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Fitting for a semi sloping bike(9 posts)

Fitting for a semi sloping bikeMCCL
Dec 13, 2001 10:06 AM
Help I am about to buy a semi sloping bike and don't have a clue on how to size it up. I have a 29 inch inseam and the bike frame is measured from top of bottom bracket to top of seat tube. Any clues in what I should be buying. Thanks MCCL
re: Fitting for a semi sloping bikemorrison
Dec 13, 2001 10:23 AM
It's extremely difficult to give advice w/out more info. Inseam can be measured in many different ways. The real measure is stand-over height. Stand up against a wall and jam a broom stick under your crotch so that it is parallel to the ground and perpendicular to the wall. (Note: I said under the crotch, not into the crotch =)) The stick should be pressed up against the bottom of your crotch. Then make a mark on the wall where the stick hits. Measure that mark to the ground. That is a decent stand-over measurement that you can use when evaluating frame size. Opinions differ, but you should have 3/4 to 1 1/2 inches of standover clearance on the frame.

The problem with a bike with a sloping toptube is that the standover height of the frame varies along the tube. Try and take a mid-tube measurement.

Make sure that this is not the only factor you take into account. How tall are you? What is your reach? Stem length and rise will make a huge difference in the fit of the bike. Width of handlebars will also make a difference.

Do you have a LBS near you? See if they can fit you.

From your inseam, I am guessing you are 5'5"-5'6"? You may be looking at a 50 to a 52 cm frame?
re: Fitting for a semi sloping bikeMCCL
Dec 13, 2001 12:00 PM
Thanks for your reply. I guess I should of been more specific. And if this adds any light to the my post please reply. Yes my height is 5'6 and broom stick is a 29 inch. I am going to a LBS store but I really don't want anyone that has only normal bike spec trying to attack a whole new animal. Semi sloping I would spectulate that are two different animals when it comes to sizing. I don't know what Giants are spec. at but I know the bike I am going to get is measured at the top of the BB and the top of the ST. The material if it matters is Carbon. Bike is so new that it just came out this yr.or 2002. I can only assume that the bike shop will know what they are sizing me up for as long as I give them the specs on the bike. But I don't want them sizing me up to some standard spec. bike unless that is how it is done for this kind of bike. I would figure that some one had a plan when they made this kind of bike and it is written somewhere. Maybe Sheldon Brown. What do you think? Thanks MCCL
Fitting a sloping bike...TJeanloz
Dec 13, 2001 2:34 PM
Fitting for a sloping bike is exactly the same as fitting for a regular bike. One just needs to take into account the 'effective' dimensions instead of the real ones. You just pretend, for the sake of fitting, that the top tube does not slope. So if you ride a 55cm regular bike, you should ride a 55 "effective" cm sloping bike- which will measure something like 51cm.

Of course, I have found the definitive treatise on sizing, written by one of the world's largest bicycle makers, here:

http://www.huffybikes.com/shop/size.html

I am kidding. Do not take their advice.
re: Fitting for a semi sloping bikeguido
Dec 13, 2001 12:08 PM
First, you have to determine the right saddle height. You can get really close by using Saddle height = inseam x .883. Saddle height is measured from the bottom bracket axle to the lowest part of the saddle.

Second, determine saddle set-back from the crank. Your knee should be over the pedal spindle holding at 3:00, measured by a plumb bob.

Now that you know your body's relationship to the crank, you can fit your upper body and arms over the top tube and handlebars. This is more touchy-feely because there are too many dimensions and angles to account for: length of torso, upper and lower arms, hands. All bike manufacturers build bikes with reach dimensions proportional to seat tube dimensions, so that if you can get your saddle at the right height, the handlebars and stem will be very close to right. A short legged person will fine tune with a longer stem to accomodate his longer upper body, and vice versa. Once you raise the saddle to the right height, preferably on a trainer where you can work the pedals, you should be able to reach the handlebars without a big stretch, but your chest should have room to expand. You should be able to make your back flat without moving out over the handlebars. They should obscure your view of the front wheel hub.
post the specific model..C-40
Dec 13, 2001 4:06 PM
If you post the exact model being considered, it would be easier to evaluate the geometry, assuming the manufacturer has a geometry chart on the web.

If your inseam is only 29 inches, you have short legs and a long torso. In a conventional frame, you would only ride a 50cm frame. The top tube will be too short on this frame size. The sloping top tube could help you achieve the necessary top tube length while maintaining some standover clearance.

I prefer to use a bike (with a horizontal top tube) to check inseam. Block up the wheels until the top tube exerts very firm crotch pressure when you stand over it in bare feet. Measure the top tube height for an accurate inseam dimension. This method might yield a larger and more accurate number. A 29 inch inseam is really short for a 5'-6" height.
post the specific model..MCCL
Dec 13, 2001 7:17 PM
Ok I just got back from the Good Guy Bike Shop. Here is What we came up with. I have a 79.5cm stand over or broom stick height. The torso is 58.5cm and the arms are 58. Yep as the bike shop attendant was saying I have a torso of a 6ft person but legs like Mr. Mini Me. The bike I am refering to is the DeRosa King. The geometry chart is with the bike on there web page. If you go to Http://www.wrenchscience.com/ and hook up to links it will get you to DeRosa. I figure that the 46.5 or the 48cm is the right one for this body. I really appreciate everyone helping. Also another problem is collar for the seat, some are 2cm. and others are 3cm. Now I don't know if this is included in DeRosa spec. but they measure from the top of the BB to what looks like the top of the seat tube. Don't know if this includes the collar or not. See my problem. I have sent DeRose a E-mail so I have some clear ideal on this matter and gave him my measurements to see if he can put this thing to rest. Thanks All and Happy Holidays. MCCL
poor geoemtry info....C-40
Dec 15, 2001 7:38 AM
The info on the DeRosa web site is pretty poor. It's not clear if the top tube length is the actual (sloped) or "virtual" (horizontal) top tube length. Also no listing of head tube length, which is an important indicator of the frame's true vertical size.

The sizes offered cover a very short range of top tube lengths. They don't appear to be well suited to a long torso rider, which would be typical of Italian and most all other stock frames.
46cmCT1 Guy
Dec 14, 2001 12:53 PM
I too have a 29 inch inseam and have a 46cm sloping frame (Colnago) - also make sure that the top-tube length is the right length. About 51-53cm is about optimum for this size of frame. I've seen some really stupid long top tubes on compact frames (Airborne)which anyone less that 5'6" would find way too long. Most of the top end Italian frames - Colnago, Pinarello, DeRosa share fairly similar geometry in the small sizes - no doubt due to many years experience of building frames for smaller riders.