I'm currently riding a 58cm tarmac and I love it overall except the steering is a little twitchy at times.

When I bought the Tarmac last year, my shop debated the fit between a 56 and 58. We ended up with a 58 but with a 90cm stem. As a reference, I'm about 6' tall and my cycling inseam is around 34". I like to sit back a little (knees behind spindle by a little) if that matters.

I 'need' a second bike and have been watching craigslist, almost did a bikesdirect deal, etc., but then just decided to pull the trigger on a new Roubaix today.

The fit calculator calls for a 58, but I liked the 56 fit compared to the 58 on my test ride (but it was only around 5 miles). My LBS ordered me one in a size 56 and it should be in next week.

My only apprehension is spending this kind of $ without going for a long test ride to make sure the 56 is really the right frame size. My LBS said I can fit a 56 or 58 and has a 30 day guaranteed fit, but I want to make sure I am getting the right frame size and will not regret switching to a smaller frame size. This is a frame I want to keep for a long time.

Assuming my reach is OK with a reasonable length stem, what are the main drawbacks of a smaller frame? I read a lot about how a larger frame is more comfortable on longer rides, but I'm not sure I really understand if the reach and bar drop are similar to a larger frame.

Also, I am hoping the Roubaix will be a little less twitchy and smoother than the Tarmac and I'm not sure if I'll cancel out the difference by going down a frame size and ending up with a similar wheelbase, etc.

I guess I'll figure all this out on my first long rides, but I'd like to know what possible drawbacks I should be looking for and try to resolve anything up front in the setup if possible.

Assuming the seat height can be set up correctly on both frames and the reach can be set to a comfortable length then the real difference as i see it will be the head tube length. The HT length will be smaller on smaller frames resulting in lower bars or the need for spacers.

Having said that I havent looked at the geometry for either bike and I think the roubaix has a higher front end anyway for the same size frame. In that case it may not turn out to much of an issue.

how much seat post will be showing on the smaller frame?

90 cm stems are for jnr's

Or guys who have frames too big.

Kick your LBS in the ass!

The seat and reach seemed good.

I just looked at the geometry and it looks like the 56 roubaix has a 190 head tube while my 58 tarmac has a 205, so I guess I'd have a bit more drop with the Roubaix. It felt pretty good on the test ride, but the stem was flipped up with some spacers (and my Tarmac is down with spacers), so I imagine it was pretty close to what I'm used to. The 58cm Roubaix has a 225 head tube, so yes it has a taller head tube for the same size frame. I guess it would be nice to get rid of some of the spacers with the 58, but I'm not sure that's a big deal.

What about handling/ride characteristics? Would the 2 sizes ride significantly different?

Smaller frame: Smaller, stronger/stiffer (at least in theory), lighter, twitchier ride, maybe a bit more road buzz.
Larger frame: more comfortable ride.

Dont listen to the people who disagree with 90mm stems, they have never looked at the basic numbers and dont understand the issues. I can get the same reach and ballence on my bike with either a 110 stem or a 90 stem without moving the seat. How?......I have run the numbers......

oh and it is impossible in most cases to move to a longer stem by getting a smaller frame. The reach between the biggest and smallest frame in a range of the same bike will usually not vary more than 25mm

I use a 75mm stem. My body isn't proportioned "normally".

Not quite.

Not sure I understand what you mean here.

Wouldn't a 2cm shorter top tube mean a shorter reach (maybe not all 2cm, but at least a good portion of it depending on the seat tube angle, etc)?

I figure on my Tarmac, I'm back about 1cm from 'typical' on my seat positioin so that affects my stem length by about 1 cm on the other end. If I set up with knee directly over spindle, I get knee pain. I assume I will have the same issue on the Roubaix. They set the 56cm Roubaix up with a 110cm stem for me and the reach seemed good, but could maybe go a little longer.

You'll be fine. Make sure the fork is uncut if possible.

Incorrect.

A lot depends on how much saddle to handlebar drop you have. I am 6'0", 35.4" inseam and have a 56cm frame - and I am happiest with a 90mm stem on it (130mm of drop).

V.

^^^^^ correct

[QUOTE=grwoolf;10596540]Not sure I understand what you mean here.

Wouldn't a 2cm shorter top tube mean a shorter reach (maybe not all 2cm, but at least a good portion of it depending on the seat tube angle, etc)?

QUOTE]

Not really
Frame reach is measured from the centre of the bottom bracket to the centre of the TOP of the head tube. Assuming you always have your knee set with the same relationship to the pedal spindle then the reach measured like this allows you to compare different bikes independently of tube angles.

Specialised don’t quote their reach unfortunately so I’ll use trek as my example but lots of other companies quote reach also.

Look at the geometry table here:
https://www.trekbikes.com/us/en/bikes/road/madone/madone69/

And you will see that the difference in top tube length between the largest and smallest size is 77mm but the difference in reach is only 24mm. Dropping a frame size will only get you 4 or 5mm on reach which is not a full stem size. Anyone who tells you that you should have got a smaller frame so you could use a 110 or 120 stem has no idea what they are actually saying. Therefore you need to size a frame to get the seat post length within range and just fit a stem that is comfortable. Spesh frames will be similar.

If the 56 you rode had a 110 stem flipped up you will probably want to flip it down again for looks, in which case you will probably find that a 100mm is more comfortable. Std spesh stem comes with the +8 and -8 degree shim, so flipping it will stretch you 9mm (a whole stem size). If you remove the spacers off either bike you will stretch yourself about 6mm. All these things make a difference

Anyway you may find with the different geometry of the roubaix that you end up with a longer stem than your Tarmac

At the end of the day if the seat is within range with enough left in the frame if you decide to change pedals with a lower stack height (read speedplay) and the front end is not too low for comfort then there is no problem.

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On a 58 cm frame.

quite. And correct. And the #'s are all in the wrong places.

The best way to compensate for varying seat tube angles is to add 9mm per degree to the TT lenght of the frame with the steeper STA and then compare the two lengths to get a reasonably accurate reach difference.

Comparing the published reach values of two frames with different stack dimensions results in the wrong conclusion. Reach can only be compared at ONE stack height. If only frame has a 2cm lower stack, you need to add 6mm to the reach of that frame when comparing.

As an example, the 51 and 54cm Cervelo R3 frames have a 10mm difference in reach, but that is not the stem length difference required to make the two frames fit the same, with the bars at the same height. It's really 16mm.

If a bike is fitted properly there is no draw back to a smaller frame. If its your size its your size. Keep in mind that any individual can ride 3 different sizes or more depending on the manufacturer. Personaly, I can up to 4 different sizes as long as the frame has a 54cm C2C top tube.

Thanks for all the responses, very educational.

Based on my concerns, my LBS has me set up to ride both a 56 and 58 roubaix from their rental fleet this friday on extended test rides (I can take them all day). It's not the same frame as what I'm buying, but they have the same geometry. I'm 90% sure the 56cm is the right bike, but I figure a nice long ride will help me finalize the decision. It sounds like the key thing I want to make sure of is that the saddle to bar drop is comfortable on a long ride without needing too many spacers.

If you mean a 90mm stem on a 58cm frame suggests something is proportionally wrong, I can generally start to agree.

But let's be clear, 90mm stems have their place (and not just for jnr. riders)

I do.

Yes they do, It was tongue-in-cheek.

+1

Yeah, I have long legs and a short torso. I'm 5'7'' and and 32 inch inseam. Dad says no to a smaller stem. I have a 110mm stem on a 55cm frame, so long rides can be a *****

i'm 5'10 and ride a 54cm frame with a set back saddle and a 90 stem.

i arrived at this after a lot of screwing around and finally a pro fit, and its the comfiest i've ever been on my bike.

i have shortish arms and torso and longish legs, for the record.

que?

Yeah, I'm a junior. With no money.