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Bike Fit Revisited - Bigger vs. Smaller Frame Differences(20 posts)

Bike Fit Revisited - Bigger vs. Smaller Frame DifferencesFez
Jan 9, 2003 2:18 PM
Recently I posted about trying to choose between 2 identically equipped Litespeeds that I had set up.

The 55 Litespeed has a 73 degree seat tube. Very comfortable ride - I feel great after 2-3 hours.

The 53 Litespeed has a 74 degree seat tube. Not very comfortable - my rear and back tend to get fatigued quicker. The ride does not feel as smooth Feels stiffer and feels like a more responsive climber. Standover is plenty, as I have a 17-18cm saddle height to top tube difference.

According to the formulas, I am probably closer to the 53. All the contact points (saddle height, KOPS, reach, bar height) are set up identically on the two bikes (the 53 has 2 cm of spacers, the 55 none). The saddle, seatpost, tires, and wheels are also the same. I have "normal" proportions and have good flexibility and conditioning.

So does a 2 cm shorter seat tube and a 74 degree angle really make that much of a difference in the ride? It feels much less comfortable, and I have the fatigue narrowed down to my back and butt area (no problem in the neck, shoulders, or arms).

Don't know whether to blame the fitting formulas in general as being too aggressive, or whether a seat tube angle really makes the difference.
It's not the seat tube angle...C-40
Jan 9, 2003 2:48 PM
The seat tube angle has little to do with the difference in ride. The difference in the force transmitted due to the change in angle is 0.5%.

What you're neglecting to consider is the fact that all of the tubes, except the chainstays, are shorter in length. Unless the diameter and/or wall thickness of the tubes is reduced, the shorter tubes will be stiffer. No mystery.

This is a common problem for folks who ride small frames. Combine a light rider with a frame that has tubes that are the same as used on larger frames, and you get an overly stiff ride.
Did consider itFez
Jan 9, 2003 3:08 PM
I just cannot believe it makes that much of a ride difference. I noticed this early on, but I decided to ride each bike several hundred miles (alternating between the 2) and see if the smaller one still bothered me.

BTW, the published weight difference is minimal between the 2 sizes - approx 20 grams, or less than 0.1 lb.

If the engineers did not compensate for wall thickness, I would think the difference between standard and compact frame geometry would produce a more dramatic difference in ride stiffness than the situation here, which compares 2 identical frames sized 2 cm apart.
compact is more complicated...C-40
Jan 9, 2003 5:35 PM
A "compact" frame has a sloping top tube that is 1-2cm shorter than the same size conventional frame, depending on the amount of slope. Typically, the TT is dropped by 5-7cm.

The rear triangle is basically identical to a 5-7cm smaller conventional frame.

The downtube and headtube are no different in length than a conventional frame of equal size.

The rear of the compact frame should be more rigid, but the the front won't be much different.

Unless you had someone build a compact frame and an identically sized conventional frame from the same type of tubes, and rode them both for comparison you can't draw any conclusions about the effect of compact geometry on the ride.
compact is more complicated...Fez
Jan 9, 2003 7:27 PM
You MIGHT be able to do this for a Seven, I think. Isn't the Axiom Ti offered in both geometries?

But otherwise, when a mass production manufacturer introduces compact frames, it usually is a substantial redesign as compared to a prior full-sized model.
Not questioning your experience, butKerry
Jan 9, 2003 5:02 PM
The differences you have felt are VERY hard to attribute to the frame. I would suggest you repeat this experiment by swapping wheels. IOW, always ride on the same wheels/tires. This is the item most likely to make the kind of difrerence you've experienced. And double check those measurements to insure that both bikes truly are identical set up.
Already done thatFez
Jan 9, 2003 6:01 PM
Measurements are the same.

Saddle, seatpost, bars, everything is the same, including tires and wheelset (except for the color).

And just for grins, I have swapped the wheelset - no difference - same harsh ride on the small one, smooth ride on the bigger one.
identical setup question...C-40
Jan 9, 2003 5:42 PM
If your bikes are set up nearly the same, both should be using the same stem length since the STA difference requires a saddle position change that is within 2-3mm of the difference in TT length. Do you use the same length stem on both bikes?
identical setup question...Fez
Jan 9, 2003 7:22 PM
Yes, same length stem (Zepp 100mm). Saddle position has been adjusted accordingly to account for the 1 degree difference in the STA.

Interestingly, the nominal measurements of top tube have a 1.5 cm difference between the big and small frame. However, I have to adjust the difference by 1.2cm or so because or the 1 degree STA difference. Therefore, the reach is nearly identical (or within 2-3mm).

So yes I am using the same stem.
Saddle-to-bar drop?...jtolleson
Jan 9, 2003 9:21 PM
Generally going to a 2 cm smaller frame drops the bars by nearly an inch. That's A LOT. Why wouldn't that be an explanation?

Sorry, can't buy the STA theory.
IdenticalFez
Jan 9, 2003 9:50 PM
In the orig post I said I had to use spacers in the smaller frame and none in the bigger one. The handlebar position is the same.
Same frame model and year? (Had to ask) nmBruno S
Jan 9, 2003 9:57 PM
re: Bike Fit Revisited - Bigger vs. Smaller Frame DifferencesTrent in WA
Jan 9, 2003 11:44 PM
Here's my hunch: The wheelbase on the 55 is longer, and since you've moved your saddle somewhat forward on the rails to acheive the same knee/pedal aspect, you're sitting more in front of the rear axle. I'm finding it harder to visualize what's going on at the front of the bike, but I suspect that your hands are farther behind the front axle as well. The net effect of that would be to provide you with more buffering from road shock. Are the chainstays on the 53 and 55 the same length?

Fitting formulas (of which there are many, some more aggressive than others) are great, but they all are based on assumptions about a rider's fitness, preferences, riding style, flexibility, and intended bike use. If the assumptions of a particular formula fit you, so will the bike. If not, there's always eBay or the classifieds here.

Trent
that's not it...C-40
Jan 10, 2003 5:20 AM
The chainstays are the same length on 53 and 55cm frames. With the saddle positioned the same relative to the BB, the rider will be the same distance in front of the rear axle.

The hands will also be the same distance relative to the front axle, since the HTA and fork rake should be the same. Spacers were used to make up for the shorter HT length.
But Trent said...KeeponTrekkin
Jan 10, 2003 8:10 AM
the wheelbase on the 55 was longer. If so, the riders position relative to the wheels must be different in some way, right?
nope...C-40
Jan 10, 2003 8:55 AM
The wheelbase on these bikes should be identical. Look at Litespeed's geometry charts. They list the same wheelbase for most 53 and 55cm models.
Ride the comfortable one. Sell the small one. Geeze. NFMSpunout
Jan 10, 2003 10:51 AM
A Very Real "NO BRAINER"crosscut
Jan 10, 2003 8:10 PM
Get the larger bike. Period. I could go on and on about the falacies of today's fit mechanics, but, suffice it to say, always stay with the larger frame.
Had the same problem...bought the bigger bikejwhite480
Jan 10, 2003 12:47 PM
Recently went through the same dilema. The bigger bike felt more comfortable and rode better than the bike I should have been just right for (according to the formulas). A number of folks said buy the bigger one and I did.....smart move as I have a bike that I can ride a lot further and longer with more comfort.
Had the same problem...bought the bigger bikeFez
Jan 10, 2003 2:24 PM
And I'm assuming you had the bar height/reach and saddle position exactly the same on both. Guess those formulas are for hardened racers who would rather trade comfort for a little more stiffness.

Which bike model and brand was it?