Size up or size down? Geometry geek out.

[rolandtiangco]

Hello all.
I'm falling in between sizes (M & L) for a bike I am interested in, and wanted to see what you would do if you were in my situation. I'm currently riding bikes for which I have been professionally fitted, but the "new" bike sizes are less granular and t-shirt sized (S, M, L, etc.)

See this screenshot from geometry geeks.

• The first 2 columns are the M and L sizes of the bike I'm interested in.
• The last 2 columns are my current bikes, which you'll see are quite close in geo.

The options are: a) size up, b) size down or c) fuggedaboutit and go look for a different bike.


WWPLD?

Thanks,
Roland

[jtakeda]

Size down.

It’s much easier to get a longer stem and change stack. Much harder to shorten a top tube

[robt57]

2 cents/Nut shell for me. Road race geom I go up, CX I go down.

If road TT is long to the point you'll use short stem/reach bars the shorter tiller/lever will change steering response. You may or may not notice or care. CX is slower up front to start, and longer tiller make it snappier to me, which I like personally and you may or may not.

Over all the 'up' may well be a faster HTA/front end than the size under as to negate. But not so much in m/l as xl on average generally.

If going up makes for too much stack, factor in -17 stem etc. And of course you gotta stand over it...


Other consideration might be, like me.. I have more inseam than I should for my height via longer femurs than avg. Long TT are less an issue for me. So if you similarly are of non avg proportions asking folks of avg proportions about sizing up of down may be a fools errand.

I was put on 56CM road bike when I came over to road from MTB in the 90s. I ride 58-61 for last 20 years. My CXs are 58, my roads are 60s, and even have a 62 still with a short tiller.

[Wolfman]

I would size down… we don’t know the head tube sizes of the new bikes, but your current bikes and the new M are nearly identical on stack and you can better adjust reach through a combo of stem length and post set back.

Going through this myself and I understand that thinking about a too-small frame versus one that’s just a hair too big is a scary proposition for a long-time rider… it’s agonizing to think you fall between sizes and the perceived cost of being on the wrong size is high!

[djg21]

If you are comfortable with your older bikes, I’d say ignore the size designations and focus on the effective toptube and reach of the newer bikes, and try to come as close as possible to the bikes you now have and have found to fit correctly. Use a stem length to address any differences.

[zero85ZEN]

As mentioned in an earlier post, the missing head tube numbers on the two sizes of the new bike you're interested in is an important piece of missing information.

The two most important measurements I look at when sizing a frame are top tube and Head tube lengths. Then I look at wheelbase.

Based on the info provided, I'd size down (as has already been recommended) if I was in your shoes making this decision. Hard to lower position from a head tube that is too tall even if the top tube length isn't too long.

EDIT: The top tube length on the larger frame is a centimeter longer (approx.) than both of your current bikes. The medium is only a half centimeter shorter. There's your answer right there.

[tv_vt]

Originally Posted by zero85ZEN

As mentioned in an earlier post, the missing head tube numbers on the two sizes of the new bike you're interested in is an important piece of missing information.

The two most important measurements I look at when sizing a frame are top tube and Head tube lengths. Then I look at wheelbase.

Based on the info provided, I'd size down (as has already been recommended) if I was in your shoes making this decision. Hard to lower position from a head tube that is too tall even if the top tube length isn't too long.

EDIT: The top tube length on the larger frame is a centimeter longer (approx.) than both of your current bikes. The medium is only a half centimeter shorter. There's your answer right there.

Agree with the need for HT length. But another thing that might concern me is the shorter chainstay length on the new frames. That would increase heel rub and might just also make for a harsher ride. Also would want to know front center on the new frames to insure toe overlap isn't going to be a problem, especially with the smaller of the two frames.

[rolandtiangco]

Crap. Life got in the way of my quest here. Thank you to all who replied.

I neglected to include head tube length for the new frame, so I'm including it in a new screenshot in red. Also including front-center.

It sounds like the wisdom here is pointing me toward sizing down, and now that you all have highlighted head tube length, it looks like the new frame does indeed have a taller HT, making the smaller size look closer to my current frames.

Many thanks.

[Alistair]

Maybe I'm missing something, but...

Looks like the new L is longer in both reach and ETT. At the same time, it's also taller in stack and HT.

If your current bikes have spacers to bring them to the effective stack of the new large, your effective reach will be even shorter. And if you're running few/no spacers on current bikes, the new bike will be too tall in front.

Unless there's a reason you want to go longer/taller, I'd size down. The new M looks pretty close to your current fit, maybe with a 10mm longer stem.

[tv_vt]

Are headtube lengths apples to apples? All external headsets, none of them have partial integrated headsets that screw things up, geo-wise? If all the same, looks like the M in new frame is way more in line with your current bikes. HT on the L is much taller than what you have now.

[giordana93]

Originally Posted by tv_vt

Are headtube lengths apples to apples?

No, most definitely not.This is main reason the "stack" measurement has become a metric for comparing frames from different manufacturers in this day of aero and integrated front ends.

[giordana93]

stack by the way can be thought of as the "size" of the frame in traditional (non sloping TT) geo: where a "Small" is around a 52, M a 54-55, etc.

when comparing sizes, don't forget to correct for seat tube angle influences, although in the middle range (54-56) you have a good chance for a neutral 73 without too much range between brands/builders. Still if you like a more forward and low-ish front end, go smaller. I always have to size down to a 52 despite my 5'9" because I have short legs and still cling to (and like) my 80s positiion, although it may start having less drop as I hit 60 soon. I can't get my bars low enough on bigger or even "endurance" geometries, and that is what it comes down to really.

[robt57]

Originally Posted by giordana93

I can't get my bars low enough on bigger or even "endurance" geometries, and that is what it comes down to really.

Same here. Which is why the Race Shop SLR Domane works for me where a 60 normal Domane has way too much stack.

[whatshubdoc]

The head tube length only has meaning if:
- you are slamming the stem
- you are at the limit of steerer length (ie. head tube length + headset + 40mm spacers + stem)

Let's say your bike currently has 30mm spacers under the stem, and you use a 10mm headset cap. For arguments sake let's say your head tubes on current bikes are 160mm. This would mean the bottom edge of your stem would be sitting at 160+30+10 = 200mm when measured from the crown/head tube + fork junction. That would make the larger bike with a 186 head tube feasible, if it uses a 10mm headset cap, 5mm spacers, and integrated headset. If it uses an external bearing headset then of course go smaller.

Another thing that was not mentioned so far is that your seat tube angle on the "56" bike is 73.5. For every 1 degree change in STA for the same stack and reach, you are looking at approx 10mm change in top tube length. So if you want to extrapolate that value to even the comparison, then your "56" bike will have a virtual top tube of 567, which is closer to the larger bike, while being 12mm longer than the smaller bike.

So depending on how your bars are set up (reach, height), how your saddle is set up (height, setback) and a host of other variables, it would be quite hard to tell you definitively which is "better".

What I would do is bang out the 4 geometries in the tool below (you will need 4 separate tabs and you need to copy/paste each URL into a document to save the schematics. Every time you make a change and want to "save" you will have to copy/paste the URL or update your bookmark. Sounds PITA but worth the effort because the tool is VERY very useful.)

https://www.bikegeocalc.com

[rolandtiangco]

Thanks all, and especially to @whatshubdoc. This tool helped tremendously. (https://www.bikegeocalc.com.) Just wanted to add that the "shadow" and "swap" features in this tool allow you to overlay 2 frame geometries and bike fits, which helped me visualize that sizing down is indeed the way to go.

This image shows two of the geometries in question (current bike and sized down frameset) overlaid on top of each other. A little hard to make out (dotted line is the other frameset) but you can see that I can get the fit pretty much identical by adjusting the cockpit reach.

I have a hard time translating geometry numbers to how fits are experienced, so visualizations are invaluable to me. Thanks everyone for your input.