BarracksSi

Let's say you've got two frames. For this comparison, take Specialized's Tarmac and Roubaix.

The Roubaix has a taller head tube -- 2 cm taller in a size 58.

Say that you set up the Roubaix with the stem all the way down, and the Tarmac with 2 cm's worth of spacers. The saddle-to-bar drop would be the same, so it seems possible to set up the exact same body position on each bike.

What will still feel different between the frames' geometries, then? I know that the head tube angles are different, along with wheelbase, chainstay length, fork rake, etc.

Bianchigirll 

depending on the difference in the geomentries it would be hard to tell, unless you road alot. however the "racing" frame should accelerate and climb better/faster than the relaxed and may require a bit more effort/concentration to keep in a straigh line. like wise the "relaxed" bike will climb and accelerate a bit slower but follow a straigh line easier.

since I am not training for the Olympics so I do not notice a huge differenc in performance between my more relaxed frames (Equinox, Sport SX) and the racer bikes (Proto, RIGI (Criterium)) but the ride of the relaxed bikes is a bit softer due to the slightly longer chainstays.

as for steering the RIGI with the 78.5 ST and 79.5 HT is the "twitchiest" bike I ever rode

BarracksSi

Hah Googling for Rigi Criterium gave me your Flickr post as the second hit:
https://www.flickr.com/photos/bianchigirll/3992322909/

I can imagine how twitchy it is. Sheesh...

scirocco

I don't buy the acceleration bit unless the Roubaix has been built up with boat anchor components. If the weights are the same there will be no noticeable straight-line difference between these two high-end bikes from the same manufacturer. Handling differences, yes, probably.

echappist

the spacers don't weigh as much as the head tube. twitchiness is normally defined by how easy it is for the bike to drift horizontally. the longer the headtube, the more force/torque is required to make it move from side to side. the flip side of the coin is that you get more responsive handling on a bike with shorter headtubes. i used to think my cannondale six13 as quite twitchy (14.5cm HT) compared to my giant OCR3 (17cm HT), but with some quality times on the rollers, i feel a lot more comfortable handling my six13.

echoing scirocco, i also don't buy the acceleration stuff. horizontal acceleration, yes, but not propulsion forward.

mrvile

I can't figure out what is going on with the rear wheel. Where does the tire go? Is there a massive cutout in the round profiled steel seat tube?

carpediemracing 

I think that if the bikes fit similarly they'll feel like, well, you're riding a bike that fits.

I haven't ridden the two examples you mention, but I figure the Roubaix got to have some more flex, both from the bouncy inserts as well as the bigger main triangle (which gives it a taller headtube). So, given identical wheels and such, it'll feel softer when you're accelerating sharply. It'll probably feel a bit softer when you're riding over bumps too.

But as far as difference in ride or performance? I doubt you'd feel too much of a difference, nor would you ride much better or worse. Okay, that's not true for certain limited conditions, like if you were climbing for 90 minutes and one bike was a few pounds lighter. Or if you were doing a crit and sprinting for the finish.

I figure that in certain types of courses, where I know how my body will react, my bike will make up to a 100 foot difference in how well I finish in the sprint. Meaning, with totally unoptimized stuff, I'd be a good 100 feet behind where I would be if I was on totally optimized stuff. This is gauging myself against other riders who are hopefully using consistent equipment.

Since I've raced more races at Bethel than anywhere else (over 18 years of holding/racing the event, about 108 weeks of racing, 1-2 races a week, 30-80+ laps each of those weeks), I have a pretty good idea of when I'm going well, when I'm not, and how a particular bike affected my performance. I used equipment that I was convinced would help, but then I'd find it didn't. I'd use equipment I was convinced wouldn't help (trying it just to try it), and then I'd find it did. I've tried three different bikes (with three different frames) in one series, with an absurd amount of wheel experimentation throughout the years. Different fronts, rears, both, clinchers, tubulars, you name it I tried it. "Mass start aero bars" - I used Scott Rakes, Cane Creek Speed Bars. Carbon monocoque-type, lugged carbon, aluminum, alum/carbon. Alum forks, carbon forks.

I found that the wrong frame will limit my potential. I adapt to the best frame I have. The others become "a little less fast at Bethel". This just means the frame feels a bit mushy, or that when I go to surge a bit to respond to a slight move the bike seems just a touch reluctant to respond.

Wheels make the biggest difference, huge, in a hard acceleration uphill sprint.

Stem, bar make a big efficiency difference in the sprint.

Saddle and post make the bike feel lighter in (my out of saddle) sprint.

Mass start aero bars make me more aero but it doesn't help me. I feel the weight of material clinging to my bars in the sprint.

Derailleurs, shifter quality, who cares. Ditto bearings, hubs, brakes, blah blah blah.

Between those two frames, at similar weights, with identical wheels + fit, I figure I'd have a hard time thinking one was better than the other at Bethel. Just a feeling, based on fiddling with some of each.

cdr

mzeffex 

+1

Jurgen

I feel like a fool adding anything after CDR... But for the record in your particular example it's more than just the head tube. The BB is lower on the Roubaix, so IN THEORY if you needed a lot of spacers on a Tarmac your centre of gravity would still be a tad lower on the Roubaix. Maybe it matters. Maybe it doesn't.

I (personally) found this interesting. (I think Red Kite Prayer is trying to do good, honest work.)

kindablue

Wow, thanks. Your post was insanely helpful. (I just added the ellipsis and took out the text so I wouldn't hog screen space with a giant quote.)

umd

Assuming that you set both bikes up so that you are in the same position, the biggest difference will come from the chainstay length and possibly a slight difference in weight distribution. I don't know if the steering trail is the same or not and I'm too lazy to look it up, but that could be another difference that would affect the feeling of the handling. I suspect that the Roubaix would feel a little mushy/cushy if you really tried to throw it around in fast & hard corners, sprints, etc. but the differences will be minor and really just affect "feel" not performance.

I've never ridden a Roubaix but I have a Tarmac SL2 and had an older SL, and the difference between those two were like night and day. Same (well, very similar) geometry, just different materials.

guadzilla

Why is that? I can see how a shorter stem would make a bike twitchier, but I dont understand how a taller headttube would - if anything, I'd think taller headtube = more upright = less weight forwards = easier to turn the wheels.

V.

umd

I don't think the headtube is the factor, it's the ht angle, trail, etc. as well as the wheelbase that make a bike "stable" vs. "twitchy"

guadzilla

^^ Ok, that makes sense. Thx.

BarracksSi

That's actually a two-piece seat tube, with two skinny tubes making a gap just wide enough for the tire to poke through. Imagine a mixte frame with the split "top" tube, and switch that to the seat tube.

siberian

Having literally just performed the EXACT same experiment you describe (I was shopping for Tarmacs and Roubaix) and had compared both side by side to get the riding position the same on both... I can definitely say that both do feel very different to ride.

Not being experienced enough of a rider, I can't quite accurately describe why they felt different. The best way I can describe it is like comparing driving a sport sedan to a sport coupe. Both bikes felt good to ride, but the Tarmac just felt like it had a more immediate response to rider input. The Roubaix, while also sporty, just felt a little bit more disconnected from the road. I would guess that this is attributed to two major things... one, the Tarmac has a shorter wheel base, and two, it had skinnier tires. The Zertz inserts may have had a *little* to do with it but the reality is, they are designed to dampen vibration over long rough roads. They are not shock absorbers that flex as some people might assume.

I ended up choosing the Tarmac because it was more fun to ride.

Fullforce

I did the exact same thing.

echappist

it was the reasoning given to me. i guess it may be flawed. if an engineer could explain this, i'd greatly appreciate it.

arshak

78.5 ST and 79.5 HT - Yowza, Not even track bikes have that kind of angles. Are you sure about the STA and HTA? Even TT bikes average 76 degrees.

foresthill

If you looks close, you can see that it's got a doubled "seat tube" (each of similar diameter to the seatstays) so the tire goes between the two seat tubes.

BarracksSi

Check out her picture that I posted -- that looks crazy steep.