Bigger front ring - Bosch Active Cruise 2014

[xena]

Hi all. I'm really not techie (but I can work my way through stuff slowly and dilligently) so would appreciate a little guidance.

I'm planning on changing the front chain ring on my 2014 Cube Reaction Hybrid Pro (specs) from 15t to 18t to increase my speed on the flat as I have a job keeping up with OH on his racer (cadence too high). I think 20t would be too high as my climbing speed will be too fast and he will get lonely?!

I have been reading this thread but I wanted to check if the info still applied to my motor. My rear cassette is 11-34 I think. Will I need a new longer chain? I already have a crank puller, and I'm not sure if I need a spider tool but I'm thinking to buy one anyway as I'll probably need it at some point for the other bikes.

Does my shopping list look ok and do I need to consider anything else?
18t connex chainring
Spider tool? (says puller?)
Silicon grease

Is the fitting procedure the same as for the CX so I can refer to the video in the linked thread?

Thank you in advance!

 

[Woosh]

I'm planning on changing the front chain ring on my 2014 Cube Reaction Hybrid Pro (specs) from 15t to 18t to increase my speed on the flat as I have a job keeping up with OH on his racer.

you need a new bike with a stronger motor.
Changing the front chain ring to a bigger one will simply reduce the torque you have at the moment, not increase it.

 

[Artstu]

you need a new bike with a stronger motor.
Changing the front chain ring to a bigger one will simply reduce the torque you have at the moment, not increase it.

Sorry to say you've completely missed the point of xena's post.

The issue is the 37.5 x 11 gearing which is too low.

 

[xena]

you need a new bike with a stronger motor.
Changing the front chain ring to a bigger one will simply reduce the torque you have at the moment, not increase it.

It feels to me that it is the cadence that is the limiting factor rather than the power (OP edited to clarify). There is apparently 48Nm of torque available to me, so I think that should be enough.

 

[Artstu]

Don't worry about your climbing speed being too high because it won't make you climb any faster. The 20T front will give you in effect a 50T front chainring, so think about any really steep hills you need to tackle on your lowest gear of 50x34 which even with the motor might be a tad high for very steep inclines.

 

[Woosh]

It feels to me that it is the cadence that is the limiting factor rather than the power (OP edited to clarify). There is apparently 48Nm of torque available to me, so I think that should be enough.

that 48NM figure is the normalised figure for comparison between crank drive motor, if you were to have a 1:1 front to rear, that is when your cadence RPM is the same as the the wheels' RPM. If your cadence is 100 RPM for example, you have 48NM if the wheels also move at 100RPM, which is about 8.3mph for a 700C wheel.
If your OH can hold on to 25mph for example, you need about 40NM at the rear wheel, assuming you run on turbo, the motor will need to be rated about 80NM derestricted.

 

[xena]

Don't worry about your climbing speed being too high because it won't make you climb any faster. The 20T front will give you in effect a 50T front chainring, so think about any really steep hills you need to tackle on your lowest gear of 50x34 which even with the motor might be a tad high for very steep inclines.

I have a proper mtb for off road, so this bike is for road riding. What I'm finding at the moment is that on the steep bits, my OH goes slow (obviously), and if I stay with him, I feel that I am turning the cranks too slow (even in bottom gear), and I worry that I will overheat the motor as they don't like working under low cadences. Am I right in thinking that with an 18t front chainring the cranks will be spinning even slower so worse for the motor?

 

[Artstu]

I have a proper mtb for off road, so this bike is for road riding. What I'm finding at the moment is that on the steep bits, my OH goes slow (obviously), and if I stay with him, I feel that I am turning the cranks too slow (even in bottom gear), and I worry that I will overheat the motor as they don't like working under low cadences. Am I right in thinking that with an 18t front chainring the cranks will be spinning even slower so worse for the motor?

Ah yes you will indeed be spinning the cranks slower with the higher gearing, I've run some really tall gearing on my Bosch Classic at times and I've had to tackle some really steep climbs (circa 30% at times) with a low cadence, not once in 11,000 miles have I ever had any overheating issues. Go for the 18T and see how it goes I'd say.

 

[xena]

that 48NM figure is the normalised figure for comparison between crank drive motor, if you were to have a 1:1 front to rear, that is when your cadence RPM is the same as the the wheels' RPM. If your cadence is 100 RPM for example, you have 48NM if the wheels also move at 100RPM, which is about 8.3mph for a 700C wheel.
If your OH can hold on to 25mph for example, you need about 40NM at the rear wheel, assuming you run on turbo, the motor will need to be rated about 80NM derestricted.

I'm afraid that's gone woosh over my head. Does it also account for the grunt that I'm adding? Doesn't the motor just need to pick up the difference between mine and my OH's power output plus a bit extra for my bike being less road biased? Everyone else seems to get by with just changing the front chainring? Sorry if I'm being blonde!

 

[soundwave]

18t will give you a higher top speed i have 2 on mine 18t and 20t

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[Woosh]

I'm afraid that's gone woosh over my head. Does it also account for the grunt that I'm adding? Doesn't the motor just need to pick up the difference between mine and my OH's power output plus a bit extra for my bike being less road biased? Everyone else seems to get by with just changing the front chainring? Sorry if I'm being blonde!

assuming that your bike is already derestricted, give me your maximum speed on flat roads at present and your target speed, I will detail the calculations for you.

 

[xena]

assuming that your bike is already derestricted, give me your maximum speed on flat roads at present and your target speed, I will detail the calculations for you.

I can't remember my normal max speed, but say for instance it's 20mph and I'd like to be able to maintain 25mph for a mile or two to get a section of fast road out of the way, how would that work out?

When I next get a chance to go out, I will see what my actual top speed is, and also how much effort the motor says it is putting in at that speed (intuvia display so has bars to show).

Cheers

 

[soundwave]

with a dongle and bigger front sprocket you will use more of the cassette gears but how fast a avg top speed you will get depends on how fat you are and how fast you can spin the cranks.

the faster you go the less assist you will get but still be pulling power from the motor and after 28 mph there is not much assist at all and then there is the wind also slowing you down that just gets worse the faster you go.

 

[Woosh]

say for instance it's 20mph

Bike: 29" wheel, assist ratio: 250% (max), front cog: 15T * 2.5 (internal multiplier), rear cog: 11T
cadence to wheel RPM ratio: 11T/37.5T = 29.3%
maximum wheel torque at 29.3% front to rear ratio with 40NM motor: 11.7NM
maximum wheel torque at 29.3% front to rear ratio with 50NM motor: 13.7NM
(This means that if you need more than 11.7NM from a 40NM motor, you will have to pedal for the difference yourself).

at 20mph:
wheel RPM: 233
Cadence: 68.5RPM
Angular speed: 24.5 radians/s
Load: 325W
Required wheel torque to achieve desired speed: 13.3NM
from motor at maximum assist: 9.5NM
from yourself: 3.8NM

I'd like to be able to maintain 25mph

at 25mph:
wheel RPM: 292
Cadence: 85.6RPM
Angular speed: 30.6 radians/s
Load: 610W
Required wheel torque to achieve desired speed: 19.9NM
from motor at maximum assist: 11.7NM
from yourself: 8.2NM

You would have to pedal more than twice as hard with your current motor to maintain 25mph.

If you have an 80NM motor:
maximum wheel torque at 29.3% front to rear ratio with =80NM motor: 23.6NM
at 25mph:
wheel RPM: 292
Cadence: 85.6RPM
Angular speed: 30.6 radians/s
Load: 610W
Required wheel torque to achieve desired speed: 19.9NM
from motor at maximum assist: 14.3NM
from yourself: 5.6NM

5.6NM is much more manageable compared to 8.2NM.

 

[xena]

Those are really interesting figures, thanks for putting them up. I do have a CX powered bike as well but that has knobblies on. I hadn't appreciated how much the active cruise assist would taper off, or the extra power required to achieve a small increase in top speed, so we'll have to see how it goes, and if I'm struggling, may have to repurpose the CX!

 

[Woosh]

Those are really interesting figures, thanks for putting them up. I do have a CX powered bike as well but that has knobblies on. I hadn't appreciated how much the active cruise assist would taper off, or the extra power required to achieve a small increase in top speed, so we'll have to see how it goes, and if I'm struggling, may have to repurpose the CX!

change the tyres and wear a bodysuit.

 

[Nealh]

change the tyres and wear a bodysuit.

Xena could scare every one and ride in the birth bodysuit, oh natural .

 

[xena]

Xena could scare every one and ride in the birth bodysuit, oh natural .

Sounds chilly, I think I'd be better in my motorbike leathers TBH!

 

[Phil Dryden]

Hi Xena,
I have a Cube bike with the CX motor and standard 15 tooth chainring, and like yourself find the same 2 minor irritations. Number 1 - I spend 95% of my time on the 3 smallest sprockets, which will wear them out while the other 7 are unused, and the chain runs well out of line for most of the time, increasing chain wear. Number 2 - I twirl my legs off going downhill or on the flat with half a breeze behind me.

I too intend to replace my chainring with an 18 tooth one, but I'll wait till I need to replace the chain and cassette as well for maximum benefit and economy. Better to replace all 3 together. I reckon this will ease things considerably and I should spend more of my time in the middle sprockets which will keep the chain happy. Whilst I will perhaps need to use the larger sprockets for steeper hills, I'll be able to go a bit faster on the flat, albeit above 15.5mph it will all be my own effort.

My only uncertainty is knowing before I start the work what length chain I will need. Reckon it will be maybe a couple of months before I do it. If you change yours before then, I'd be interested to know how you get on.

 

[Deno]

I have an 18T front cog on a CX motor with an 11-36 cassette on a 29er wheel. That will spin comfortably to 50kph with reasonable cadence. I also climb a 10% hill without needing the lowest 2 gears. The bike is just about usable with no power on the flat or on a gentle incline, not up steep hills but the CX is pants with no assist anyway.

The CX has a 1 to 2.5 gearing ratio (I think) so going from 15 to 18T is an actual equivalent 7.5 tooth difference.