runninupthathil

I recently got a deal on two Trek Aluminums. One is a 54cm one is a 56cm. I'm about 5'10" with a 30" inseam (I think). Both bikes seem to have 110mm stems. I have the seat and bars higher on the 54cm, and lower on the 56cm.

I've always ridden a 56cm, but I think the 54cm feels a lot more comfy. The 56cm feels sort of cramped, basically like the top tube isn't long enough. I can't particularly figure out what the feeling is due to, but my back doesn't feel angled correctly, or that I'm just too scrunched up.

I tried to do some research, but seeing as how the stems are seemingly the same length, I'm confused. Does anyone have input on why the bigger frame would be feel more cramped? I measured the top tubes as well and I believe that they are the same length.

Any input would be appreciated! Thanks in advance!

Moisture

If the top tubes are the same length, then the only other factor here would be the reach measurement.

By cramped and scrunched up, do you mean like you are sitting too upright? You can try sliding the seat backwards a little to make some space.

runninupthathil

Thanks for the response. Reach as in stem reach length? Sorry, figuring out sizing is somewhat new to me. I was looking at possibly getting a 120mm stem.

Yes I guess it is feeling too upright. I'm having a hard time describing it I guess. Essentially like there is not enough length between my seat and my bars, so my back is angled oddly. I'll try to slide my seat back a little in the morning as well!

Carbonfiberboy 

One seat post has setback, the other doesn't? It's quite odd that the larger frame would have the same TT length. First thing, with the cranks aligned with the seat tube, make the pedal spindle to saddle distance the same on both frames. With a level, make both saddles have the same angle to their tops. Look to see if the bar shapes are different and the brake levers are set up the same. Here's my bike fitting primer: https://www.bikeforums.net/21296948-post3.html

Moisture

The three main considerations to make when determining frame size would be:

Stack: how high the bars will be
Reach: how far the pedals are from the bars (this affects reach, weight distribution and stability when taking weight off your saddle, such as riding over a bump
And top tube length: which determines how your "reach" when sitting in the saddle normally.

the stem will obviously change the "reach" and "top tube length" according to how long it is.

I think your best bet would be to post up a picture of you sitting on the bike normally, with your back straight and your leg at the 6 o clock position on one of the pedals.

Try taking the bike out for a ride, and push the bike as hard as you reasonably can into any turn of your choice. If you feel like the front tire tends to give up traction first, and early, then you likely are already leaning forward too much due to your 100mm stem.

If you clearly feel like there is too much weight centered over the rear axle and you find yourself needing to constantly lean forward a little to compensate, then yes, maybe a 120mm stem will help. But I think that with some corrections with your posture and riding technique, its hard to say for certain. Your best bet would be to try and eyeball what sort of stack, reach/top tube lengths will work best for you, paired with the right stem/handlebars, and post a picture of you sitting on your current setup normally.

Moisture

One thing I must mention, that if you are not able to get the fit right by simply sliding your seat forward or backwards along the rails and trying different length stems the frame(s) you're riding clearly isn't the right size for you. Keep in mind that the seat tube length of a bike does little to gauge fit.

For example, i am a taller, heavier rider and require a pretty upright position to feel comfortable and well balanced from a performance perspective. I need a tall seat tube (63.5cm) and a very short top tube, along with a very tall yet stubby 40mm stem so I can properly slide my seat back a little bit to bias my weight rearward and get the right riding position.

For reference, using a shorter stem and sliding the seat around on my two previous bikes, which not were too small seat tube wise, but on top of that has much too long of a top tube for me to sit upright as I prefer, in this case its clearly useless to play around and youre best off doing more research on frame fit and getting another bike.

shelbyfv

Line your two bikes up against a wall. A tape measure and a critical eye should be all you need to determine what's different.

runninupthathil

I thought it was odd that the larger frame would have the same TT length as well. I'm going to get out there and measure again today, but fairly sure they were the same. I did notice the bars were tilted a little upwards on the smaller frame, and I'm sure that makes some difference, so I am going to move the bars around as well. I'll also make the pedal to saddle distance the same on both frames. Thanks for the response and the fitting primer!

runninupthathil

Thank you so much for both of the responses. I'm still somewhat of a novice with all of this so it took me a bit to digest. I'm going to get back outside and remeasure and adjust some things and ride both around a bit. I guess my main confusion just stems from the fact that the measurement of both frames (and components) seem to be the same aside from crank to seat post. The 54mm feels pretty comfy with how its set up at the moment, but I thought that might be on the smaller side for someone my size.

I'll have to slide the seat around and adjust the angle of the bars. I'm hoping that at least one of the frames will end up being the correct one to keep. I've just always assumed that I should be riding a 56cm based on my measurements. Thanks again! I'll update after doing some measuring/adjusting.

runninupthathil

That's essentially what I did yesterday evening, and left me thinking WTF! From what I could tell they have the same measurements aside from crank to seat post. Now I'm guessing seat positioning and bar angle may have some factor in things? Thanks for the response!

philbob57

If your measurement is correct your legs are a lot shorter than those of most people who are 5' 10". Are you sure you measured your 'cycling inseam' correctly? The cycling inseam is from the floor to the bottom of your pubic bone.

Iride01 

Possible that head tube angles and seat tube angles are different between the two frames. That might possibly give a reason for them being different size frames yet have top tubes the same length. As well, if these aren't vintage enough to have perfectly horizontal top tubes, then that can be another reason.

A 2 cm difference in frame sizes isn't going to make either bike totally wrong for you. It's more what you want your fit and ride to be like. A personal preference thing.

I do wonder why you say your saddle height is different on each. If they are both road bikes, I'd think your saddle height measured to the pedal at the bottom of it's stroke would be pretty much the same.