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Giant frame size?(19 posts)

Giant frame size?gregflynn
Dec 12, 2003 1:03 PM
I am considering buying a Giant TCR Carbon frame but am concerned about the size,

I am 5 11 1/2 with an inside leg of 34" (86.5cm). I currently ride a Look 381i 56cm which a find a touch long.

Does anybody ride a giant size M with my size???
Dec 12, 2003 1:34 PM
What size frame other people ride is of no value to you.

A geometry comparison will tell you exactly what is different about the fit.

According to my charts, your LOOK has 56.9cm TT length and a 72.5 degree seat tube angle.

The medium size Giant has a 55.5cm top tube length, but a steeper 73.5 seat tube angle. To properly compare these two frames, .6cm must be added to the TT length of the Giant to compensate for the different saddle position, making the Giant's effective top tube length 56.1cm, or .8cm shorter than the LOOK. If the LOOK feels long, then you could probably run the same stem length on a medium Giant.

You don't mention the stem length on your LOOK, but if it feels long, why don't you try a shorter stem?

The other thing that should be closely compared is the total head tube length, including the headset. With the limited number of sizes in the Giant frames, the head tube may be shorter than desirable, resulting in the need for a high rise stem and/or many steering tube spacers. To avoid a goofy looking setup, it's important to evaluate the vertical dimensions of the frame along with the horizontal. Unfortunately, Giant does not list the head tube length in their geometry charts.
Dec 12, 2003 1:46 PM
I have a 110mm stem length (reduced from 120).

The question of head tube length is a concern. As you say there is no info on these dimension in the Giant brochure and whats the point of a compact 'rigid' frame if you have a 60mm head tube length?

Maybe I'll opt for the new Time VXR as I understand it's a touch longer (5mm) and has a higher head tube. The only problem is the price 50-60 % more expensive and 3 month delivery time.

My last choice is the LOOK 486 but is a bit heavy from what I've seen on Weight weenies.

Whats the best choice?
ask a Giant dealer or owner...C-40
Dec 12, 2003 2:19 PM
It shouldn't be that tough to find out the head tube length of a medium Giant carbon frame. Someone reading this forum may have the answer. LOOK frames also tend to be as short as anyone's for a given frame size.

Time frames are awful pricey, but hey, it's your money. For that kind of price I'd look at a Colnago C-50 myself. A lot more paint selection, but delivery would not be quick either.

You might also look at Fondriest frames. They have some new carbon models out, but the geometry varies among them.

As for the weight weenie thing, we just got done with discussion that showed the minute savings from a full pound of weight. According to analyic cycling, you save about .3 seconds on 300 meter hill and less than 30 seconds on a 3-mile, 6% grade, (if you have real mountains to climb). If you're already in single digit body fat, maybe the frame is the place to worry about weight, but for most of us, a pound off the middle is a lot cheaper.
Dec 12, 2003 7:40 PM
I thought the change was 1.2cm. for every degree in seat tube. Have I got this screwed up?
right, I goofed...C-40
Dec 13, 2003 6:38 AM
Thanks for catching my error. The two frames acutally have nearly identical TT lengths.
right, I goofed...koala
Dec 13, 2003 8:49 AM
No biggie, thought I was losing it.
Dec 13, 2003 6:37 AM
Oops, I goofed. The Giant has an effective TT length of 56.7cm which is only .2cm less than the LOOK.
Don't quite get it?gregflynn
Dec 13, 2003 12:03 PM
I have both the Look and Giant brochure in front of me and from how I read it the Look (56cm) frame has a top tube length of 56.9cm where as the Giant (M) has a top tube length of 55.5cm how do you calculate the 'effective' TT length to be .2cm different ?
most folks don't...C-40
Dec 13, 2003 12:55 PM
To compare the top tube length of two frames with different seat tube angles, you have to compensate for the fact that the saddle must be moved further back on the bike with the steeper STA, to place the rider in the same position relative to the bottom bracket. If you don;t do this, you're comparing one frame with the rider further forward than the other. Most people want to duplicate a specific saddle location that places their knee in the desired relationship to the bottom bracket or pedal spindle.

The formula for calculating the exact difference is: saddle height x (cosA-cosB), where A and B are the two seat tube angles. The amount will vary from 1-1.5cm depending on the saddle height, but 1.2cm is close enough average value for most comparisons. The value can either be added to the TT length of the frame with the steeper STA or subtracted from TT length of the frame with the more slack STA.

Failing to take the seat tube angle into account is one reasom people buy frames with what they think is a shorter top tube and then find out it's not.

A good example is found when comparing many 53 and 55cm frames (litespeed). A 53cm might have a 54cm TT with a 74 STA and a 55cm may have a "longer" 55.5cm TT, with a 73 STA. After compensating properly, the real difference is only .3cm. I honestly don't think some manufacturers realize that the smaller frame with the steeper STA should have a TT length that is about 1cm shorter.
what if we use front center as a parameter as well?colker1
Dec 13, 2003 4:40 PM
cause a bike with a laid back stube may also have a slack head tube which could "ask" for a longer stem to place your weight over the front axle thus making for a longer cockpit than it's short top tube suggests...
so, using set back and front center as measures, as the italians do, we could have a more precise idea of fit and balance over different bike. top tube as a measure of fit is rather unsubstantial. what's the big difference between a 54cm and a 54.5 cm top tube if you swap a 110 for a 130 stem... see what i mean?
(front center= measured from bb axle to front hub axle) nmcolker1
Dec 13, 2003 4:43 PM
another misconception..C-40
Dec 14, 2003 8:42 AM
A laid-back seat tube angle really has nothing to do with a long front-center dimension, since the BB is the only fixed point from which all other frame dimensions are derived. The head tube angle has a big impact on front-center as does the top tube length (for a given STA).

As an example, My 54cm Colnago has a nearly 59cm front-center with a 74 STA. Compare that to my Fondriest which has a 73.5 STA and a MUCH shorter 57cm front-center, but the same chaintay length. With the saddle set to place my body in the same position relative to the bottom bracket, the weight on the front wheel is increased by about 1.5lbs and reduced by the same amount on the rear wheel. That's not enough to make a perceptible difference in handling. These bikes do handle quite differently, but it's due to the shorter wheelbase and less steering trail on the Fondriest, not due to the minor difference in weight balance.

The front center dimension would indeed allow a person to calculate the difference in weight balance, but personally I would not adjust my riding position on the theory that there is a "perfect" weight balance that should be achieved and ignore the relationship of the body to the BB. As the example above shows, even a pretty extreme change in the front center dimension has little impact on weight balance.
what i was trying to implycolker1
Dec 15, 2003 9:34 AM
was the possibility of using longer stems on bikes with laid back head angles... have not ridden many bikes but i see sometimes advice (from serious folks) to downsize with colnagos. then i looked at colnago charts and saw the longer front center (balanced with very short cs).. so i figured (i haven't ridden them) colnagos could be set with longer stems. therefore, the idea of colnagos having short toptubes is a misunderstanding of the it's intended use. the actual cockpit would be the same as a conventional italian bike(w/73.x hangle) but the laid back head angle would throw the headset closer to the seat tube and a longer stem would compensate the shorter top tube. what do u think?
Dec 15, 2003 11:58 AM
A slack head tube angle would only move the bars about 2-3mm closer to the rider. Although it's something to consider if you're splitting hairs, for the most part the head tube angle differences can be ignored. The head tube angle on frames of eqaul size rarely differs more than 1 degree.

The front center has absolutely nothing to do with the fit of the frame. It affects the wheelbase and the weight balance.

The TT length of Colnagos varies from longer than many others in the small sizes to shorter than many others in the large sizes, so there is no generalization that can be made about Colnago frames. The majority of complaints come from large riders who legitimately complain about the shortenss of the Colnago TT and the need for a long stem.

If you want to compare the horizontal fit of two frames, all you need is the formula: saddle height x (cos A-cosB) to calculate the difference in effective top tube length. If you know that there is 1 degree or more difference in the head tube angle, you can reduce the TT length of the frame with the slack HTA by another 2-3mm. Occasionally this might suggest one stem length over another, if you're in between sizes.
I get it!!gregflynn
Dec 13, 2003 12:11 PM
If I had read your message right in the first place I would have understood. The higher the seatpost the longer the 'effective' TT length.

Obvious really just not concentrating hard enough.

Has anybody seen or ridden a Ridley Damoncles frame out there?
not quite..C-40
Dec 13, 2003 1:24 PM
Read my explanation above, then you'll get it.
Medium Giant headtube about 145mm (nm)undertrained
Dec 13, 2003 9:28 PM
re: Giant frame size?toronto-rider
Dec 15, 2003 11:39 AM
I have the same dimensions as you and ride a medium 2003 TCR Carbon frame. I use a regular size seat post and still have a lot of the post in the frame.I tried the large, and it way to big. YOu should have no problem with the medium.
Here is the picture of the bike
toronto-rider "Poll: Tell us about your ride" 12/14/03 7:51am