[Nick09]

I'm looking at the Fuji Roubaix 1.0, Fuji Roubaix ACR 2.0, Specialized Allez Elite, and the Bianchi Via Nirone 7.

I would like to know how to read the geometry charts of theses bikes. Specificlly I have heard that some bikes are have more of an aggressive geometry. What do I look at to compare how "aggressive" a bike is compared to another?

 

[Nick09]

I'm looking at the Fuji Roubaix 1.0, Fuji Roubaix ACR 2.0, Specialized Allez Elite, and the Bianchi Via Nirone 7.

I would like to know how to read the geometry charts of theses bikes. Specificlly I have heard that some bikes are have more of an aggressive geometry. What do I look at to compare how "aggressive" a bike is compared to another?

EDIT: And what will a more agressive geometry do for me?

 

[ultimobici]

EDIT: And what will a more agressive geometry do for me?

Aggressive geometry generally means a shorter distance from BB to front axle giving twitchier handling. Generally achieved by steepening the head angle a tad. Check the fit first and then look at the handling by riding the bikes that allow you to get your position correct without extreme stem lengths, extra layback seatposts etc.

 

[wim]

" Aggressive"could also mean "rider is more bent over." To check for that, compare head tube lengths of frames of equal vertical size. The shorter the head tube, the lower the handlebar in relation to the saddle.

 

[C-40]

info...

The most important thing to look at is the steering trail. Trail is a function of both the head tube angle and the fork rake(offset). Steeper HTA and more offset both reduce trail and quicken the steering.

The top of the line racing frames will have normal head tube lengths. The lower level models may have 20-30mm taller head tubes to produce a more upright position.

When comparing the fit, you must look at both the TT length and the seat tube angle. Steeper angles increase the reach. As an example, a frame with a 56cm TT and 73 degree STA would have not more reach than a frame with a 55cm TT and 74 degree STA, since the steeper STA increases the reach by about 1cm per dgree. This is often overlooked and leads to fitting errors.

 

[Allez Rouge]

When comparing the fit, you must look at both the TT length and the seat tube angle. Steeper angles increase the reach. As an example, a frame with a 56cm TT and 73 degree STA would have not more reach than a frame with a 55cm TT and 74 degree STA, since the steeper STA increases the reach by about 1cm per dgree. This is often overlooked and leads to fitting errors.

I know you know your stuff, but I'm not following. Assuming the head tube stays where it is and only the ST angle and TT length vary, wouldn't a steeper ST angle decrease the reach? Because the 74deg ST is more vertical, which means the TT need not be as long to connect to the head tube?

(This may be a matter of semantics. To me a 74deg ST is steeper than a 73, because it's more vertical. Is that the way you see it?)

 

[RagbraiNewB]

I feel so close to sort of comprehending this almost.

 

[temoore]

I know you know your stuff, but I'm not following. Assuming the head tube stays where it is and only the ST angle and TT length vary, wouldn't a steeper ST angle decrease the reach? Because the 74deg ST is more vertical, which means the TT need not be as long to connect to the head tube?

(This may be a matter of semantics. To me a 74deg ST is steeper than a 73, because it's more vertical. Is that the way you see it?)

It is a little counter intuitive. You would think that if the seat tube was more vertical (steeper), the reach would decrease. But because most riders would want the same (from one bike to another) distance from the seat nose to the center of the bottom bracket, you would have to move the seat further back with a bike with a steeper seat tube to accomplish this. A bike with a 56 cm TT and 74 degree seat tube would have a longer reach than the same bike with a 72 degree TT as you would have to move the seat further back on the 74 degree bike to achieve the same distance from the seat nose to the centerline of the BB. Here is a good explanation:
http://www.rivbike.com/assets/payloads/271/original_Excerpt_Top_Tube_Ruse_Flash.swf

 

[wim]

Sketch.

With the saddle in the same position over the bottom bracket, a shallower seat tube angle shortens reach. As you can see, x is shorter than y. More of the top tube disappears under the rider, so to speak. Top tube and seat tube (heavier lines) are the same length both left and right bike sketch. Dimensions / angles exaggerated for clarity.

 

[C-40]

I know you know your stuff, but I'm not following. Assuming the head tube stays where it is and only the ST angle and TT length vary, wouldn't a steeper ST angle decrease the reach? Because the 74deg ST is more vertical, which means the TT need not be as long to connect to the head tube?

(This may be a matter of semantics. To me a 74deg ST is steeper than a 73, because it's more vertical. Is that the way you see it?)

Others have provided an explanation. The mistake is in thinking that a steeper STA forces the rider into a different position, relative to the BB. It does not. If you have a steeper STA, you probably need more seatpost setback, but the idea is to position the saddle the same, relative to the BB.

 

[Nick09]

Wow. Tons of info. Thanks to all. I think I am starting to actually get this stuff. The more times I read it, the more it makes sense. haha.

Thanks to all!

 

[dcl10]

The top of the line racing frames will have normal head tube lengths. The lower level models may have 20-30mm taller head tubes to produce a more upright position.

That is not necessarily true, it is for some brands like Look for instance, but not for others. For example Felt's top range AR1 has a head tube that is more than 20mm taller than the F series, and that is also the case with many top end aero bikes. Or if you look at Specialized, Tarmac vs. Roubaix, the Roubaix is not a lower end model, you can get either with any given grade of carbon or components. The geometry is independent of price or relative standing within the lineup. Also what is normal is relative, Ridley, BMC and even Trek use head tubes which are similar or even taller than other manufacturers relaxed geometry frames.

 

[Allez Rouge]

The mistake is in thinking that a steeper STA forces the rider into a different position, relative to the BB. It does not. If you have a steeper STA, you probably need more seatpost setback, but the idea is to position the saddle the same, relative to the BB.

This is going to sound like a "The check is in the mail" story, but as I was laying in bed this morning, waiting out those last few minutes before it was time to get up, the light went on -- and I'm not talking about the light on the bedroom ceiling. The line of reasoning that went through my head was almost exactly as temoore expressed it. I was going to draw it out with CAD this morning, but wim saved me the trouble. Thanks to everyone for your answers.

Another question: does head tube angle affect fit in any way? Or does it only affect how a bike handles?

 

[C-40]

well...

The Roubaix is a special case, in that they offer a pricey version, but the frame is still designed for comfort.

Come to think of it, LOOK makes a pricey frame with a tall head tube - the LOOK 585 Optimum.

The Felt F series geometry is a bit odd too. The miniumum head tube length of 110mm is normal, but the reaches are long for a given head tube length and the STA is much too steep. Increasing the reach but not the head tube length is stupid - as in the 50-52cm sizes. Can't say much good about the F series.

The smallest AR series has a head length of 120mm and the frame reach is just what I would expect. The issue with this series is a lack of sizes. Five is just not enough.