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Discussion Starter · #1 ·
I'm buying a new bike and it comes with 170 mm cranks. Previously I had 172.5 mm.
I searched the forum and some people feel the difference, others dont. I made a 30 km test ride with some climbing and didn't feel anything. The only difference was - perhaps - that it was easier to maintain higher pedaling frequencies (placebo effect?).

I haven't decided yet - try 170 mm or install 172.5 mm.

But if I stay with 170 mm - what should I do with the saddle?
 

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The shorter crank arm (pedal) will be 2.5mm higher at the bottom of the pedal stroke. You can compensate for that by raising your seat that much if you wish.
 

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The shorter crank arm (pedal) will be 2.5mm higher at the bottom of the pedal stroke. You can compensate for that by raising your seat that much if you wish.


You can, yes. But you really don't need to if the 172.5 fit you before.

Also, what bike is this that you are buying? Generally speaking, bikes are matched with crank arms depending on frame size as well as bottom bracket height. A size 56mm road bike generally has 172.5mm crank arms. The same size mountain bike may have 175mm crank arms while the same size touring bike may have 170mm crank arms.

Be careful you don't increase your crank arm length that it will compromise ground clearance.
 

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You can, yes. But you really don't need to if the 172.5 fit you before.

Also, what bike is this that you are buying? Generally speaking, bikes are matched with crank arms depending on frame size as well as bottom bracket height. A size 56mm road bike generally has 172.5mm crank arms. The same size mountain bike may have 175mm crank arms while the same size touring bike may have 170mm crank arms.

Be careful you don't increase your crank arm length that it will compromise ground clearance.
Not sure what your points are here. The OP simply asked how to compensate seat height due to a new bike that comes with 170mm cranks. Presumably he is talking about seat height from the BB axle, and so BikeLayne's answer was correct.

Crank arm length that comes on a given bike is a mixture of fashion, frame size, and the cost structure the manufacturer sees. Somehow the whole world of performance bikes did just fine with 170 mm cranks until MTBs came out and stocked 175 mm as standard. This "trickled backwards" into road bikes, along with riders like Indurain riding 175s in time trails.

The reality is that crank length is 1) personal preference and 2) not much of an influence on performance. That is what the research shows and it matches my personal experience. I rode 170 mm for 20+ years, then 175 for 17 seasons, and am now back on 170s. They all work fine. It's a topic people get worked up about for no reason.
 

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I searched the forum and some people feel the difference, others dont. I made a 30 km test ride with some climbing and didn't feel anything. The only difference was - perhaps - that it was easier to maintain higher pedaling frequencies (placebo effect?).
Sounds like you already have your answer. Just go by feel. By the way the feeling you could keep a higher cadence might have been from the new bike being set up slightly different. I had that feeling, big time, when I moved my saddle forward just 1cm on the same bike/same cranks/same saddle height/same everything else.
 

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Not sure what your points are here. The OP simply asked how to compensate seat height due to a new bike that comes with 170mm cranks. Presumably he is talking about seat height from the BB axle, and so BikeLayne's answer was correct.
I am not disputing what BikeLayne said. Note also that BikeLayne said "if you wish".

The reality is that crank length is 1) personal preference and 2) not much of an influence on performance. That is what the research shows and it matches my personal experience. I rode 170 mm for 20+ years, then 175 for 17 seasons, and am now back on 170s. They all work fine. It's a topic people get worked up about for no reason.
So it appears you do have a preference for 170s. Is there a reason for this? My only reservation for running longer crank arms is possibly greater knee stress - with an emphasis on "possibly". There are people who swear that getting shorter crank arms made their knee pains so away.
 

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Discussion Starter · #7 ·
Sounds like you already have your answer. Just go by feel. By the way the feeling you could keep a higher cadence might have been from the new bike being set up slightly different. I had that feeling, big time, when I moved my saddle forward just 1cm on the same bike/same cranks/same saddle height/same everything else.
I just got an e-mail from the guy that fitted me on my previous bike. He looked at my data and thinks 170 mm will be fine in my situation. He suggest to raise the saddle 2.5 mm.
But there are other parameters (knee angle etc.) and since my fitting I'm obsessed by millimeters.

On a related note: one of the bikes I tried was a Specialized Roubaix SL4 (2016). It felt sluggish compared to my old Giant TCR and I don't understand why.
Is it the geometry? The other bike was a Bianchi Intenso, which is also sold as a "relaxed geometry", but didn't feel sluggish at all.
Is it the wheels? The stock wheels of the Roubaix don't have a good reputation, but neither do the wheels on my Giant.

Sometimes I wish I didn't know how difficult buying a new bike is.
 

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Are you REALLY going to raise your seat by 1/10 of an inch????
That's worth doing in my opinion, I would for sure. It's going to take a few minutes and it's free, why not if you have paid good money for a good bike fit already?
 

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All I know is I have 170 on my roadie, 175 on my MTB, and have never been able to get comfortable on the latter no matter the seat position; the former fits like a glove. 31" inseam. Replacing the 175 with 170 shortly.
 

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I just got an e-mail from the guy that fitted me on my previous bike. He looked at my data and thinks 170 mm will be fine in my situation. He suggest to raise the saddle 2.5 mm.
But there are other parameters (knee angle etc.) and since my fitting I'm obsessed by millimeters.

On a related note: one of the bikes I tried was a Specialized Roubaix SL4 (2016). It felt sluggish compared to my old Giant TCR and I don't understand why.
Is it the geometry? The other bike was a Bianchi Intenso, which is also sold as a "relaxed geometry", but didn't feel sluggish at all.
Is it the wheels? The stock wheels of the Roubaix don't have a good reputation, but neither do the wheels on my Giant.

Sometimes I wish I didn't know how difficult buying a new bike is.
What was the quality of the tires? A less supple tire will ride sluggish if compared to a supple tire. Tire quality makes a Big Time difference in ride.
 

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Discussion Starter · #14 ·
What was the quality of the tires? A less supple tire will ride sluggish if compared to a supple tire. Tire quality makes a Big Time difference in ride.
The Roubaix had Turbo Pro 25 mm (or is that 26 mm?)
The Intenso had Vittoria Zaffiro Pro Slick 25 mm.

Both 7 bar, front and rear.

The guy at the LBS that let me try the Intenso, told me I should keep in mind that new bikes in general come with mediocre tires. Specialized says that the Turbo Pro is "fast, light and strong".

Never tried the Turbo Pros before, but I know the Zaffiros and I think they are OK.
 

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So it appears you do have a preference for 170s. Is there a reason for this? My only reservation for running longer crank arms is possibly greater knee stress - with an emphasis on "possibly". There are people who swear that getting shorter crank arms made their knee pains so away.
A little easier to maintain higher cadence with shorter cranks and less likely to ding them. If I'd had a significant problem with the 175s, I wouldn't have ridden them for over 180K miles.
 

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Track bikes , for the same nominal frame size, have crank arms that are 5-10mm shorter than a road bike.
The riders have no issues going back and forth.
You might notice a 5 or 10 mm difference for the first few hundred meters of the ride, at most.
 

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Track bikes , for the same nominal frame size, have crank arms that are 5-10mm shorter than a road bike.
The riders have no issues going back and forth.
You might notice a 5 or 10 mm difference for the first few hundred meters of the ride, at most.
I heard of a fellow riding with a miss matched set of crank arms, 2.5 or 5mm, I don't remember, and he rode without realizing for a coupla few weeks before discovering.

I rode on 170's until trying 172.5's and now that's what I prefer, but even though I prefer the 172.5's I can ride my 170's and it don't make no difference.
Shorter is easier to spin, but 2.5mm don't amount to much.

But others say they feel the difference. I don't know, maybe they can, maybe they're just puffing up their chest. I think that maybe my preference might just be in my head, again, I don't know.
 
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