m_michael50

Dear All,

First I would like to thank you all for the valuable info you post in this forum.

My name is Michael, I am from Cyprus, close to 50s, around 85Kg and 184cm height. I have a Ghost 56 carbon road bike with Shimano 105 10sp groupset. My bike came with compact crankset (50-34) with 175mm arm length. Ever since I bought it 5 years ago I performed my own bike fitting based on what I read on the net and I have always enjoyed every ride without feeling any pains on my knees or back and although getting older year by year, my performance was getting better.

However, recently some people I go cycling with some times (and consider them to be "expert") convinced me to change my crankset to semi-compact (52-36) and what is more, they advised me to switch to 172.5mm arm length to increase my casence. After checking about the arm length I repaced my 105 5700 crankset with R7000 semi 172.5mm.

BTW, My cassette is 12-30 and I mostly do Hills with average gradient 7% with slow cadence of around 60-70 as physically can't keep on higher cadence (perhaps because I was a smoker in the past).

Ever since i changed the crankset and although I replaced my cheap shimano pedals with R8000 pedals, no matter how much effort I put on my rides, I am always slower and on top of that I started getting low back pains.
I don't believe it is the switch from compact to semi as I always have a spare cog on the cassette. My suspicion is due to the crank arm length change.
I was so desparate that I begun doing panic changes like raising the seat and as a result I got a back knee pain from my first ride.
I am at total loss here. I am thinking of buying another crankset with 175mm and if this solves my problem sell the other one.

What do you think all?

Thanks in advance,
Michael.

RShantz

When you put on shorter crank arms, you increased the distance from pedal to saddle. So to get back to the original set up you'd need to lower the saddle height. Don't know if this is your issue or not, but wanted to point this out.

m_michael50

You are wright. However did not think 2.5mm would make such difference. Will try it though.
Thanks.

WhyFi

First things first, take a breath and slow down - throwing things against the wall to see what sticks isn't a very good strategy.

I'd look elsewhere before heaping the blame solely on a crank arm length change that's about the thickness of a coin.

You've got new pedals (were the old pedals SPD-SL as well?) and slightly shorter crankarms - individually, both of those things can affect fit a teeny bit. Together, it has the potential to be significant. Adjust saddle height and fore/aft based on the new set-up.

While you're at it, make sure that everything is hunky dory with the install of the new equipment. Is everything spinning freely?

Also, not that I'd blame any decreased performance on it, but is the front derailleur shifting well? The spacing between the large and small chainrings changed between 5700 and 7000, and the FD might need to be swapped out for best shifting performance, too.

GailT

A shorter crank arm results in a shorter distance to the pedal when the leg is fully extended, so you need to raise the seat.

100bikes

This is a case where the true body dimensions make a difference.

Can't speak to your overall bicycle frame(dimensions).
Nor, can I speak to the dimensions of your body.

That presents the challenge.

Not every bicycle(frame) is suitable for every rider of equivalent height.

Geometry, especially top tube length and head seat angles will affect fit, and
you cannot affect those.

Short torso, long leg, long torso short leg or equivalent body parts - which are you?
Did it match you, basically when you bought the bicycle- that is did you consider frame
dimension when the bicycle was originally purchased?

Changing crank length, especially after mentioning issues may be the root.
All "compact drive" means, from a bicycles standpoint, is smaller gears/ tighter ranges.

Because all modern production bicycles use basically the same components,
all that differentiates a bicycle from one company is frame geometry/philosophy.

No magic to it. A bicycle that fits is the result of mating the frame first and
fine tuning the components dimensions.

Also, if you have been spinning a 175 mm crank at about 60 rpm for a long time,
changing to a shorter crank without a complete revision of your fit has potential
to results as you are finding. Not only muscles, but ligaments and joints will have
developed a memory, and now you are asking them to change.


On to one other point:
I would suggest pushing a gear at 60 because you feel that you cannot develop a
faster cadence you explore this idea.

An increased(ie faster) cadence is much more efficient use of your bodies resources.
-Less strain on the muscles with each rotation.
-Slower lactic acid build up from not tearing at the muscles or consistently drawing upon the reserves.
-Better use of all the gears the bicycle has to offer -generally pushes rider to manage the middle
of range rather than living at the upper end.
-Higher cadence - more aerobic action- with lower stress per rotation.
Pumps more blood and oxygen thru the body.

Find a stretch of flat road and trail that you can concentrate on riding in a single
gear for a while. Put the gear in the "normal " for you on that surface.

Gear up - that is make it easier to pedal a bit a time, perhaps one cog or two, and ride
in that gear for awhile. I find the brain will automatically compensate for what is "sense"
should be your riding speed in that situation by allowing for an increased cadence.

I encourage riders, especially newbies to ride in the easiest gear possible and
develop the smoothest spin they can.
This process continues almost to the point of seeming to be to easy to pedal/spin smoothly.

Your body will settle into a cadence which (i believe) will be higher than it is
now for the equivalent gear combination, and perhaps beyond what you even
thought possible.

One simple test is to maintain a conversation with someone after a couple miles
of spinning at this rate.

(Professionals can hold conversation while spinning over 90 rpm, in gear inches you and I
can only dream of pushing -through the mountains!)

As the strength increases, the natural move to the larger gear will happen,
and so on. All the while, settling into a higher, more efficient cadence.

A smooth cadence will encourage ride comfort and develop the physical benefits available
far more than strength developed from pushing to big a gear.

Hope this helps.

popeye

As for the performance difference, power = cadence x torque. So to take advantage of the crank change you would need to up the cadence. This may or may not be possible with or without training due to your limitations. Torque stresses muscles, cadence stresses cardio so go with your best abilities.

RedBullFiXX

Remember, when you adjust seat height, you are also changing for/aft position at the same time, due to seat tube angle

Maelochs

Your cadence is not dependent on crank length. I have several bikes, some with 170s, some with 175s, one with 165s .... but I am always the same rider and I always pedal about the same cadence. I set my cadence based on my weak lungs ( I feel you there) and bad heart, and then on my weak legs. I find that even with heart and lung issues I still do better spinning faster. I used to be a big-thigh masher, but over time I began to change (magazine articles ... not sure if you are old enough to remember magazines ) and I found that as I aged, I got more out of a higher cadence.

I find my cadence is almost always the same regardless of what bike I am riding---it changes based on road conditions and weather (headwinds, tailwinds, hills, and such) but it isn't a lot faster on the short-crank bike .... because I shift to get into that sweet spot for my lungs, heart, and legs.

Also ... 2.5 mm seems like almost nothing, but if your bike was set up really well with one set of cranks, it is wrong with another .... and anyone who cannot notice 2.5 mm of change in saddle height ... well, I can. Whatever. Also, with new pedals you get new pedal stack height (the distance from the center of the spindle to the top of the platform) which is also going to change things.

I do not want to be too critical ... but I think those "experts" read to many articles online---and not the right ones. Cadence is Not a function of crank length---it is your personal cycling habit. Changing how you ride .... not what you ride, makes that particular difference.

I used to ride with a body-builder---ridiculously huge thighs. he noticed I was spinning really fast, and remarked that he was training by riding to a cadence---80 rpm---all the time. I was spinning over 90. Thing is, he could turn 80 rpm up an 18% hill---he had the legs for it. I need to hit 90-100 rpm to make it up a 7 % hill .... I don't have the power.

He didn't change his cranks---he just lowered his cadence to make better use of his body (big muscles spinning more slowly with not much load = not much blood flow compared to trying to spin those huge muscles really fast at almost no power.) But he didn't change his cranks, his pedals, his seat .... All he changed was his cadence. He did what worked for him.

If you have thighs as big around as most men's waists, low cadence might be most efficient. If you have normal legs, your heart and lungs can carry you farther, probably---as @100bikes explains.

Either adjust your seat to the new cranks and pedals, or go back to your old cranks .... and adjust the seat for the new pedals. get your fit dialed back in. Then decide what you want to do about your pedaling style.

RShantz

I stand corrected. Should have thought first!

Racing Dan

I think you should change down a gear or two and try again.

wphamilton

I'll also point out that changing the crank length has no impact on cadence other than psychologically. That's not to mean just you; the tendency for most people is to increase their cadence at first when they shorten the crank. Because the foot speed is less for a given rpm. So the "cadence fixing" guys didn't really help.

m_michael50

Yes my previous pedals were SPD-SL cheap ones. Perhaps there is also an axle difference between the two because with the ultegra ones I find my feet rubbing the arms from time to time where as this did not happen before.
Regarding your question about the derailleur, yes I too read that there could be shifting problems because of the .4mm difference but it seems ok. However I do keep in mind getting a new FD.
I did lowered my seat a bit and pushed it slightly at the back. I will check it out and adjust accordingly.

m_michael50

Thank you all for your valuable contributions. I have already adjusted my seat as per your recommendations and will see how it goes and adjust accordingly. In the meanwhile I will try to exercise in getting my revs up and hopefully I will manage to do so, otherwise may well consider switch back to my old crankset and get a comparison with the new pedals. I know I made too many changes at one before getting the necessary feedback.

Michael.

Sy Reene

<> about 2.5mm.. Hell, chamois pad thicknesses vary by a lot more than this in my bib selection

Dean V

True.
Also it isn't practical to keep changing saddle height to match different shorts.
So you get another bike set up at the different height.

Litespud

I think you’ve got it backwards - shortening the crank length will bring the pedal (at the 6 o’clock position) closer to the saddle, so you would have to raise the saddle to maintain the same max leg extension. This would also mean less flex at the knee and hip when the pedal is at 12 o’clock

m_michael50

This is exactly what i thought which is based on pure Logic. Also regarding crank arm length, I know from physics that a longer shaft needs less power to produce the same torque. So in a effect, for climbing a longer crank arm requires less power to spin.

Seattle Forrest

Wait, you're saying I need a bike for every pair of shorts I own? 🙂

Branko D

Don't bloody change crank lengths if they fit you. Saddle height needs to go up with a shorter crank and the saddle also needs to be now placed slightly forward (since moving the saddle up moves it also back, too), and then your reach and saddle to bar drop are also affected. Essentially you change the entire coordinates on a bike. Don't listen to "experts", you don't get more power by changing crank length, and suggesting a change to 52/36 to deliver more power is beyond daft.

My road bike came specced with 52/36 as stock, and whenever the gradient gets up to 10% I curse the idiots who spec bikes for normal riders with anything bigger than a compact and with short cage derailleurs, because I really don't fancy buying yet another set of high quality chainrings. Will do it anyway before the next uphill race, but that's some months away. They use compacts with 11-32 cassettes in the Vuelta and the Giro on mountain stages, but self proclaimed experts don't need no granny gears, go figure.

Dean V

Absolutely.

WhyFi

This difference is on the order of what, ~1.4%? But it's still no free lunch as you have further to push on each revolution. In terms of performance, no one's noticing a difference of 2.5mm in crank arm length (assuming both arms fit within the usable range of the rider).

Maelochs

Also, you will need a new bike as your shorts or bibs age and the padding compresses. That is why so many of us have multiple bikes in our sigs---trying to keep up with aging chamois pads.

SactoDoug

IMO, if you want to increase your cadence, the best thing you can do is get a bicycle computer with a cadence meter. Before you change out other components such as gears and crank arms, you should know what your current cadence is, and where you want your cadence to be. I did so and found that I started out around 80 rpm cadence. I then would adjust my gearing until my cadence was just a little uncomfortable but sustainable. I also would throw in a few high rpm sprints during my commutes. I am now comfortable with a 100-110 rpm cadence. 90 rpms now feels like I am mashing my pedals. I am very happy with the higher cadence since it eliminated my knee pain.

In short, without a cadence meter, you don't have a good idea where you are or when you have gotten to your goal of a higher cadence.

Iride01 

I confused by your description of your gears front and back. Did you change the size of your chainwheels and cassette? If so, what were they before and what are they now?