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

[ThatDippyTwat]

If you only get 50 on GPS, especially with a capacity boost, something is fucked.
I'm, 6'3" and my stock YBR hit 70 if you had a long enough road, even before replacing the motor that the previous owner had fucked. Assuming you actually redline it on the way up the gears (It's a 125, you need to).

I suspect that with a "sports" exhaust, free flowing airfilter and larger bore, you may be outside of what the FI system can adjust to.
____________________
'98 VFR800 (touring) - '12 VFR800 Crosrunner (Commuting) - '01 KDX220 (Big Green Antisocial Machine)

[-]

If you're over 20 stone, that's going to make a huge difference to performance on a bike that, which even after modification, is probably still making under 15bhp.

It's like two skinny adults, so 56mph sounds about right to me.

My skinny ex could easily get her full power NSR125 (de-restrictedd, upjetted, full Arrow system) over a ton, when I was 17 stone, it only did around 85mph with me on it.
____________________
TZR250 2MA road, TZR250 1KT road, TZR250 2MA race, TDR250, YZF-750R Boost colours.
Jaguar S Type 3.0 V6 Sport R, VW Transporter T5 GP LWB Shuttle 140ps DSG.

[The Shaggy D.A.]

YBR speedo is driven from the front wheel, not the gearbox, so the lack of change in what that is displaying is probably down to the cable needing a lube

I'm not surprised that a tooth drop on the front sprocket gives a higher MPH, they only produce around 10bhp so need all the leverage they can get. The earlier carbed version has slightly larger ports and kicked out a whopping 11bhp, so if you're in the mood for fettling you might want to source a head from one of those and see if you can rig it for FI, or just swap to carb for more options, if your mate can't remap the FI.
____________________
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

[The Shaggy D.A.]

Oh, and (a) welcome to the forum, and (b) lube up, you're in for a heavy Teffing
____________________
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

[Sister Sledge]

[stevo as b4]

Fuck Tef!

He's probably first been wanking himself off over a possible green enough newbie that he feels he can get away with bombarding with poorly spelt Gospel script. (Although OP doesn't come across as a chump to me based on his first post and background). Teff might not get as far with his "assumption is the mother" essay.

I've been around here long enough to know, he'll only try it on the conscientious clueless newbie prey, and not someone who's too wise to suffer fools, or even a stroppy teen that knows everything and will just give it a load of "fuck you grandad" at such a telling.

To OP, I agree with the others (no insult intended) that with 20St a YBR will get nowhere near 65mph. A true measured 56mph on the flat sounds bang on for a stock bike, given the circumstances. If even with a 10St rider a std YBR could reach a true 60mph on the flat then that's really good with 10bhp and no fairing. The YBR is doing itself proud even!

Bear in mind a best case (guess) for an indicated 65 or 70mph being 60/65mph is very good. Some speedos on small bikes are 10mph out at 60mph. (I think the KMX speedo is 7-8mph out as an example).

Your imminent heavy Teffing will go on about cylinder filling, valve sizes and cam timing etc, but the bottom line is that the 150cc kit as bolted on with a different exhaust and air filter will be lucky to make 2bhp extra. If it makes this much or more then again it's working as well as can be expected.

Out of interest what's your top speed down a long hill? If it's going to over 70mph on the speedo (on the standard gearing) that will tell you what you need to know.

Lastly expect some frowned upon comments if your planning on handing the bike down to your son as a 150cc, as people will be screaming to tell you how illegal this is and how irresponsible of dad etc.

[Travis Bickle]

Thanks guys all so much for your input. Sounds like this Tef character is quite popular round these here parts? LOL I'm a noob to 2 wheels and not afraid to admit it, although I do have a keen interest in motors in general and am always keen to learn as much as possible... Which is why I'm here with all these questions; the pursuit of knowledge!

[ThatDippyTwat]

[Travis Bickle]

[ThatDippyTwat]

[The Shaggy D.A.]

<duplicate post>
____________________
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

[The Shaggy D.A.]

[Travis Bickle]

That's nice, I like that. Mine's a bit of an old dog really but I love it.

[Travis Bickle]

[The Shaggy D.A.]

Probably worn speedo drive or sticking cable. Either replace or stick a bicycle speedo on, I had one of these on my Enfield :-

https://www.amazon.co.uk/gp/product/B000KC2402/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1

[edit] Or buy used for much cheap, very wow.

https://www.amazon.co.uk/gp/offer-listing/B000KC2402/ref=dp_olp_used?ie=UTF8&condition=used
____________________
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]

[Travis Bickle]

The thought occurred to me... given that no matter what I do to this bloody bike I still don't seem to be getting any more or any less than 56mph out of it... Does anyone think it could actually be power restricted from the ECU or top speed limited? I can't think why anyone would want to do that, unless it was an import as I know different countries have different rules on 50cc and 125cc bikes with regards to power and top speed restrictions etc. But it just seems odds to be as 56mpg just so happens to be exactly 90kmh...

[ThatDippyTwat]

[Travis Bickle]

[ThatDippyTwat]

https://www.bikechatforums.com/viewtopic.php?t=326544
____________________
'98 VFR800 (touring) - '12 VFR800 Crosrunner (Commuting) - '01 KDX220 (Big Green Antisocial Machine)