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Between sizes - frame geo considerations

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Between sizes - frame geo considerations

Old 09-19-21, 09:19 AM
  #1  
sanmi
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Between sizes - frame geo considerations

Many of us can fit more than one size frame of a given model. Conventional wisdom is generally to size down unless you want a particularly relaxed/upright fit. My question is whether this calculus changes for frames on the smallest end of the spectrum, where manufacturers often make geometry compromises that may adversely affect handling characteristics.

For context, I am 5'5" with an approximately 29.5" cycling inseam. For the last few years I have primarily ridden a 50cm Bianchi Infinito CV (circa 2014) and a 51cm Jamis Renegade (circa 2019). Both frames have an effective top tube around 525-530mm and relatively slack head tube angles of 71-71.5*. I have both fitted with 0 offset seatposts with saddles fairly forward. I'm running relatively short 160-165mm cranks on both rigs. I have sometimes wondered whether my need for 0 offset posts on these bikes indicates that they are slightly large for me, though I have ridden both comfortably for several thousand miles.

I'm interested in trying a new bike with racier geometry and handling. Looking at geometry tables, it seems that manufacturers differ in how well they preserve certain measures (e.g., trail) in the smallest sizes. Of course there are tradeoffs but it seems like a large part of the variation is whether the manufacturer uses multiple fork rakes across sizes. For example, Giant uses a single fork rake across TCR sizes resulting in trail ranging from 59-72mm from largest to smallest. On the other side, Cannondale uses 2 fork rakes and manages to preserve a narrow trail range of 58-60mm across sizes. This makes me wonder if certain brands are particularly well suited for shorter riders.

My LBS is a Specialized dealer and I'm quite interested in the Aethos. Given my measurements, Specialized recommends a 52cm frame. Yesterday I test rode both the 52 and 49cm. I think I could make either frame work, but for the 52cm I would certainly need a 0 offset seatpost and to push the saddle forward. I recognize this is likely not ideal in terms of weight distribution over the wheels. The pedaling position on the 49cm fit much better out of the box, and the complete build also comes with my preferred crank length and bar width. However, the 49cm has a significantly longer trail of 63mm compared to 58mm on the 52cm, which is more in line with larger sizes. Here is the geo table for reference: https://www.specialized.com/us/en/ae...=299423-187066

My question is whether to follow conventional wisdom and go with the smaller 49cm frame that I can ride with the stock seatpost (12mm setback) or if the altered geo at this size will compromise the pin sharp handling I'm looking for, suggesting I should either size up or look at a different model/brand. How compromised do you think the handling characteristics will be on the 49cm given longer trail vs the 52cm?

Last edited by sanmi; 09-19-21 at 11:19 AM.
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Old 09-19-21, 09:39 AM
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Increased trail in the small bikes is common. Even in very expensive bikes. Imo you shouldn't compromise if either of two sizes doesnt fit or have weird handling from compromised frame geo. Try them again and if they still both feel off get something else. At that price point you should be able to get an exact fitting bike with no corners cut.
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Old 09-19-21, 09:49 AM
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You rode them. Which feels better to you? I will say that my pick of two different size bikes was opposite between my initial test ride in the parking lot and a 10 mile ride on each the next day. I liked the smaller frame for so many more reasons. One being it was sportier and more fun feeling. However if you are wanting a more luxurious ride, maybe the larger frame.

There isn't any one formula that can be applied to everyone to give them their perfect feeling bike size choice.
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Old 09-19-21, 11:26 AM
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I took both sizes on the same 7 mile test ride. The 49 fit much better around the pedaling axis, but I didn’t test the 52 with a 0 offset post or 165mm cranks. Both bikes were also fitted with 155mm saddles vs my preferred 129mm. With the contact points off, I found it hard to compare handling feel over that distance.

I have a good relationship with my LBS and they offered to spend 2-3 hours doing some fit work on both frames. I plan to take them up in the offer as well as try some other models (pending availability). In the meantime I’m interested in hearing experiences from other shorter riders who have been through similar fit and handling exercises.
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Old 09-19-21, 11:42 AM
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The difference in STA has nothing to do with weight balance. Both bikes have the same chainstay length and nearly identical wheelbase. Your body should be placed in the same position relative to the BB on both sizes. I'm only an inch taller, but my legs are 3 inches longer. My wingspan is also about 2 inches greater than my height. I could ride either size, but the steep sta on the 49 would move the top of the seat post about 18mm further forward. I might get my saddle back far enough with a 32mm setback post. I would use a 25mm setback on the 52. I wonder how you select such a forward position - KOP maybe? I select setback based on my balance over the saddle, not any knee to pedal relationship. With your shorter saddle height, that alone moves your saddle quite a bit further forward.
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Old 09-19-21, 02:05 PM
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When bike frames were made from steel the desire to have the lightest and therefore the smallest frame possible may have made sense. But with CF frames there is little to be gained by having a smaller frame and the end result is adding more weight with a longer seat post and longer stem for the handlebar and a more confined posture when riding. A difference oe 1cm in seat tube length is meaningless. It is the length of the top tube and head tube angle and fork rake that will have the most impact on the ride.

"Racier geometry" in the past meant steeper head tubes and less fork rake and much less stability at speed. The bikes were designed after criterium racing bikes and of no real value for much else. During the Tour de France the riders will have other bikes for use on the open road that were more stable at the very high speeds at which they routinely travel downhill.
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Old 09-19-21, 04:34 PM
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Originally Posted by DaveSSS
The difference in STA has nothing to do with weight balance. Both bikes have the same chainstay length and nearly identical wheelbase. Your body should be placed in the same position relative to the BB on both sizes. I'm only an inch taller, but my legs are 3 inches longer. My wingspan is also about 2 inches greater than my height. I could ride either size, but the steep sta on the 49 would move the top of the seat post about 18mm further forward. I might get my saddle back far enough with a 32mm setback post. I would use a 25mm setback on the 52. I wonder how you select such a forward position - KOP maybe? I select setback based on my balance over the saddle, not any knee to pedal relationship. With your shorter saddle height, that alone moves your saddle quite a bit further forward.
Thanks for sharing your numbers and experience. Our 3” difference in leg length seems pretty significant and certainly enough to explain your preference for a 25mm offset on the 52 vs my 0mm offset.

I’d be curious to hear your thoughts on the difference in front end geometry of the two sizes.
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Old 09-19-21, 05:15 PM
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Originally Posted by sanmi
Many of us can fit more than one size frame of a given model. Conventional wisdom is generally to size down unless you want a particularly relaxed/upright fit.
Really? I thought sizing up makes you more stretched out, which is the opposite of relaxed, which, to me, is having my back at its most natural angle.
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Old 09-19-21, 06:03 PM
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Sizing up might be done to raise the bars 20mm and only require a 10mm shorter stem to get the same reach.
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Old 09-19-21, 06:13 PM
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Originally Posted by DaveSSS
Sizing up might be done to raise the bars 20mm and only require a 10mm shorter stem to get the same reach.
Now I get what you were getting at: If in between sizes, pick the larger one to get more stack but use the next shorter version of the stem to maintain approximately the same reach.

Do note that some carbon fiber seatposts, e.g., Cannondale Save, Canyon VCLS 2.0, require a certain length of exposed seatpost to maximize their dampening effects. Picking the larger frame reduces this effect.
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Old 09-19-21, 06:15 PM
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Originally Posted by sanmi
Thanks for sharing your numbers and experience. Our 3” difference in leg length seems pretty significant and certainly enough to explain your preference for a 25mm offset on the 52 vs my 0mm offset.

I’d be curious to hear your thoughts on the difference in front end geometry of the two sizes.
Actually, it's the opposite. Your lower saddle height moves the saddle forward, even though you have a longer torso, then you want the saddle even further forward, with little seatpost setback. In total, you're much further forward. I have a shorter torso, but place the saddle further back.

As for the geometry, my current CInelli superstar frames have a slack hta and lots of trail at least 68mm. They handle great. I wouldn't want only 58mm of trail.
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Old 09-20-21, 05:45 AM
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Originally Posted by SoSmellyAir
Now I get what you were getting at: If in between sizes, pick the larger one to get more stack but use the next shorter version of the stem to maintain approximately the same reach.

Do note that some carbon fiber seatposts, e.g., Cannondale Save, Canyon VCLS 2.0, require a certain length of exposed seatpost to maximize their dampening effects. Picking the larger frame reduces this effect.
Yeah, I tend to go more off stack height than reach when looking at frame sizing. If I was on the fence between two sizes I would tend to size up for a longer wheelbase and higher stack for endurance riding. A lot of racers do the opposite and size down to get lower. Stem length is pretty easy to compensate for within reason. It's a good point about seatpost extension vs comfort. Definitely something to bear in mind if sizing up, but wouldn't be my primary concern as long as the post wasn't slammed right down - which would indicate a frame way too big anyway on a modern bike.
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Old 09-20-21, 08:50 AM
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On my bikes that were way oversize for my I was more upright on them. The saddle in relation to everything else is lower. So stack height relative to the rider is higher.

So yes, a larger frame size might give one a more relaxed position. Still, it depends on a lot of other stuff whether the rider will be comfortable or not. I was. But I'm also more comfortable on a much smaller frame in a more aero position for both long and short rides.

I did try stretching the stems out farther on those oversize frames, but that never was ideal as it left my arms straight at the elbows.
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Old 09-20-21, 03:15 PM
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Originally Posted by Iride01
On my bikes that were way oversize for my I was more upright on them. The saddle in relation to everything else is lower. So stack height relative to the rider is higher.

So yes, a larger frame size might give one a more relaxed position. Still, it depends on a lot of other stuff whether the rider will be comfortable or not. I was. But I'm also more comfortable on a much smaller frame in a more aero position for both long and short rides.

I did try stretching the stems out farther on those oversize frames, but that never was ideal as it left my arms straight at the elbows.
I agree and would only size up if I was genuinely right on the fence between sizes. I wouldn't deliberately size up if I was in the middle of the fit range of my appropriate size.
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Old 09-20-21, 06:37 PM
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I appreciate the responses but want to refocus on my original question, which is whether the "between sizes" decision changes at the smallest end of the spectrum where manufacturers begin to make compromises in geometry to maintain 700c wheels. For example on the frame I'm considering, 52-58cm sizes all have a trail between 55-58mm but this jumps to 63mm on the smallest 49cm frame. Since I think I can fit on either the 49 or 52 by adjusting setback, does it make sense to size up to preserve handling characteristics?


After further reading, I think I'm getting close to answering my own question. In this case, it seems Specialized is using this strategy of a super steep seat tube to shorten the effective top tube. Given DaveSSS's observation that I prefer a fairly forward position, this may actually work for me, but then I'm left with the suboptimal head tube angle and trail. In fact, comparing the complete geo of the 49 and 52, the reach is only 5mm longer on the 52. The rest of the 23mm difference in top tube length is behind the BB and thus adjustable via setback. Swapping the stock 12mm offset post for a 0mm offset leaves only 6mm to be adjusted for by sliding the saddle forward. So I'm thinking the 52cm with a 0 offset post is likely best provided the saddle rails allow for sufficient adjustment.


What am I missing?



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Old 09-21-21, 03:40 AM
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Originally Posted by sanmi
does it make sense to size up to preserve handling characteristics?
Have you considered whether you would prefer more or less trail? It actually makes some sense to increase trail on a smaller frame because it will naturally have quicker handling due to the shorter wheelbase. Increasing trail will slow the steering slightly, but increase stability.
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Old 09-21-21, 05:29 AM
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Originally Posted by PeteHski
Have you considered whether you would prefer more or less trail? It actually makes some sense to increase trail on a smaller frame because it will naturally have quicker handling due to the shorter wheelbase. Increasing trail will slow the steering slightly, but increase stability.
It's a great question. I currently ride an endurance and gravel bike that each have trail between 65-68mm. I like both bikes but am interested in trying something different.

Even if I end up learning I prefer more trail, I'm still a little hung up on the relative differences across the geometry table. For example between the 49 and 52 there is a 5mm difference in trail and a 2mm difference in wheelbase whereas between the 54 and 56 there is a 3mm difference in trail and a 13mm difference in wheelbase. Going to the 58mm trail doesn't change but the wheelbase is another 14mm longer.
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Old 09-21-21, 07:38 AM
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A 63mm trail is not suboptimal. It's done to avoid handling that would be too quick with a short wheelbase. My Cinelli superstar has a 71 degree sta and 45mm fork offset for a 67mm trail and it handles great. I've done 56 mph on it. It handles great on mountain descents. I was impressed with the handling, right from the start.

Also, some brands never use a small 58mm trail on any of their frames, except maybe the largest.

Last edited by DaveSSS; 09-21-21 at 11:26 AM.
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Old 09-21-21, 08:31 AM
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Until you ride them both, you'll never know. The things I don't like about one bike that makes me favor the other might be just the opposite of the things that are important to you. And a lot of these things are probably aspects we can't really begin to describe with words.

Focusing on one part of a bikes geometry without taking everything else into consideration will lead you to just trying one bike and bias.
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Old 09-21-21, 11:04 AM
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Originally Posted by sanmi
It's a great question. I currently ride an endurance and gravel bike that each have trail between 65-68mm. I like both bikes but am interested in trying something different.

Even if I end up learning I prefer more trail, I'm still a little hung up on the relative differences across the geometry table. For example between the 49 and 52 there is a 5mm difference in trail and a 2mm difference in wheelbase whereas between the 54 and 56 there is a 3mm difference in trail and a 13mm difference in wheelbase. Going to the 58mm trail doesn't change but the wheelbase is another 14mm longer.
It's hard to say whether or not you would notice a change in trail of 5 mm. Trail is quite sensitive, so you might well feel it in a back-to-back test. 2 mm difference in wheelbase would be insignificant to the handling, so at least you can rule that out as having any effect.
But in any case I would go with the frame size that fits you best and not worry too much about these relatively subtle differences in steering geometry.
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Old 09-21-21, 01:11 PM
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Not being trite, but can I mention aesthetics? If I'm sure that I can make either size fit, I'm going with the one that's closer to the middle of the bell-curve, 'cause they look better - the further away you get from 54cm or 56cm, the less pleasing I find them to be.
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Old 09-21-21, 03:13 PM
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Originally Posted by WhyFi
Not being trite, but can I mention aesthetics? If I'm sure that I can make either size fit, I'm going with the one that's closer to the middle of the bell-curve, 'cause they look better - the further away you get from 54cm or 56cm, the less pleasing I find them to be.
We're talking about the two smallest sizes. I'd rather have a bike that fits. I'm really pleased with my bike when it fits. My saddle height is 7-8 cm greater than the OP's, so I'd buy the 52 and slam it with a -17 stem. It would fit just like my previous Colnago, but I wouldn't like the small 58mm trail. With the OP's short legs, the smaller size is a no-brainer. Even with no spacers and a -17 stem, the saddle to bar drop would be quite small, since the stack is only 13mm less on the 49. The seat tube is only 3cm shorter than my current Cinelli, so there wouldn't be too much exposed post either.

Small frames look how they look. There's no magic that can change that.

https://forums.thepaceline.net/showt...elli+superstar

Last edited by DaveSSS; 09-21-21 at 03:20 PM.
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Old 09-21-21, 03:41 PM
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Originally Posted by DaveSSS
I'd rather have a bike that fits.
Didn't I preface it with that?

Originally Posted by WhyFi
If I'm sure that I can make either size fit...
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Old 09-21-21, 10:34 PM
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The larger frame will have quicker steering due to less trail and a shorter stem.
Which one is preferable is totally up to you.
Personally I would ignore the handling difference and get the one that fit best.
i.e, the one that when set up to my fit has the more "normal" amount of stem spacers and stem length.
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