Frame Size Question - Help!
12-09-09, 10:11 AM
#1
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Frame Size Question - Help!
Looking into purchasing a new '09 BMC Pro Machine for my wife. Right now she's on a Trek 5500 size small frame (52cm) that runs a top tube length of 53. She's got a Ritchey stem on the bike that I believe is 120 mm. Here's a link to a website with the geometry of her bike
https://web.archive.org/web/200210150.../5500_geo.html
I'm looking into building up an '09 BMC Pro Machine for her. She VERY briefly rode both a size 49 and 51 in the Pro Machine at a shop here in town that no longer carries those bikes. She by no means had a formal fit on the bike, but said she thought she preferred the 51. Unfortunately there's no shops in town any more that carry these bikes. As many of you know, BMC has odd sizes when you look at the actual frame "size" compared with the top tube length. Here's a link which shows the geometry of the BMC bikes:
https://www.competitivecyclist.com/ro...c-01-5429.html
The guy at our local shop here in town as well as the dealer I'm going to purchase the bike from recommended putting her on the 51 BMC (which has a 54 top tube) and shortening up her stem a bit, which would make the fit similar to her Trek. Before I pull the trigger was just hoping to get some thoughts from others on here who may have had similar fitting experiences. Any thoughts/advice would be much appreciated! If it helps, here's a few measurements from the CC fit calculator. These measurements would seem to say go with the 49 BMC......so who knows.
Measurements
-------------------------------------------
Inseam: 31.5
Trunk: 22.5
Forearm: 13
Arm: 25
Thigh: 22.5
Lower Leg: 20.5
Sternal Notch: 54.5
Total Body Height: 65.5
https://web.archive.org/web/200210150.../5500_geo.html
I'm looking into building up an '09 BMC Pro Machine for her. She VERY briefly rode both a size 49 and 51 in the Pro Machine at a shop here in town that no longer carries those bikes. She by no means had a formal fit on the bike, but said she thought she preferred the 51. Unfortunately there's no shops in town any more that carry these bikes. As many of you know, BMC has odd sizes when you look at the actual frame "size" compared with the top tube length. Here's a link which shows the geometry of the BMC bikes:
https://www.competitivecyclist.com/ro...c-01-5429.html
The guy at our local shop here in town as well as the dealer I'm going to purchase the bike from recommended putting her on the 51 BMC (which has a 54 top tube) and shortening up her stem a bit, which would make the fit similar to her Trek. Before I pull the trigger was just hoping to get some thoughts from others on here who may have had similar fitting experiences. Any thoughts/advice would be much appreciated! If it helps, here's a few measurements from the CC fit calculator. These measurements would seem to say go with the 49 BMC......so who knows.
Measurements
-------------------------------------------
Inseam: 31.5
Trunk: 22.5
Forearm: 13
Arm: 25
Thigh: 22.5
Lower Leg: 20.5
Sternal Notch: 54.5
Total Body Height: 65.5
12-09-09, 11:33 AM
#2
Senior Member
Your dealer is wrong about the adjustment needed to make the 51cm BMC fit like the 52cm Trek. The seat tube angle on the BMC is only 73.5 degrees, compared to 75 degrees on the Trek. The slack STA shortens the reach so it is actually about 3mm shorter, not longer, once the saddle position is set the same on both bikes.
I can provide a very accurate comparison of these frames if you want it, but I would need a little more information. The Trek geometry chart dating back that far is incomplete, since it does not list the head tube length. I have some of the lengths for the larger sizes, but there is a limit to how short the HT can be made and I don't want to guess. IIRC, that model still used a conventional headset, not an integrated? That matters too, since the length of a conventional headset is in the 25-35mm range, while integrated models are 10-20mm.
While your wife's inseam is valuable information, an actual saddle height would be far more valuable, since people with the same leg length can have a significantly different saddle height, depending on how much heel rise they prefer.
The angle or the brand of that 120mm stem would be good to know too; either that, or the actual height from the floor to the top of the bars.
With the limited information, all I can compare is the frame reach. The 51cm BMC has about 3mm less reach than your Trek and the 49cm has about 5mm less reach.
It's simple to compare the 49 and 51cm sizes of the BMC frame. The reach on the 51cm is only 1-2mm longer than the reach on the 49cm, once corrections are made for the different seat tube angle. That means that both frames would use the same stem length, to produce a nearly identical fit. The head tube on the 51cm is 15mm taller, so it has the potential to raise the bars that much higher. In practice, either frame could be made to fit the same, but if it's important to have the bars fairly high, the 51cm would be the better choice. That's why I asked for the bar height information.
You really can't tell much about the fit of a bike from a test ride unless there was a considerable effort made to position the saddle properly (both fore/aft and height). These two frames should also have the same stem length on them if they are to be compared properly. The handlebar height is typically set as high as possible on a bike sitting on the shop floor, with maximum spacers and a flipped up stem. That may be quite different than the current bike's setup. Handlebar reach and the position of the brake/shift levers also make a big difference in a person's riding impressions.
FWIW, I'm an inch taller, but my legs are also an inch longer. I'd still choose the 49cm because it has nearly identical dimensions to the 51cm LOOK 585 that I ride. The HTL is the same, the STA is the same and the TT length is the same. That said, I also want an aggressive fit, so I want a relatively short head tube. A person wanting a more upright fit would choose the 51cm, since the reach is about the same.
For one of my fans who's lurking - isn't it amazing that a 49cm BMC has ALL of the same dimensions (that affect the fit) as a 51cm LOOK? Just goes to show you that it pays to look at more than just the frame size number.
I can provide a very accurate comparison of these frames if you want it, but I would need a little more information. The Trek geometry chart dating back that far is incomplete, since it does not list the head tube length. I have some of the lengths for the larger sizes, but there is a limit to how short the HT can be made and I don't want to guess. IIRC, that model still used a conventional headset, not an integrated? That matters too, since the length of a conventional headset is in the 25-35mm range, while integrated models are 10-20mm.
While your wife's inseam is valuable information, an actual saddle height would be far more valuable, since people with the same leg length can have a significantly different saddle height, depending on how much heel rise they prefer.
The angle or the brand of that 120mm stem would be good to know too; either that, or the actual height from the floor to the top of the bars.
With the limited information, all I can compare is the frame reach. The 51cm BMC has about 3mm less reach than your Trek and the 49cm has about 5mm less reach.
It's simple to compare the 49 and 51cm sizes of the BMC frame. The reach on the 51cm is only 1-2mm longer than the reach on the 49cm, once corrections are made for the different seat tube angle. That means that both frames would use the same stem length, to produce a nearly identical fit. The head tube on the 51cm is 15mm taller, so it has the potential to raise the bars that much higher. In practice, either frame could be made to fit the same, but if it's important to have the bars fairly high, the 51cm would be the better choice. That's why I asked for the bar height information.
You really can't tell much about the fit of a bike from a test ride unless there was a considerable effort made to position the saddle properly (both fore/aft and height). These two frames should also have the same stem length on them if they are to be compared properly. The handlebar height is typically set as high as possible on a bike sitting on the shop floor, with maximum spacers and a flipped up stem. That may be quite different than the current bike's setup. Handlebar reach and the position of the brake/shift levers also make a big difference in a person's riding impressions.
FWIW, I'm an inch taller, but my legs are also an inch longer. I'd still choose the 49cm because it has nearly identical dimensions to the 51cm LOOK 585 that I ride. The HTL is the same, the STA is the same and the TT length is the same. That said, I also want an aggressive fit, so I want a relatively short head tube. A person wanting a more upright fit would choose the 51cm, since the reach is about the same.
For one of my fans who's lurking - isn't it amazing that a 49cm BMC has ALL of the same dimensions (that affect the fit) as a 51cm LOOK? Just goes to show you that it pays to look at more than just the frame size number.
Last edited by DaveSSS; 12-09-09 at 04:08 PM.
