Making sense of aero vs tt/tri frame geometries

[alcatraz]

Hi wws

I'm trying to make sense of tt frame geometry.

I have currently a cervelo s5 vwd frame in 51cm that I have setup as a tt bike. It's got the double clamp seatpost so the seat can be moved forward nicely.

I'm looking to get a bit more high speed stability.

From what I've read it's in the head tube angle. People have told me tt bikes have completely different ride characteristics. But when I look at the head tube angles, tt frames are around the same angle as an aero frame.

S5 = 72.2 degrees. P2 = 72.5.

In fact the P2 should then be less high speed stable than an S5 because it's got a steeper head tube angle. Or?

I've tried to make sense of rake and trail too but not many geometries specify them. All I got to go on is head tube angle.

Also because I run short cranks I'm thinking it's good to have a big bb drop value to keep me extra low and out of the wind.

What do you think? Any comments are appreciated.

/a

[Karvalo]

From what I've read it's in the head tube angle. People have told me tt bikes have completely different ride characteristics. But when I look at the head tube angles, tt frames are around the same angle as an aero frame.

It's not the headtube angle. The seattube angle is a lot steeper, and as a consequence the reach, and therefore front centre, and therefore wheelbase values are all a lot longer. And yeah, BBs are generally lower. All that adds natural stability to the bike, and keeps your weight more centred over the bike instead of becoming too front wheel biased.

[alcatraz]

Thanks for your reply. I really appreciate it.

To obtain a longer wheelbase, doesn't the headtube angle have to be shallower? Seems all of the steep angle ones have short wheelbases for their "reach".

Just measured the wheelbase of my 51cm s5 and it's 97cm. I see that most small tt frames don't go over 97cm.

I have a short torso, and thus a reach problem. That's why I have to size downward to not use like 50mm stems. My seats are unfortunately ridiculously high. Naturally someone like me would complain about high speed stability due to the undersizing problem. For this reason I think that maybe a shallow head tube angle is one of few choices that can help me. It expands wheelbase without affecting reach.

What do you think?

[Karvalo]

To obtain a longer wheelbase, doesn't the headtube angle have to be shallower?

If all else is equal, yes. But all else isn't equal. All the other stuff I said was different is different.

Just measured the wheelbase of my 51cm s5 and it's 97cm. I see that most small tt frames don't go over 97cm.

Ah, that does change things slightly, since smaller road bikes are already longer in the front than they 'should' be just because of issues related to fitting a 700c wheel. You can still see that a 51 P3 for instance is biased further forward than you S5, with shorter rear centre and longer front centre, but yeah it's much, much less difference than on a 54cm even.

[RocketRacing]

I ride 51, so frames can be surprisingly different to fit around the wheels vs their larger breathen. 650b is kind of dead... but cervello did it for the 48’s.

If you have the flexability, i wonder if a 48 tt is best? Most people lean to smaller for tt.

Either way, get a fittment.

[alcatraz]

Thanks...

I see there is an old and a new cervelo tt geometry. Old being 650c for 48cm and new being 700c.

The new one looks intriguing. I asked a guy and he measured 97cm wheelbase which means the same as the 51cm s5.

Head tube angle 71 degrees. Bb drop of 80mm which is extreme. You would need some short cranks and great care when leaning the bike.

But it seems like a very fast geometry. I'm a bit disappointed that the wheelbase isn'f longer.

If I'm unsatisfied with 97cm wheelbase. Should I aim for 98 or 99? I don't want to go too big on the reach so I basically just want enough to improve a notch on the s5.

I love the idea of a big bb drop on the cervelo frame. It sounds rally fast.

Also for my fit in general. It helps give my longer legs more space without sending my ass into the sky. You know what I mean...

Cannondale Slice Black inc edition also looks to fit ok. 7.7cm drop and relatively long wheelbases.

I've even looked at women frame geometries.

What would you do in my sitiation? Thanks for reading and writing.

[reedplayer]

From what I've read it's in the head tube angle. People have told me tt bikes have completely different ride characteristics. But when I look at the head tube angles, tt frames are around the same angle as an aero frame.

It's not the headtube angle. The seattube angle is a lot steeper, and as a consequence the reach, and therefore front centre, and therefore wheelbase values are all a lot longer.

thats only partially correct. the steep (usually +/-78°) seat angle is a characteristic of triathlon-frames. but we are talking of tt-frames, and those should have a flater seat angle, to enable a setback of +5cm without an, resulting, "exploding" reach.
thats, why, for example, specialized offers two models, shiv triathlon and shiv tt. shiv tt has a seat angle only slightly steeper as with a "normal" roadbike.

[alcatraz]

I don't mind getting a tri frame. I don't plan on joining any uci events.

[Shrike]

Was wondering this too. I have an S5 and recently built up a P2. The missus asked why I didn't just build a fast road bike by using road bars on a P2 frame if the P2 frame is more aero.

Said it wouldn't handle as well as a road bike tended to need. But I am wondering how much of a difference there would be.

Could be an interesting thing to test if it wasn't such a massive pain in the arse setting up a clean TT front end

[Nefarious86]

Trail is probably the biggest part of the equation.

https://www.velonews.com/2018/10/bikes- ... inn_480109

[alcatraz]

Obviously they are all interconnected but how would you rank the order of importance for high speed stability?

trail
wheelbase
head tube angle

I've kind of exhausted my option of running a long wheelbase because that really streaches my reach out too much. What's the second place I can try and improve things?

I don't feel very relaxed on the aero bars at higher speed. At 50km/h it's quite uncomfortable. I'd like a bike where I feel confident to push even if I'm doing a shallow descent at 50km/h.

[Karvalo]

Trail is probably the biggest part of the equation.

Cervelo have made a big thing about trail values for their past few bikes, and the new S5 is the first one they've produced enough different forks to keep trail exactly the same for every size. They claim it's made a huge difference in the handling of smaller sized bikes. It's interesting when you look at the comparison chart in their whitepaper you can see that Cannondale have done the same thing with the same trail value with the SystemSix. In future I wouldn't be at all surprised to see more of the bigger companies focus on consistent trail values across each family of bikes.

The problem for the OP though is that trail values and the things you can easily calculate trail from are often ommitted from geometry charts.

[alcatraz]

Can a 1cm increase in wheelbase be felt?

/a

[Jugi]

Can a 1cm increase in wheelbase be felt?

If two otherwise completely similar bikes are tested back to back, yes. When a direct comparison can’t be made, 10mm difference in wheelbase will most likely get lost in midst of all other minute variances.