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FantasticMrFo...
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PostPosted: 19:00 - 10 Jun 2012    Post subject: Hello! How better to introduce myself than with a question? Reply with quote

Hello All!

I've just registered on this forum and have to say it's really well laid out and run Smile

I thought I'd start my introduction with a question, It's one you may have been asked many times before so please be a little patient with me.

I own a Honda CBF 125, which I got from brand new. Around London it's the business, but since I moved out of the city into a more rural setting where the roads are faster, it's clear that it needs a bit more top end.

I commute to London quite frequently, and have to go through about 40 miles of 70mph zone, which my bike can manage at a push but it really doesn't like it, if I drive that stretch at a speed my bike feels comfortable with, its usually about 60mph.

So I've been thinking of changing the final drive ratio, to take a bit of the acceleration from low speeds (Which will make 1st and 2nd a bit nicer to use, I have to change into 3rd rather fast) and stick a little on the top end, hopefully making it so the comfortable speed for my bike to take the 70mph limit section is 70mph.

My question is this, Is it going to be possible to achieve that from changing the cog's? If so which way do I need to go about it, smaller front and larger rear or the other way around?

Could anybody talk me through the maths involved in working out which tooth combination would be best (on paper at least).

Being a mechanic, I understand a little about gear ratio's, but not enough to be able to work out the best gearing combination for getting 70mph in 5th gear at low rev's (The CBF doesn't really rev highly in 5th gear, which is whats making me thing that it has the torque to be able to pull me along at 70 fairly easily if given the right gear combination.)

Sorry for the first long thread, and for asking such a tedious question.

Thank you for having the patience to read this without falling asleep lol!
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.....
Quote Me Happy



Joined: 15 Jan 2005
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PostPosted: 19:04 - 10 Jun 2012    Post subject: Reply with quote

You can look on www.gearingcommander.com for an idea.

Bear in mind though, gearing for a higher top speed can make the bike slower as it might not have the power to rev out at the different ratio.
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neil.
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PostPosted: 19:58 - 10 Jun 2012    Post subject: Reply with quote

On Gearing Commander, the CBF125 is listed incorrectly as the "CBF250 '09-'11" (I know because I submitted the gear ratios to them).
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FantasticMrFo...
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PostPosted: 20:26 - 10 Jun 2012    Post subject: Reply with quote

Joe wrote:
You can look on www.gearingcommander.com for an idea.

Bear in mind though, gearing for a higher top speed can make the bike slower as it might not have the power to rev out at the different ratio.


Thanks for the replies lads! I appreciate it!

What do you mean by rev out? are you talking about it hitting the limiter?

I tend to drive in the lower rev's anyway because the bike sounds really poor and rattly when the rev's get higher, and because of this I don't mind the bike being slower on the acceleration because I never accelerate as fast as I can anyway, I'm happy to sacrifice the acceleration for the top end speed.

Edit, Which would be safer to change, the front sprocket or the rear one? Thanks for your help feller's!
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Marmalade
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PostPosted: 20:55 - 10 Jun 2012    Post subject: Reply with quote

Usually changing the rear gives finer adjustment.
If you changed to 1 tooth larger on the front it would be like changing to 2 or 3 teeth smaller on the back.

What one of the posters above means by not having the power, if you dropped the rear sprocket size by a quarter you may well get 20mph out of 1st instead of 12 mpg but by the time you get to 4th or 5th it won't have enough engine power to maintain any speed meaning you'll have to constantly rag the life out of it in 3rd to go anywhere

You need to find a happy medium that keeps the bike happy in top at the best sensible speed you can get
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FantasticMrFo...
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PostPosted: 21:07 - 10 Jun 2012    Post subject: Reply with quote

Marmalade wrote:
Usually changing the rear gives finer adjustment.
If you changed to 1 tooth larger on the front it would be like changing to 2 or 3 teeth smaller on the back.

What one of the posters above means by not having the power, if you dropped the rear sprocket size by a quarter you may well get 20mph out of 1st instead of 12 mpg but by the time you get to 4th or 5th it won't have enough engine power to maintain any speed meaning you'll have to constantly rag the life out of it in 3rd to go anywhere

You need to find a happy medium that keeps the bike happy in top at the best sensible speed you can get


Thanks for that post mate Smile Clarified a lot.

I'm well due new sprockets and chain, so I'm going to order a 17 tooth and 18 tooth front (only about a fiver each) And try them both to see which gives me the better compromise.

That gear commander site is saying that I'd need a 17 tooth front and 37 tooth rear (originals are 16 and 42) to reach 75 mph. but somehow I don't think the engine would cope with that for too long, if it had the poke to get me there at all.

I'm only looking to make the commute easier until I can afford to do my full licence and get either a new hornet or a gsr750 Very Happy

Edit: I don't suppose anyone knows whether the cogs on the CBF and the CBR 125 of the same year are the same?

I'm seeing a lot of CBR parts but hardly any CBF parts, but figured they'd be very similar if not identical right?
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U_W v2.0
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PostPosted: 21:21 - 10 Jun 2012    Post subject: Reply with quote

what year is yours?

http://www.gingerparts.com/parts/chains-sprockets?cat=126

thats a really good place to order from. if anything goes wrong they'll see you right. they also have all the dimensions of the sprockets so you can simply measure your existing one (the hole in the middle) and get one with the same hole size in the teeth you need.

or drop them an email and ask which one.
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FantasticMrFo...
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PostPosted: 21:23 - 10 Jun 2012    Post subject: Reply with quote

Dilligaf_NO wrote:
what year is yours?


Its registered 2010 but think its still the 09 model mate. It definitely has the old dash with no rev counter.
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U_W v2.0
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PostPosted: 21:38 - 10 Jun 2012    Post subject: Reply with quote

well i cant find a specific sprocket for you but im sure you can go through the link i sent and you'll find the right one Smile
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FantasticMrFo...
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PostPosted: 22:09 - 10 Jun 2012    Post subject: Reply with quote

Dilligaf_NO wrote:
well i cant find a specific sprocket for you but im sure you can go through the link i sent and you'll find the right one Smile


Thanks for your help dilligaf! (Loving the name BTW lol! What's the name of the ozzie bloke who sang that?)

I'm going to get a 17 and 18 tooth front to start with, as they're only about a fiver each and see which I get on with, then once I've found a set up close to what I like I may put a 16 back on the front and put a 39 or 37 on the rear, then tweak the combination until I find the one I'm happy with.

Thanks for everything lads Very Happy
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J.M.
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PostPosted: 00:17 - 11 Jun 2012    Post subject: Reply with quote

FantasticMrFox wrote:
What do you mean by rev out? are you talking about it hitting the limiter?


The engine just doesn't have enough power.

On the CBR125 I used to have, switching from 5th to 6th gear, unless I was actually on about 10-11k RPM would actually result in the bike losing speed.
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karthead
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PostPosted: 01:14 - 11 Jun 2012    Post subject: Reply with quote

Hi there,
I know that you only want to change gear ratios, but it may do you good if you upgrade the engine (bore out, bigger carb, tuned exhaust ect if possible) to give you're engine a bit more oooooouumph Wink if you do that, combined with a small change in gear ratios (possibly 2-3 teeth smaller on front sprocket), then you will gain speed, but likely not lose any acceleration because the upgraded engine will give you more power.
I understand its a new bike, so if you dont want to risk tinkering with it thats fine, just another idea for you to consider Thumbs Up
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FantasticMrFo...
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PostPosted: 03:41 - 11 Jun 2012    Post subject: Reply with quote

andresoccer10 wrote:
Hi there,
I know that you only want to change gear ratios, but it may do you good if you upgrade the engine (bore out, bigger carb, tuned exhaust ect if possible) to give you're engine a bit more oooooouumph Wink if you do that, combined with a small change in gear ratios (possibly 2-3 teeth smaller on front sprocket), then you will gain speed, but likely not lose any acceleration because the upgraded engine will give you more power.
I understand its a new bike, so if you dont want to risk tinkering with it thats fine, just another idea for you to consider Thumbs Up


I had considered that, tinkering with the bike isn't a problem, new or not, as I'm a mechanic by trade albeit with cars, stripping an engine is no problem, the issue I'd face with that is the legalities of it all, that and cost, for the price of a bore kit etc I'd be able to afford to do my full licence.

I had been considering putting a CG250 lump in it as I've been told (all though not checked myself so could be wrong) That the engine mounts etc are the same so an engine swap wouldn't be impossible, but I intend on giving the bike to my missus when I've got a bigger one, so I don't particularly want to do too much to it.

I'm not trying to make it something that it isn't, just hoping to squeeze a little more top end out of it to make cruising at 70 easier rather than having to thrash it silly to keep it there.

Thanks though, all of those are valid points, and ones I'll be more inclined to consider when she turns round and says "how the hell did you put up with this for 2 years, its too bloody slow, make it quicker!" lol
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bodger
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PostPosted: 07:58 - 11 Jun 2012    Post subject: Reply with quote

Gearing the bike up may give you a small increase in top speed when you have favourable conditions, but you,ll find you have to keep dropping down a gear as soon as you go up a incline or hit a head wind. The bikes just dos,nt have the power.
Better stick with original ratios or get a bigger bike.
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