Geometry

Stylo26

Hi ,
This is my first post on your forum as I am doing some research before buying my first road bike.
I currently ride a Trek 7.5 as my everyday commuter, and am looking to spend between £1100 and £1500 on a bike for Sportives and weekend rides.

In my search I hear the word "Geometry" bandied about a great deal.
I guess a more upright riding position would be beneficial (as opposed to a nose to the bar position)
And would obviously need the correct height frame to suit my inside leg.

I realise that all good LBS's will offer a bike fitting service, but I would like a better understanding myself for piece of mind...... That and I was going to purchase online (looking at Ribble new sportive/ Grand fondo, Pinnerello or Willier)

Do many people out there know what geometry to look for when searching out a bike, or is it just choose and hope?
How do you relate a bikes geometry to the riders physical size (i.e reach for stem and top tube, crank length) and needs (i.e sportives or racing etc?)

crankycrank

Well, I'll have a go at this. First of all these are all broad generalisations but give a good idea of some of the terms used to describe geometry in different frame designs. The best way to determine what is best for you is to test ride a few different styles to find what works for you.
Touring Geometry-Generally has a long wheelbase, gives a very stable, straight line ride and can handle a heavy load. Some people are content using this for doing everything (touring, commuting, sportives, audaxes, fast training rides, etc.). Next to a race bike these can feel slow to maneuver and cumbersome and are usually at the heavy end of the fleet.
Beginner Geo- Usually found on the cheapest bikes and has a slightly more upright position longer wheelbase and a stable ride but not as much as a tourer.
Sportive Geo-Uses closer to a race geometry but with a higher headtube to facilitate raising your handlebars higher than a pure race bike without having to use a large stack of spacers under your handlebar stem. Generally has a quick steering, responsive ride and sometimes designed with a more cushy, bump absorbing ride that comes with a tradeoff of less responsive acceleration.
Race Geo-Quick steering, short wheelbase and will have a low handlebar height (which can be raised by using spacers under the stem if necessary but not ideal if you need say, 50mm of additional height) A racer is basically like a Ferrari. Built for speed with a stiffer and more harsh ride and needs a little more attention to stay on top of but is the most thrilling to ride in all of the road cycling kingdom.

Again, I have to stress that these are very broad generalisations and many frames tend to straddle two or more categories plus using different frame materials and different designs can create vastly different ride characteristics within each category such as a cushy racer or a harsh and stiff tourer.
A bike fitting would be a good investment as it's not just the length of the seat tube that determines your size and it may all feel a bit alien to you at this point until you spend some time getting used to a road position. I think for most people it is very hard to know just what type of bike will suit you best if it's your first road bike. After a few thousand miles of riding you may decide you value comfort over speed or slow steering over quick, etc, but a test ride can narrow your choices down a bit. I have been riding quite a few years so I know what position every part has to be in right down to the millimetre so it makes it easy to find the right size frame but I still prefer a test ride because only I can determine how a bike will feel to me rather than relying on blueprints or others opinions to guess how a bike will feel.

maddog 2

what CC said.

The main measurement is the effective top tube length, measured horizontally between the centre of the seatpost to the centre of the steerer/stem. This determines the stretch of the bike. Obviously you can tweak this to some extent with longer/shorter stems. 12cm is long, 8cm is short, 10 is what most folks go for, roughly speaking.

After that the headtube length matters, as a racey frame will have a short one which drops the bar height. Again you can tweak this with spacers and flipping the stem but then it can start to look a bit dodgy if you're not careful.

After that you're into ride characteristics, which can be tweaked to some degree with a softer seatpost (carbon), bigger tyres, better shorts, different bar tape, and even different wheels.

After that, you can talk about frame materials.. and whatever else.

Evil Laugh

If this is your first road bike you'd do well to budget for a bike fitting, up to about £200 depending on where you live, a bit more if you need lots of foot correction. I mean a proper bike fit too, should take at least 2.5 hrs. There are many benefits for you.

1. You get the right size bike. Potentially saves money long term if you screw it up.

2. You get it it set up perfectly for you. This will save you a lot of time messing around.

3. You'll get your cleats/shoes set up for you. This is where the fit begins so affects all other factors, seat height, stem length etc. Done well this could also save long term injury/pains caused by improper set up, collapsing arches, leg length discrepancy etc.

4. A well fitting bike is a beautiful thing. You'll ride with more power in more comfort. Will be much more enjoyable riding a perfectly fitting £1300 bike than an ill fitting, poorly set up £1500 one.


My 2p.

Evil Laugh

Stylo26 wrote:

Do many people out there know what geometry to look for when searching out a bike, or is it just choose and hope?
How do you relate a bikes geometry to the riders physical size (i.e reach for stem and top tube, crank length) and needs (i.e sportives or racing etc?)

The short answer is you don't relate the size of bike solely to size of person. Lots of shops and fitting systems will but it's bullshit. Proportions, structural strength, functionality and flexibility all need to be considered which is why it's best to get fitted. When you know your position and have a point of reference then you can look at geometry charts and work out what is good for you. Also when you have a grasp of how handling is affected by geometry and what your preferences are here you can again use charts to choose a frame. Otherwise its all guesswork.

maddog 2

I'd say geometry&sizing on MTBs is more crucial, personally, as handling is more important. Within any type, road bikes are all pretty similar these days. If you're prepared to experiment a bit with different setups (stem length&height, crank length, saddle position etc.) then it's pretty easy to get a good fit. These things can be bought cheaply on ebay to try out.

When I build bikes for friends I usually ask them initially to describe what they want to do with it, and any physical issues (flexibility, injuries etc. as Evil Laugh says) then get the right size frame then tweak from there. I can usually get something pretty close after a hour os so of riding and tweaking.

Evil Laugh

I'd disagree with that somewhat. Sizing is important and small differences can affect handling quite a lot. This comes down to steering, stability and front end feel. Choosing the size is a balance between fit and handling. Fit should not be compromised so you need to choose a frame that fits and best suits your handling preferences. Saddle to bar drop is a big factor, as is top tube length. Both will have a bearing on length and height of stem (via spacer stack).

In my experience, let's presume the stem, frame and forks are constant. A bike with less spacers under the stem will have a better front end feel. This is hard for me to describe but road feel is better, the bars and wheel feel more connected and confidence in steering/descending is better.

A bike with a longer stem will be more stable in a straight line and steer slower. Sometimes this is described as twitchiness. You'll notice a short stem for sure when climbing holding the tops of bars, sprinting on the drops, the bike will quickly veer off line. Steering will be quicker with a short stem, for me not a desirable trait, but this is personal preference. Fast steering can make handling on a fast descent less predictable. Headtube angle, fork rake/trail also affect steering speed and stability. I would slightly disagree with above and say 100mm is short for a roadbike stem. 110/120 is desirable for me but I find this difficult due to long inseam proportion. Stem length should also be proportional to the frame/rider. So a 110 on a massive frame could be considered short but on a tiny frame would be long for eg. Stems can also be bought with different angles of rise that can make up for spacer height. So a -17 120mm stem with 20mm spacers is gonna put the bars in the same place roughly as a -6 120mm stem that is slammed.


I ride a Colnago which can be bought in 1cm frame increments. Most makes will be 2cm increments if you are lucky. I could ride a 55,56 or 57 Colnago with an acceptable bar/stem arrangement. Each choice has clear effects on setup and handling even though the changes in top tube and headtube are much less than 10mm between sizes. A 55 puts me on a long stem but with a spacer stack under the stem, a 57 on a shorter stem but slammed stem. Both have advantages and disadvantages. I have a 57 now with a 100mm stem, the choice made by the price I got it for, I prefer the handling of the slightly smaller 56 on a 110 stem. Luckily the carbon Colnago I'm awaiting I bought in a 56.

My point being to slightly disagree with maddog in that small changes in frame/stem size can make all the difference to how a bike rides. I could setup a 54 o even a 59 Colnago to fit exactly the same but it's about finding that right blend that suits your wants and needs.

Evil Laugh

To illustrate my point about steering speed think of a car with decent power steering. Most nowadays will automatically adjust the influence of the power steering in relation to the speed of the car. Lots of assistance (fast/light steering) at low speeds and the opposite at high speeds to keep the car stable. On a bike you can't vary this in ride, you pick your stem length and headtube angle obviously this is what you have at all speeds. A steep headtube angle has the similar effect to short stem. I rode an enigma with a stupid steep headtube angle, it was the most horrible handling bike I've ridden.

Most road bikes will have a sensible hta though.

Monty Dog

Half the terms bandied-about re geometry are purely marketing bull - sportive geometry is a longer headtube for those with lack of flexibility / large midriffs that rather than face the aesthetic ridicule of a 4" stack of spacers, makers fit longer headtubes instead - the Litespeed Sportive for example is an ugly manifestation of this phenomena. Even Cervelo have jumped on the "fat-dentist" bandwagon with their extended headtube designed for "aero-benefits". The same goes for some of the tri/TT geometries where you see people riding expensive frames with flipped stems and a stack of spacers in the mistaken belief that it's the bike that makes them go faster.

siamon

Geometry is a big red herring to me.

One one test I rode two different bikes with very similar geometry, one was a frigging nightmare, the other felt perfect. If you buy a bike based on a geometry drawing and without riding it, you are taking a MASSIVE risk.

The difference in stiffness & comfort between something like a Scott CR1 or a Giant Defy and a proper race bike like the Scott Foil or Giant TCR Advanced SL is immense yet the difference in speed will be barely detectable.

If you ride the bikes (and I don't mean for 5 minutes) your decision will be easy, if you rely on reviews and geometry may as well flip a coin.

maddog 2

"fat-dentist"

I like it. My dentist is a tad chubby....

I agree that small changes make a big difference - I don't think I said otherwise. I aim to fit a bike to someone with a 10cm stem, maybe a tad longer if it's a flat bar.

Evil Laugh

Yeah sorry maddog read your bit about MTBs wrongly so I was disagreeing with something I thought you wrote!

Sportive bikes are what they are, road bikes with taller headtubes, maybe a longer wheelbase, whatever. But surely it's a good thing to have the choice for those with knees hitting stomachs, poor flexibility/postural strength or saddle height heavy proportions, not something to be ridiculed. The bull is the implications that these are the only bikes suitable for a sportive.

gwillis

really interesting thread and its come at a good time for me...
Im lucky to have a few bikes CR1 / Bianchi Infinito and now Kinesis TK2.
Of the 3 the kinesis has been the hardest to get to fit right . According to my stats I suit a 51cm frame but its been a real ball ache to get the fit right swopping between 90 / 100 stems and even the bar height which at present looks a little high. Ive gone from pain in my lower neck to lower back and so on.

Im at the point now where its time for a proper fit as something isnt quite right.

saying that i have just moved the saddle forward to see if this helps.

Next time just like my other bikes ill get a fitting session as its worth it IMHO.

Stylo26

Thanks top everyone for their input to this thread, I was quite suprised with the quantity and quality of reply.

I think what I have gathered (rightly or wrongly!) and for my riding purposes and price range is:-
Geometry obviously relates to riding position which is generally a function of the genre of the bike, be it sportive, race or touring.
Wheel base will have a bearing on handling, and stiffness will relate to the handling and comfort.
But apart from seat tube and head tube angle, once a bike is chosen by its size, generally governed by the seat tube length. There is only so much that can be done with a frame of two triangles, considering the wheels are a fixed dimension. (stiffness and flex aside)


I can't help think that any bike fitting service would be beneficial but shop would be prone to steer the customer towards a bike that they themselves sell?
Whether this is right of wrong we only have the choice of "off the shelf" frames to choose from. unless we go down the custom frame route which is out of my price range.

I think at this price range I will just have to chose a bike on genre and size it right to my leg length!
Any further adjustments will have to be done by way of saddle height, saddle position, and stem length changes.
After all I'm only riding for fun/fitness.

Thanks again for your wisdom.