

Considered as such, it's fairly obvious that even as you move the top tube and seat stays further up the seat tube it gets stiffer. Will be diminishing returns for sure, but the effect will still be there. What's very sure is that lowering it will never make it stiffer.

Off road there are other drivers that take priority; the set of circumstances you're optimising for are utterly different; hence the divergence.

Edited by upsidedownmark on Tuesday 31st March 03:19

[gazza285]

Herman Toothrot said:

gazza285 said:

Now apply the broom handle analogy to the 400mm seatpost on a smaller, sloping top tube frame.

And when you are sat down is when you are putting the least power through the system.

And when you hit a bump and all your weight goes through the seat post? Google cracked seat tube and see how many hits you get.

[fausTVR]

gazza285 said:

Herman Toothrot said:

gazza285 said:

Now apply the broom handle analogy to the 400mm seatpost on a smaller, sloping top tube frame.

And when you are sat down is when you are putting the least power through the system.

And when you hit a bump and all your weight goes through the seat post? Google cracked seat tube and see how many hits you get.

Oof! Done just that, all those failures where the seat post enters the frame. Too much stress at that junction caused by having too long a lever in that long seat post. I always saw the back sloping top tube as a fad carry-over from moutain bikes and never liked the look anyway.

[anonymous-user]

So why are integrated seat posts falling back out of favour if they appear to be the solution?

[Jimbo.]

pablo said:

So why are integrated seat posts falling back out of favour if they appear to be the solution?

Because they're a PITA and not one person on this earth has a good thing to say about them!

[dave123456]

similar to the wheel size debate with mountain bikes.

I would argue it makes a little more difference than others are making out. yes there are 3 main contact points for rider interface, seat, handlebars and pedals. a smaller frame equals a longer seatpost, so more flex.

consider though when the most loads are placed on the frame, in a lot of cases the compromised contact point, the seat, is often no longer used, as the rider is out of the saddle, then the smaller triangle of the cranks, rear axle and seat stay / seat tube / top tube junction is preferable.

obviously I await someone who will assert how poor form leaving the seat is...

[Dammit]

Stiffness is sometimes not the characteristic you want out of a certain part of the frame, so for example if you drop the top tube you end up with a longer seat post, which can flex/deflect more than a shorter one would - this is a good thing.

Also with seat stays, these don't really contribute much at all to positive stiffness - that's the chain stays. But very stiff seat stays can contribute to negative stiffness - i.e. a frame that will beat you up rather than taking the edge off the bumps.

It's the old, old cliche, but you want the frame to be laterally stiff yet vertically compliant, some of which (a lot of which, in some cases) is down to materials (CF layup for e.g.) but also the layout of the frame comes into this.

We use very large diameter, bi-axially ovalised tubes for the downtime and top tube, a PF30 shell to give a very large brazing area and a swaged seat tube that is a larger diameter at the BB junction, plus hugely ovalised and very stiff chain stays - but the seat stays are very modest.

Putting that to one side, here's our new TT frame - note the seat stays:



This is designed for seated, constant high output, it's a prototype at the moment (we'll be racing this and a larger one over the next few months), but it's based on what we know from our other frames.

That said, it could of course be an horrific disaster - but if it is we'll take what worked and bin what didn't.

[scherzkeks]

yellowjack said:

Are we on about 'horizontal' top tubes here?

They ought to be retained because they simply look correct. The world managed just fine without this modern fad for 'sloping' top tubes for over a hundred years, since the invention of the 'Safety' bicycle. We don't need any of this new-fangled, funny angled nonsense, thank you very much...

Slopers are certainly less aesthetically pleasing.

Manufacturers like them because they free them up from offering a proper range of sizes, thus reducing manuf. costs. There is a positive however, in that short people have more options for getting a halfway decent fit. Try finding a traditional frameset smaller than 46 cm c-c.

[gazza285]

Interesting article about frame stiffness.

https://janheine.wordpress.com/2011/10/03/science-...

[TheFungle]

Dammit said:

Stiffness is sometimes not the characteristic you want out of a certain part of the frame, so for example if you drop the top tube you end up with a longer seat post, which can flex/deflect more than a shorter one would - this is a good thing.

Also with seat stays, these don't really contribute much at all to positive stiffness - that's the chain stays. But very stiff seat stays can contribute to negative stiffness - i.e. a frame that will beat you up rather than taking the edge off the bumps.

It's the old, old cliche, but you want the frame to be laterally stiff yet vertically compliant, some of which (a lot of which, in some cases) is down to materials (CF layup for e.g.) but also the layout of the frame comes into this.

We use very large diameter, bi-axially ovalised tubes for the downtime and top tube, a PF30 shell to give a very large brazing area and a swaged seat tube that is a larger diameter at the BB junction, plus hugely ovalised and very stiff chain stays - but the seat stays are very modest.

Putting that to one side, here's our new TT frame - note the seat stays:



Hubba Hubba.

Can't wait to see the final article.

For it's testing phase would you consider running it completely naked? No 'cleaning' of the frame at all, would be the bicycle equivalent of a development mule.

This is designed for seated, constant high output, it's a prototype at the moment (we'll be racing this and a larger one over the next few months), but it's based on what we know from our other frames.

That said, it could of course be an horrific disaster - but if it is we'll take what worked and bin what didn't.