Another MTB --> commuter conversion. Questions

[Remko]

Re:

As said above, the frame seems a little large for a 1.70 m person.
Next to that, my experience is that it's not easy to keep a high constant pace on an mtb. If you want high pace and a comfortable riding position you could buy a racing bike, mount a stem with a lot of rise, mtb handlebars, canti brakelevers and a set of bar-ends. The large wheelset, narrow tires and frame geometry will give you instant extra mph and very easy commuting.

 

[Ugo51]

I wonder what is the inherent feature of a racing bike frame that makes pedaling at high cadence so easier than on a MTB.
Maybe the seat tube more vertical on the pedals?

What you describe is a good option. I will consider switching to a road bike. My wife has a lovely old Peugeout which I have been using the last couple of days. I love it but I also miss some of the features of the MTB.
I guess I can't have it all.
Unless I do as you suggested

 

[xerxes]

Re:

Cranks are standard 175mm.

That's long for a short arse, especially one with shorter-than-average femurs.

I have 170mm cranks on all my bikes, I did have some 175mm cranks on one bike, but it felt odd compared to the others, so I changed them. It wasn't so much the issue of where my knee was in relation to the pedal spindles, but more an odd feeling that I was having to move my legs too much.

 

[Ugo51]

haha, I guess I brought that upon myself

I'm not going to invest in a new crank though. Not on a bike that is a misfit to start with...

 

[CassidyAce]

Re:

Some rather disjointed points:

For anyone who hasn't read it, this article by Keith Bontrager about KOPS and bike fit on Sheldon's site is worth a read: https://www.sheldonbrown.com/kops.html.

As others have said, 20.5 inches is too big for 170cm. I'm taller than that and have a 15.5 inch Palisades Trail from 1996 albeit with a long seat post: very similar geometry (just a longer head tube on the two smallest frames). It's a good fit for me but there is something quite relaxed about the ride: it's difficult to pin down but it's a less racy more chilled out ride than some. I quite like it myself.

If you want high pace and a comfortable riding position you could buy a racing bike, mount a stem with a lot of rise, mtb handlebars, canti brakelevers and a set of bar-ends. The large wheelset, narrow tires and frame geometry will give you instant extra mph and very easy commuting.

That pretty much sums up the 1996 Marin Stinson but as it came off the shelf. In general, the Marin flat bar road bikes make excellent commuters: I regularly get overtaken on the way to work by a guy on a more recent one - wears shorts even in the dead of winter as well! Mad man.

 

[hamster]

I am 170cm tall and have a 19" Marin Palisades (their sizing, c-c) as a tourer with drop bars. It's fine and i have run it for 25 years like that; but I am relatively long in the leg and run it with 170mm cranks.

175s are a definite no-no. Track riders all ride 165s to allow higher cadences. On my singlespeed (which I ride most of the time) I find that the ability to pedal and spin smoothly is much better with 170s than 175 cranks.

 

[Ugo51]

Re:

Update: I was talking rubbish.

This morning I measured again the frame because yes, it does feel a bit too big for me, but not enormous.
In fact it measures 19.5' (crank to top tube) or 20' measured crank to the seatpost bracket. So it is not as big as I claimed at the beginning on this topic.
Sorry.

I will also measure the cranks because if I confused numbers once, I might have done it twice...


Anyway this morning I went back to my Marin after I've used for two days my wife's road bike. I can keep a maximum cadence of 95rpm, but I generally stay lower than that.

Also, KOPS might not be connected with the actual biomechanics of the knee, but my knees are definitely happier staying more on top of the spindle rather than backwards.

Tonight I will try to measure how much I would need to move the seat forward in order to get a better positioning.

Surfing on eBay/Gumtree I see that it would cost much more than the £50 I paid for my Marin to replace it with a similar MTB, so adapting it might be the most cost/effective choice.

 

[RadNomad]

Re:

Hmm.. ok but..
Track riders are not climbing gradients and their disciplines require very high cadence.
For your average Joe out on the road gradients matter and cadences are much lower on ave than a track sprinter. Longer cranks provide more mechanical advantage for climbing.
I’m 181cm tall and use 175cm cranks on all bikes, MTB (19” frames) and road (56cm frames). I have used 170cm and 172.5cm but find 175cm preferable. I do race in veteran catagories and don’t have problems achieving high cadences (not that i’m spinning out a 52/11 on the flat chasing Cavendish or anything..). So to say “175 are a no no” seems rather a rigid statement. No no for you maybe.

 

[Ugo51]

Re:

I'm ignorant when it comes to bikes, but I like the size of my crank because it allows me to use the full extension of the leg. In top position the femur is almost parallel to the ground, so it mimics the movement of the squat (intended as weightlifting exercise).
High cadence is not my top priority, I like that on the road bike I can spin a bit faster because it allows me to save on gear shifts, but that's all

 

[hamster]

Re: Re:

Longer cranks provide more mechanical advantage for climbing.

In exactly the same way that gear selection changes mechanical advantage!

Cranks need to be fitted to leg length, then gears do the rest. Watching cheap kids bikes with adult cranks gives an extreme example - you see kids whose knees are lifted above their hips, unsurprisingly they have no power until way down the pedal stroke.

The only point I was trying to make about short cranks on track bikes was that if you want extreme cadences then these are only achievable with short cranks. However this trade-off is I suspect at the expense of optimal efficiency, which is why longer cranks are better generally when match to leg length is important.