tturner2

Hi, first time poster. I currently have a 2017 Giant Defy Advanced 2 size large. I usually do fitness rides 25-35 miles. Overall I like the bike fit but it does seem just a tad long which I notice at the end of a ride (achy back). Always wondered if a M/L Giant would have been better suited. I'm testing a viathon race bike in a size 56. I've only done a few rides and overall I'm comfortable with less back pain(I've had two lower back surgeries). i definitely notice the overall bike looks smaller, but is a race bike. The reach is probably more comfortable, but seems I generate less power with the new bike per stroke. Couple of questions, I'm wanting to make sure I shouldn't size up to the 57.5. Maybe the 56 and my giant are more similar in size than I understand. Could the "power" difference feel come from being a better rolling/lighter bike or possibly related to the geometry and size being less efficient? I am over three hours from the nearest bike shop, and my current work covid restrictions prevent me from traveling to be fitted again. Sorry I tried to post photos of me on the new bike, but they wont upload.


my sizing:

6' 1/2"

34" bike inseam


Giant Defy endurance large specs:

reach 391

ett 574

stack 604


Viathon race 56 specs:

reach 391

ett 565

stack 569

Carbonfiberboy 

No, photos won't upload until you have 10 posts, which you could get by looking around at interesting topics on the various subforums.

You can put your measurements into a fit calculator: https://www.competitivecyclist.com/S...ulatorBike.jsp
and see what comes up, looking particularly at effective top tube length.

You say "less power" but not what that means to you. You have a power meter on both bikes? A feeling of power difference could be from feeling cramped on the bike and less able to breathe freely.

AnthonyG

"Reach is more comfortable"? Well actually, the Reach is the same.
It's right there on the spec sheet. A Reach of 391mm for both frames.
Welcome to the topsy-turvy World of bicycle frame geometry.

So where's the difference in the effective top tube (574 vs 565) coming from? They have just moved the seat forwards on the smaller frame.
The stack on the smaller frame is 569mm vs 604mm on the larger frame.

So what is this telling us?
It's counter intuitive but maybe you like lower handlebars. Maybe.
Nominally moving the seat forwards makes people LESS comfortable but can sometimes give you more power so EVERYTHING is running counter intuitively here.

Pictures would be good.
Nominally I would leave you on the larger frame and start fitting new (shorter) stems and experimenting with the height with either spacers or stem angle.

tturner2

agree with the thoughts and hence my confusion. I was able to put 30 miles on today and overall felt better than my giant. I honestly think theres truth to the shorter stack being more comfortable and causing less back pain, but seems against common theory. I know everyone is different but just wanted more input before I went crazy thinking about it. On the giant I get a little more neck pain and "elbow" ache, this is why I was figuring reach was a little better on the knew bike. the giant has a 120mm 8 degree up stem and I have flipped the racer up with the 6 degree 120mm stem. as far as the power I meant I felt more resistance and propulsion on the giant. the viathon just hauls when I get it going. My previous statement was due to lack of knowledge and wanted additional input. On paper the racer should produce faster speeds with less weight and rolling resistance. I know some will just say ride what's more comfortable, but dont want to regret it in the future. I have many pics I will post once I can. thanks for the replies, nothing beats experience.

Iride01 

Stack as reported in the specs isn't the whole picture. My Tarmac came with 30 mm of spacers under the stem so that changes many of the other dimensions they show on the geometry charts to make the bike more of what they were showing for bikes with more stack.

I've gotten the stem down to just a 10 mm spacer and I like it. Might have to play with stem length though or seat fore/aft. But only one ride since I dropped it from just 15 mm under the stem so I need to do more rides first.

But in general, I've been telling people not to overlook low stack bikes that are advertised as "Race" geometry if they can try them out. I found them more comfortable for long rides. Might be different though if I had panniers loaded to the max and was pulling a trailer. So riding style makes a difference more so than marketing terms of "Race", "Endurance","Performance" and etc. Bikes really need to be test road if you ride more than 100 or so miles a week. If a person is just a leisure and occasional rider it probably isn't a big deal.

To me, playing with formulas and calculations is not desirable until you have enough experience to know where you differ.