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LBS said this was a bad idea - is it?(20 posts)

LBS said this was a bad idea - is it?UncleMoe
Aug 20, 2003 7:48 AM
I'm 5'8", ride a 52 cm road bike that has 170 mm crank arms. I have two MTB's, both with 175 mm cranks. ON flats and on hills, I often feel like I am spinning out on the road bike. The pedalling circle simply feels too small.

I asked my LBS about putting 175mm cranks on the road bike and they said for my size and the bike size, 170 is best, and that it is normal to have larger cranks on the MTB's for better leverage.

I know it is mainly preference, but I'm looking for others opinions.

Also, this discussion came about because my middle chain ring is slightly bent. The bike is entry level, the cranks are stock "bianchi" cranks, rest of drive train is Sora (don't laugh I only communte on it). If I keep it as is and simply replace the middle chain ring, should I have any problems using a Shimano Ultegra middle chain ring?
re: LBS said this was a bad idea - is it?lemmy999
Aug 20, 2003 8:00 AM
I am 5' 11" and ride a 56cm Klein and it has 172.5mm crank arms and they feel fine for me. I am a newbie to road biking but have been mtn biking for nearly 10 years on the 175mm crank arms. Maybe you should comprimise with 172.5mm or get the bike shop to put on the 175 and ride it to see if you like it, surely they wouldn't mind your trying 2-3 different sizes.
re: LBS said this was a bad idea - is it?biknben
Aug 20, 2003 8:08 AM
The LBS gave you solid advice. There are of course exceptions to the standards but at your height and frame size I'd say the LBS was right on.

It is also common for MTB cranks to be slightly longer.

Does shifting to a higher gear relieve the "spinning out" problem? Personally. I've never noticed a difference with my cranks. I used to run 175 on both types of bikes. Now I run 172.5 on the road and noticed no change.

The Ultegra chainring will work fine.
re: LBS said this was a bad idea - is it?Mike Prince
Aug 20, 2003 8:10 AM
They probably are saying this because of the 'formula' that manufacturers use. No problem changing things to suit your body proportions IMO. The only fear would be if the smaller bike has a lower BB, you could scrape a pedal while cornering.

Personally, I wouldn't worry about it. My experience is that the advice at a bike shop is free for a reason. Hey wait a minute my advice is free too! Hmm....
re: LBS said this was a bad idea - is it?Spoke Wrench
Aug 20, 2003 8:11 AM
The only mechanical issue that I can see is that the longer cranks are going to be 1/4" closer to the street so you'll touch a pedal that much more easily in the corners.
I think that's good advice.Zonic Man
Aug 20, 2003 8:23 AM
I'm about the same height, and it was recommended by 3 custom fitters that I stick with the same crank length (170).

Note this improves SPIN and CLEARANCE.

Form and function.
give it time...spluti
Aug 20, 2003 8:54 AM
the "spinning" will probably make you a better all around rider. Need a reference? Give it 1500 miles. More free advice.
re: LBS said this was a bad idea - is it?maurizio
Aug 20, 2003 8:54 AM
For a 52cm bike, you're probably safe going to a 172.5. For that 'small' frame, 175 may be a bit extreme.
I don't think your body will suffer harm as much as the risk of scraping a pedal.
I've got a buddy that rides a 54 and he uses a 175 crank with no problems, but he's not doing any severe pedaling through corners either.
If you like to sit and power through hills, a longer crank will probably help you.
Having the same length as your MTN bike is not such a bad idea if your positioning is exactly the same as your road bike either.
If you have the extra hundred bucks, give it a shot. If it doesn't work out, you can always sell them here on the boards.
Clearances?Alexx
Aug 20, 2003 9:56 AM
Two things may be a factor here: road clearance and toe overlap.
You guys are stoned53T
Aug 20, 2003 5:19 PM
5 mm! Do you really think 5mm of extra toe overlap is going to be noticable? Do you really think 5mm of ground clearance is going to be noticable?

Even if it made a little difference, it's not worth the money.
re: LBS said this was a bad idea - is it?mapei boy
Aug 20, 2003 10:04 AM
I'm about your height and I wear the same size bike. With my road bike, I rode 170 cranks for years, but when I got a new bike in 2000 I went with 172.5. I greatly prefer the 172.5. That 1/4 inch does indeed make a leverage difference. I still use that older bike nearly everyday, too, so I'm constantly comparing the two crank lengths. My mountain bike, by the way, is a 175...which is wonderful for ratcheting yourself up a hill, but miserable for spinning.
Got to learn to spinLC
Aug 20, 2003 10:38 AM
I use a 175 on the MTB and 170 on the road too. They are different beasts so you can't really compare them. After some more miles on the 170's you will learn to adjust to it. You actually produce more power with a faster spin, so you should learn to do it if you want to improve your road skills.

Another issue is the toe overlap on the front wheel for a 52 frame. Longer cranks is going to make this even worse.
Got to learn to spinbrian n
Aug 20, 2003 10:55 AM
here's an extreme situation: i'm 6'4" and run 165mm cranks on my track bike (the standard size to get clearance on the banks). I can spin these up to about 140 rpm (about 38 mph with a 90 inch gear) for a lap or two, with short bursts a little higher. Another benefit of the smaller crank length is that at high rpms you will pedal much smoother. Mtn cranks tend to be longer because you don't really have extended periods of high rpm pedaling, and the extra leverage is sometimes all that gets you up some seriously steep and rocky hills.

I would keep the same crank size, and concentrate on learning to spin the cranks. Most mountain bikers don't have good pedaling form, simply because you don't really need it for most mtn bike riding. I found that once i picked up my road bike and concentrated on spinning a high cadence smoothly both my road and mtn pedaling improved drastically.

what speeds do you top out at? What gear are you using?
Got to learn to spin/mtbignazjr
Aug 20, 2003 11:19 AM
I think you should rephrase this to, "most cyclists don't have good pedaling form." From what I've seen on the front range of CO, as many roadies as mtbers are guilty of this.

I don't know specifically what kind of mountain biking you're referring to, but of all of the kinds I've seen, good pedaling form is always a major help. For instance, on a long, smooth climb, pedaling form is way more important on a FS mountain bike than it is on a road bike...
Got to learn to spin/mtbMShaw
Aug 20, 2003 12:09 PM
A smooth spin does wonders mtn biking too. The looser, more technical, the smoother your power delivery needs to be to maintain traction.

Mike
i'm of the schoolSteve_0
Aug 20, 2003 11:31 AM
in which 3/16th inch aint gonna make no nevermind to us everyday folk.

why would someone laugh for having practical equipment?
180 MTB & 175 RoadJFR
Aug 20, 2003 2:42 PM
I have a 36 inseem.
Been on 180s on my MTB for almost five years.
Bought my first roadie last May.
Had big dilema over crank length, was sure I needed 180s.
Test road 175s and they felt a bit short.
ButI listened to trusted experts and got 175s.
Now I LOVE EM!
My road spin has worked it's way up from 85-90 to 95-100+.
However, the 180s on my MTB now and then feel a wee bit long... not TOO long mind you, just a bit longer than whay I've become used to.

I'm riding my roadie 4 days a week (commuting) and my MTB just 1 (Sunday). I wonder if when we switch back and forth, the crank length we ride the most is what will feel "right". Do you ride your MTBs more than your roadie? If so, that might be why the 170s feel a bit small.

Maybe you should get 172.5?

Good luck.
Thanks everyone...UncleMoe
Aug 20, 2003 6:23 PM
Good comments all around. I'll stick with the 170's for now. I have been riding them for 2.5 years and since I needed to replace the middle chainring, I thought it might be something to consider, but you all made good points. I need to remember I'm only commuting 80% of the time I ride and it really won't make a noticable difference.
Thanks everyone...Roadi
Aug 21, 2003 8:46 AM
I ride a 54cm and am 66" with a 33" inseam. My current crank length on my geared road bike is 172.5. I tried 170 and 175 too. I liked the 175, but found I couldn't keep a good spin...and with the 170, I didn't have the torque I was used to from the mountain bike's 175 cranks. So, 172.5 was a compromise. Of course, this all depends on your riding style. I came from a singlespeed mountain bike, so I like to mash. However, on the road, I have found spinning to be a much better way to get lasting power. With the 172.5's, I can mash when I want to or spin when I want to.
re: LBS said this was a bad idea - is it?powergyoza
Aug 20, 2003 7:57 PM
Your height does not matter as much as the length of your legs. I haven't tried it myself yet, but component & frame builder Graeme Murray has a table of appropriate crank lenghts for a range of leg lengths:

http://www.murraytourdeforce.co.za/Cranks/pagina1cranks.html