Altering my gearing - advice needed

danlightbulb

Hi all,

I'm just thinking about what options I have for giving myself a little easier gearing to tackle some of the steeper sections of my regular trails. I find myself grinding to an almost halt on some climbs these days (I'm getting older maybe). I'm otherwise happy with the bike so don't want to upgrade.

My bike has a 10-42T 11 speed Sram XG 1150 cassette. I believe this will be on an XD driver. The derailleur is a Sram GX 11 speed.

On the front I have a single 32T narrow-wide chainring, 4 bolt style with 94mm bcd.

The rear spacing is non-boost, i.e 142mm standard.


What is the best way to give myself a little easier gearing? I don't really use the 10 tooth cog on the back so could go to an 11-46 or 11-50 cassette? Or I could swap the front chainring to 30T?

Modern bikes seem to be coming now with 10/11-50/51 cassettes either in 11 or 12 speed.

mully79

Currently you have a ratio
32T with 10-42 = 3.2 to 0.76

32T with 10-50= 3.2 to 0 64
(For comparison)

28t with 10-42 = 2.8 to 0.67

The 2.8 ratio with the 28t is about the equivalent of using your 11t cog on current set up.
The 0.67 ratio is equivalent to what a 48T would be on your current set up.

I reckon you should buy a 28T chain ring.

steve_sordy

Mully79's calcs are correct.
But to put it another way, and staying with your current cassette:
Changing from 32t ring to a 30t ring will give enable you to climb up 6.25% steeper hills.
Changing to a 28t ring will enable you to climb up 12.5% steeper hills.

If you keep the 32t ring and change the cassette to a 11-46t, it will be equivalent to being able to climb up 9.5% steeper hills in the largest gear.
If you go for a 11-50t then it will be equivalent to being able to climb up 19% steeper hills!

When I say "steeper hills" that is the same as "same hill but easier".

An average gear change is 15% for comparison. I doubt you would notice the 6.25% change, maybe the 9.5% change. But you would notice the 12.5% and deffo the 19% change.

So on cost basis alone, I would go for the cheap change to a 28t ring (12.5%) instead of the cassette. Don't forget to shorten your chain!

danlightbulb

Thanks both. I initially had thought that going to 28 T would be too small but perhaps you are right.

I know this may be personal preference, but what about going oval? I have read that a 30 T oval would have the best of both worlds of 28 T and 32 T.

steve_sordy

I have read a lot of good things about going oval, never tried it myself, although I would have done if my chainring had worn out. It is important that you get the oval in the right part of the power stroke. ie don't just bolt on the ring in any old place.
In fact I was reading the packaging on an oval ring in the LBS today while I was waiting to be served. According to the blurb you get 9% more efficiency and go up hills more easily.
That is the sum total of my oval gear ring knowledge. Oh and that you can't fit them to an emtb.

larkim

Having read stuff like this https://www.bikefitadviser.com/blog/oval-chainrings I'd be pretty certain that the gains from oval rings wouldn't equate to the difference between 28T and 30T chain rings.

steve_sordy

A 30t oval that is shaped to give the diameter of a 28t at one point and that of a 32t at right angles to it, is still a 30t gear. So it will not, and cannot, advance the bike further forward with one pedal turn than any other 30t ring would do. An oval ring does not change your gearing overall.

What it does do is to even out the power delivery. Your pedal stroke is most powerful at the point your leading foot is about to press down and for a short while afterwards. The weakest point is when the crank arms are almost vertical. So by making the chain ring smaller where your power stroke is weaker and vice versa it compensates and levels out the force you need to apply. The exact placing of the axis of ovality is important fair best effect.

One of my fellow mechanical engineering students at Uni made oval chain rings his final year thesis. That was 1974 and I didn't even know that oval rings were a thing. In fact they may not have been because he had to make his own. He played around with the amount of ovality and also where the ovality should be placed in relation to the crank arm. He demonstrated that it was a real thing and came with real benefits.

danlightbulb

larkim said:

Having read stuff like this https://www.bikefitadviser.com/blog/oval-chainrings I'd be pretty certain that the gains from oval rings wouldn't equate to the difference between 28T and 30T chain rings.

Yeah that study highlights my main concern with trying it out - is oval more suited to riders constantly pushing their power and endurance to its limits, rather than a general rider like me who drifts around the trails for a few hours at far lower than maximum effort.

I'd like to try it but it might be a waste of £45.

Its kinda the same predicament with dropping to 28T. I mean, I can still get round the trails now, I just feel like when Im knackered I could do with an extra gear. Dropping to 28T is a bit of a cop out - it would be better to get stronger/fitter instead. But I can only devote so much time to my cycling so I'm stagnant in terms of fitness.

If I drop to 28T and get used to using my new easiest gear all the time then I'll both be slower than I am now and I might even lose fitness, compared to trying to make it round in the gear I have now.

steve_sordy

Can you find a 10-46t cassette? This will give make it almost 10% easier to get up those hills, without losing any of your top speed. I know they are made, but I don't know if they will fit on your hub. Try Sunrace if Sram don't do one.

larkim

At a constant rotational velocity of the cranks, does an oval ring cause the cassette to rotate at an uneven velocity? Just trying to think this through in my head with a hypothetically "extremely" oval chainring and can't quite satisfy myself in my head how the mechanics work.

steve_sordy

Yes is the answer, but only if the wheel was unrestrained and had no inertia (just like those maths questions you used to get at school, ie not in the real world).

For the cassette to rotate at an uneven velocity, the wheel (which is directly connected to the cassette) would have to do so as well. If any perturbations were to be transmitted I very much doubt that you would notice as the vibrations would be damped out by all the other chatter that is going on.

What will happen in the real world is that the resistance to the system changing velocity is damped out by your legs which in any case wouldn't actually transmit power at a constant rotational velocity. This happens whether the chain ring is oval or round. The resilience in the entire system, (ground, tyres, wheels, chain, suspension, you) combine to smooth out the ride.

whyamihere

larkim said:

At a constant rotational velocity of the cranks, does an oval ring cause the cassette to rotate at an uneven velocity? Just trying to think this through in my head with a hypothetically "extremely" oval chainring and can't quite satisfy myself in my head how the mechanics work.

Yes, but that's partly why oval chainrings work. If you're starting from the assumption that you have a constant rotational velocity of the cranks, you won't understand the system because that's not true. There's a good image here which shows how angular velocity typically varies around a revolution of the cranks: https://cycling.favero.com/blog/tech-tests/whats-iav-power

The oval ring effectively puts you into a harder gear when the angular velocity would normally be highest and a lower gear when it would normally be lowest. This actually serves to smooth out the power delivery and I would guess, though I haven't done the maths, would also smooth out the angular velocity to bring it closer to the average.

mully79

The oval ring changes the chains position in relation to a full suspension bikes pivot.
My brain hurts trying to work out whether this would induce more pedal bob.

steve_sordy

The whole point of oval chain rings is to even out the power output, therefore you will get less pedal bob.
If you currently suffer from pedal bob, do less out of the saddle hero-heaving and cultivate a smooth pedalling style.

danlightbulb

I finally fitted a 28T blackspire snaggletooth chainring yesterday. What finally did it for me is really running out of steam on a climb on the trail the other day, and finally succumbing that I do need a extra gear. I didn't go oval, just a plain round 28T.

https://www.chainreactioncycles.com/blackspire-snaggletooth-narrow-wide-chainring-x01/rp-prod144537

That's the one I went for. Now for anyone else fitting it, it didn't come with any bolts and the existing ones didn't fit. The product description says it needs M8x11 chainring bolts, but on querying live chat to be certain they said M8.5x12mm. And some reviews also say that size. Regardless, they said they didn't sell them.

I also had no luck in my three closest bike shops.

However as luck would have it one of the LBS said "sounds like the Shimano fitting" so I popped out to the garage and lo and behold the bolts holding the granny ring onto my old Shimano geared triple-ring MTB were the right size, almost. They are about 1mm shy of the ideal length leaving a tiny bit of non-utilised thread on the chainring, but still 90% of the thread is secure and I used some locktite so hopefully will be ok. I will hopefully find some of the correct size bolts at some point.

i do not know why these things have to be so complicated. a) why cant they put the correct fitments on the description and b) if it needs bolts why not just supply the product with the right bolts in the first place. £44 is not cheap.

Looking forward to seeing what difference it makes on the trails.