YBR125 final drive ratio curiosity

[Travis Bickle]

Dear friends

I'm new to this site, and indeed two wheels altogether. I had a midlife some time in 2017 and suddenly decided it was time to fill in all those empty boxes on the back of my driving licence. C came first, then CE, then D, and finally acquired A in summer 2018.

Anyway having acquired category A, although I could have gone for a bigger bike, my son is 17 in September so I've decided to get a 125 to scoot around on until then, at which point I'll get a bigger bike and he can have my 125 hand-me-down. I got a 2012 Yamaha YBR 125cc which is fuel injected (that point may become relevant later on down this post).

However I have to say a 125 is incredibly frustratingly slow! I've done a few bits to it and seen no improvement in power (150cc bore and piston kit, free-flow sports exhaust with removable baffle, K&N air filter). I KNOW WHAT YOU'RE ALL THINKING you can't polish a turd right? That may be true but you CAN roll it in glitter!!! Ever seen Pimp My Ride? LOL well I'm after any gains possible from this little bike, almost regardless of cost and even though it would probably be cheaper and certainly far more easy to just buy a bigger bike. But like I said I'm kinda stuck with this until September now and it's kinda grown on me anyway.

Anyway the bizarre thing is I done a top speed test on a piece of dual carriageway and got 56mph on the GPS (showing about 65mph on the speedo) before all these mods. The MCN review of the bike says its top speed should be 70mph but to be honest I'm a bit of a lump at over 20st so not surprising it's not getting the quoted top speed but still 56mph seems a bit low even taking this into account???

I wasn't expecting huge power gains from these mods but I did expect at least a small measurable improvement (which is why I was using top speed as a benchmark as acceleration is somewhat more difficult to measure accurately). Done the same test on the same stretch of dual carriageway after all these mods and guess what... still getting exactly 56mph on the GPS. Not even ONE mph more after all these mods. It's been suggested to me that I would need to get it remapped in order to make use of the extra power available as it's fuel injected it may be limited??? Whereas if it had been a carb engine you'd just put a bigger jet in the carb apparently??? I dunno what I'm talking about I'm just regurgitating something I've been told by someone else. But if it were limited wouldn't it be limited to 70mph rather than 56mph as that's what the official top speed is?

I've got a guy who does the remapping on my small fleet of taxis and he's going to look into whether he can get the kit to do this for me.

ANYWAY that was just a bit of background, the main point of this post was relating to the final drive ratio. Just out of curiosity I swapped the front sprocket from a 14t to a 13t to see how much more acceleration I would get and how much of a drop in top speed I would get. Acceleration is marginally noticeable but the top speed has gone UP!!! It's now showing 65mph on the GPS and about the same on the speedo (which was the same as it was showing before swapping the sprocket but before it was actually going slower than indicated, I understand the speedo runs off the gearbox and not the wheels which is why the speedo accuracy has been affected). How can that work? Surely lower front sprocket = lower top speed?

The only explanation I can think of is that maybe the bike was only ever designed for 10st midgets and over-geared in the first place, with a 20st fat bloke sitting on it 70mph was never going to be achievable in the first place anyway, but swapping that 14t to a 13t has lowered the gearing actually allowing that under-powered engine to get somewhere near the red line in top gear, resulting in a higher top speed (whereas in a more powerful bike the end result would have been a LOWER top speed).

I've got a 49t sprocket for the rear to replace the 45t one that's on there from standard but haven't got around to fitting it yet.

Just after some thoughts and any input in general would be welcome really.

Thank you in advance.

Travis

[Travis Bickle]

Sorry this was supposed to go in the workshop section. I'll copy and paste it in there. Can an admin please delete this thread?

[M.C]

If no one responds to a post or even topic you can delete it, there should be a x in the top right of your post. Obviously I've ruined that now I'll delete this in a bit...

EDIT: Nooooo. Delete yours Ste then I can delete mine

[Ste]

[Travis Bickle]

There's no x showing there for me, I cannot delete it.
____________________
Must Go Faster
1999 Yamaha FZS600 Fazer

[Ste]

Yes, you can only delete a post if nobody has replied to it.

[M.C]

Its simple, I'll delete this post, Ste deletes his, then you delete your post, then Ste can delete his, then I'll delete my post and you can delete your post and the topic

We can fix this...

https://media1.tenor.com/images/f6b70c48ce6122eb7a658d530f658481/tenor.gif?itemid=7616588

[The Shaggy D.A.]

Go on, you can do it, I have faith
____________________
Chances are quite high you are not in my Monkeysphere, and I don't care about you. Don't take it personally.
Currently : Royal Enfield 350 Meteor
Previously : CB100N > CB250RS > XJ900F > GT550 > GPZ750R/1000RX > AJS M16 > R100RT > Bullet 500 > CB500 > LS650P > Bullet Electra X & YBR125 > Bullet 350 "Superstar" & YBR125 Custom > Royal Enfield Classic 500 Despatch Limited Edition (28 of 200) & CB Two-Fifty Nighthawk > ER5

[Travis Bickle]

https://rampages.us/gaubatzek/wp-content/uploads/sites/1242/2014/09/keep-calm-and-newb-on.png
____________________
Must Go Faster
1999 Yamaha FZS600 Fazer