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Top Tube Length(10 posts)

Top Tube Lengthmc3
Aug 1, 2001 7:40 AM
Everything I read says that Top Tube Length is more important than frame size. A rule of thumb for tt length is that the front hub should be obstructed by the handlebar. Colorado Cyclist says you should be in the drops. Others say you should have you hands over the brake levers and still others say you should have your hands on the handlebars. Which is most appropriate?
there really needs to be a FAQ or Tech 101 on bike fit.Jack S
Aug 1, 2001 7:43 AM
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there really needs to be a FAQ or Tech 101 on bike fit.Jon Billheimer
Aug 1, 2001 8:13 AM
I agree. The answer can vary, too, depending on one's relative arm and torso lengths. A more definitive answer is given in Kevin Lippert's article on bike fit, at his website. Once proper seat fore and aft position is set, he says that the major determinant of top tube/stem length is the rider's weight distribution from rear to front. Ideally this should be 55% on the rear wheel and 45% on the front wheel. And the reach to the handlebars must be comfortable.
I have a formula from Excelsports.comjw25
Aug 1, 2001 11:22 AM
I'll try to remember to post it when I get home. Basically it's something about torso length plus arm length, minus something, divided by two, equals top tube length plus stem length. Of course, it's more useful if you know what size frame you need, and that the determine by inseam length, which isn't the best way in my opinion, but short of a full physiognomy workup, is probably the best way for a mail-order company to do it.
Excel's formula is...PsyDoc
Aug 2, 2001 4:40 AM
Reach = torso length plus arm length, divided by 2, then add 4. But, this formula is a general starting point and does not take into account one's flexibility.
forget that "rule of thumb"ColnagoFE
Aug 1, 2001 11:54 AM
I don't think that "obscuring the hub" works for a lot of people. Especially riders with unusual proportions.
top tube myths...C-40
Aug 1, 2001 2:03 PM
Most folks who stress the importance of top tube length over frame size are just repeating something they've read or heard and don't have the foggiest idea why this may or may not be true.

Many frames are built with the top tube only lengthened by .5cm for each 1cm increase in frame size. Head tubes, however change almost one for one as frame size increases. If you pick a frame that's 2cm smaller or larger just to get a "perfect" top tube length, you may have greater problems achieving the desired saddle to bar height than getting the proper stem length. All you do is trade one small problem for a bigger one.

What many people also fail to realize is that top tube lengths can only be compared directly between frames with the same seat tube angle. Steeper seat tube angles effectively lengthen the top tube, after the saddle is adjusted to produce the same rider position relative to the bottom bracket.

A classic example is the Colnago 55cm frame with a 74 degree seat tube angle and 54.3cm TT. It will fit effectively the same as 55cm Litespeed (for example), which has a 55.5cm top tube and a 73 degree seat tube angle. The math to prove this is pretty simple. There will be small differences (1% or less)in weight distribution due to slightly different wheelbases.

Direct comparison of the top tubes on these two frames leads to the short top tube myth often associated with Colnago frames. In some of the larger frames sizes, Colnago top tubes can be a little shorter because these sizes have the same 73 degree as many other brands.

As for stem length recommendations, obscuring the view of the front hub is a mere coincidence that has no functional relevance. Functionally, I want my stem & top tube long enough that my knees and elbows don't hit each other when I ride low in the drops. With this achieved, most people will be pretty well stretched out. Additional stem length will create a slightly lower position, but can create too much stress on the shoulders. Lowering the stem is a more effective means of achieving a lower, more aerodynamic position. How low and stretched out you can go will depend on your flexibility and level of fitness.

The goal of perfect fit is to get your personally desired position with the saddle rails relatively centered on the seat post, a mid-length stem to permit some adjustment, and your desired saddle to bar height, without using a high rise stem or a lot of steering tube spacers. If this can't be achieved, something is not perfect.
top tube myths...Steve B.
Aug 1, 2001 7:13 PM
This part didn't make any sense, maybe 'cause it's late, so correct me if I'm wrong.

>What many people also fail to realize is that top tube lengths can >only be compared directly between frames with the same seat tube >angle.

True

>Steeper seat tube angles effectively lengthen the top tube, after >the saddle is adjusted to produce the same rider position relative >to the bottom bracket.

What we should be talking about here is "reach", particularly the reach from the saddle to the h-bar. A number of issues come into play; such as seat rail length, seat tube angle, seat post length, seat post off-set, top tube length, sloping TT ?, head tube angle, stem height and stem length.

A steeper seat tube causes the riders position to move forward over the b-bracket. The ST angle and top tube length is particular to the frame design and affects reach to the h-bar. A bike with a 72.5 seat tube angle and a 55cm TT has a seat that is somewhat further away from the h-bar as compared to a bike with a 74 degree seat tube also with a 55cm TT if only due to the ST angle pushing the seat further away from the h-bar (this assumes seat posts with similar SPO - seat post off-set). Remember that the TT measurement is at the joint of the seat tube and top tube (usually center of each). This is especially pronounced on a bike with a lot of seat tube exposed. Likewise, the head tube angle can slightly affect the reach, for the same reason, but to a lesser degree due to (usually) less height of the stem off the headtube/TT junction.

Getting a similar fit on the Colnago as is found on the Litespeed assumes using a seatpost with a lot of SPO and/or long rails on the seat, which is probably not a good way to go. Better to go the next size up on the Colnago and use a shorter stem (not that that has anything to due with the original post).

Not even going to discuss weight distro.

SB
muddying the water....C-40
Aug 2, 2001 1:29 PM
I always assume that experienced riders know the knee-over-pedal (KOP) position that allows them to produce optimum power, and try to achieve this position on any frame that they might purchase. Certainly this will require adjustment to the saddle fore-aft position, with frames that have different ST angles. There is always the risk that you will run out of saddle adjustment, making a particular ST angle undesirable. That's why it's important to be able to calculate the effect of different ST angles on saddle position (and reach).

To achieve a given KOP, a frame with a 74 degree STA requires the saddle to be moved further back than a frame with a 73 STA, therefore, it effectively lengthens the top tube. The amount is calculated by taking the difference between the cosines of the two ST angles times the saddle height (not the seat tube length). In my example this would be (cos73-cos74) x 72.5cm = 1.21cm. This calculation proves that either frame in the example would have the same reach with a given length of stem. . I know from experience that I can successfully achieve my desired KOP with a 73 or 74 STA, or anything in between.

I've owned both of the frames in my example and had no problem getting the saddle properly positioned on either one, using a traditionally offset road seatpost. Of course, the saddle was pushed further back on the Colnago and further forward on the Litespeed. Either angle can create problems depending on the choice of seatpost and saddle. I found one model of saddle (SDG Comp Ti) that I found to be quite comfortable, but couldn't get far enough forward on the Litespeed, due it's rail position and the wide clamp on the Campy Record seatpost. A change in seatpost was needed to get this saddle to fit. It would work perfectly on the Colnago, though.

A rider who needs a saddle position that is relatively far back may be wise to exclude any frame with a 74 degree STA from consideration, unless he's willing to select from a limited number of seatposts with additional offset. There is certainly nothing "wrong" with using a post like the LOOK carbon to provide additional offset, if it consequently produces an acceptable "effective" top tube length.
Top tube length & seat tube angle go hand and hand.Highgear
Aug 1, 2001 5:41 PM
If your trying to set position then you establish your saddle hight and the fore & aft. Then your drop from the saddle top to the top of the bars. Now with the correct reach your upper arms will be at the same angle as the head tube when you place your hands on the tops with the forarme parallel with the top tube. Everyone talks about top tube length being so important without understanding how the seattube angle will effect it. Say you ride with the tip of your saddle 10cm behind the BB on a bike with a 73 deg. seat tube angle and have a 58 TT length. You now set up a bike that has a 72 deg. seat tube angle and a 58 TT. The second bike will have an effectivly shorter TT due to the saddle being pushed forward causing the need for a longer stem than the first bike. Understand? Look at the bike as only the saddle and bars in a fixes position, now envision what happens to the TT when you play with the seat tube angle. Hope this helps.