Weight Weenies

What are you experiences of going up, or down, a frame size.

Moving from an "endurance" bike to a race bike, i can get the same fit on a size up (larger) with 10mm shorter stem (100mm)

On my endurance bike i went with the smaller size (between sizes) - but am an little nervous about a "bigger bike".

The only geo differences that matter AFAIK are 1 degree steeper headtube, and slightly shorter chainstays.

Should i be concerned?

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Go smaller. You can always compensate with stem/bar/seatpost. If you are genuinely between sizes, the drop from your ideal size should be minimal. The characteristics of a smaller bike will be overwhelmingly (but not entirely) positive, whereas riding something too big will always handle and look 'wrong', and there is nothing you can do about it, unlike the slightly smaller option.

Coming from an endurance bike, you'll want to carefully consider the major implication of going smaller which is the shorter headtube. Are you trying to achieve a significantly lower position than you currently have? If that is a goal, definitely go smaller. If you are running spacers under your stem on the endurance frame and/or using an up-turned stem and don't want to go all that much lower, go with the larger frame.

Minor but sometimes important detail, if you like to use big water bottles especially insulated ones, you'll have an easier time getting them in and out of cages with a larger frame.

Sizing up will usually require a 10mm shorter stem. Many race geometry frames are designed to handle best with a 110-120mm stem. You may also need a zero offset seatpost when sizing up.


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It's a tough call because you can generally get basically the exact same "fit" on a bike that's too small as well as a bike that's too big simply by changing things like bars and stem and seatposts. This was the concept behind the S/M/L sizing when Giant first introduced "compact" geometry back in the 90's. That didn't work very well because there is the handling aspects of a frame which come into play as well. Too small and you could have a center of gravity way too far forward over the front wheel and have really sketchy, and dangerous, handling. Some confuse that with "quick handling". Not the same. A too small bike is simply too small. Conversely, a too large a bike will feel sluggish and slow turning. However, if i had to errr one way or another, I'd err on the side of a larger frame simply because you can always get "comfortable" on a larger frame (within reason) even though handling may be somewhat awkward whereas if you are on a frame that forces you to be scrunched up you will never be comfortable and run the risk of sketchy handling due to a too short wheelbase for the "chassis" so to speak.
Caveat: those are some VERY general comments I just made and the bottom line is that nothing is carved in stone and at some point it becomes a very individual preference.

Another vote for going smaller. Years ago I did the opposite and it never quite worked out.

Compare the stack and reach measurements.

Are you limited by flexibility? Going from enduro>race, same size will generally have shorter stack. Smaller size may see your stack drop 50mm. Quite significant.

Of the brands I have analysed, I have noticed a pattern. Sizing follows a bell curve. If you are average size, you have more options for stack. or the middle sizes reach increments are smaller than changes in stack.

How comfortable are you with your current drop to bars? Do you want to increase or decrease the drop to the bars?

I went down a size. I have 1.5cm of spacers between the stem and headset top cap. The style police on the forum would say to size up and slam the stem. I think I made the right decision.

To me, if you are sizing up and getting down to a 100mm stem then I'd probably size down if it means a longer stem. Bikes have always felt better to me with around a 12cm stem. I had an old Litespeed that I bought one size too large. On top of that Litespeed's had longish top tubes back in the day. I ended up with a 10cm stem. I just didn't like it.

Note that the pros are running 1 or 2 sizes too small these days with lots of drop and long stems.

^If you went down a size and only have 15mm of spacers with a stem and bar that gives you a nice reach and feel for the bike then I'd say you found the "right" size, not a small size. No style points lost for that. It's pretty easy for people to fit on several size frames. But as you allude to, a big part of the equation is how the bike feels underneath you and on the road that can't be ignored either.

It's not like I went "down" a full size. I feel like I'm in the middle of a 52S and 54S in Colnagos. If they had a 53S, it would probably be spot on. I'm sure I could ride a 52S or 54S just fine. The reach is almost the same on both. And Colnago has like 8 sloping sizes. Other makers only offer 4 or 5 sizes so the differences between two sizes are larger.

To, the OP - what really helped me was putting all the frame data for both sizes in a geometry calculator. It ended up telling me exactly the reach and stack I'd have and how many spacers under the stem.

57 Traditional is calling your name.

What is the difference in wheelbase if you compare, 52s, 54s vs 57 traditional?
Just looking at front to center and chainstay length, they seem to be very close, but 54s has a stack 17mm higher.
Reach seems to be within 1-2mm between sizes.
I am also almost always between sizes. Either reach is fine, but bike is too low, or reach is longer and stack is better.
I am always fitted to the larger frame due to my overall length.
But i prefer a less stretched position due to bad back/ scoliosis and low core strength.
Out of pure curiosity i usually ask what a custom size would have costed, i hear from two brands i own, like a luxury car.
Not many brands carry such a wide platform of sizes as Colnago.

Not an easy call IMHO.

I went from a 56cm Scott Foil to a 54cm BMC TMR.
Granted they are both aero bikes, so not moving from endurance to race.
I got them both to fit with the help an excellent bike fitting.

However, I never seemed to have the same speed going up hills.

I've gone back to a 56cm frame, Focus Izalco and am much happier.

Before I bought the Izalco the vendor did a bike fitting to make sure I bought the best size.

Is a fitting a possibility before you buy?

I've been both directions fitting wise on both sloping and trad geometry frames and my conclusion is that if you can achieve a similar fit either way then it's a subjective and personal choice as conventional frame sizing dictates the opposite to what I actually prefer.

If front wheel/toe clipping is not an issue then I prefer smaller.

Smaller gives me :- Slightly lighter weight frameset (assuming it's the same model)
Better weight bias as I prefer more weight on the front wheel.
Better power delivery - my body mechanics just work better with a slightly longer stem and an rear offset seat post - I fatigue/tire less on this set up.

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Something completely different to think about.

I was riding a frame one size smaller the other day, longer stem to put my bars in the same position as my normal bike.

Whacked my knee on the stem out of the saddle a few times. Last time was hard enough for something on it to scrap and draw blood. Maybe just shitty technique.

Doesn't happen on my normal bike. Odd considering that the difference between the two is maybe 8mm of reach.