Street Twin Gear Ratio Change

I'd been delving to some of the deeper parts of the internet researching performance mods to upgrade my 2016 Street Twin. I travel along national speed limit roads for 80% of my journey and find it is nearing the top of its' comfort zone. Naturally, it needs a 6th gear, or I need to buy a bike that has a 6th gear, but they're both pretty expensive solutions. Plus I'm rather attached to the little thing...

I found lots to do with air filters and Euro 4 overrides for low-speed manoeuvring, right up to cam changes courtesy of George and co at Tec Parts, but one sprang up that seemed to make sense: change the gear ratios. Now, this thread on the Rat forum died in 2017 but all the chat rang true; make the gear ratios longer. Plus, as I needed a new chain I thought it would be worth having a look at.

Spoke to a motorbike shop near work (Chippenham Motorcycles, ask for Freddie, he is a true gent and would highly recommend) and talked through the many permutations and went for it.

The Street Twin has a stock ratio of 17 teeth front, 41 teeth rear with a 520 gauge chain. No sprocket supplier offered any different options for the Street Twin, however, the measurements for the Thruxton sprockets matched those of the Twin, just with a 525 gauge chain. Agreed we'd give it a punt and made the change to a 39 tooth rear and retaining the 17 tooth front, which should give an extra 6-7mph on each gear.

Verdict: Absolutely. Spot. On.

Where I was umming and ahhing about changing from 4th to 5th at exactly 50mph, 4th now comfortably runs to 56/57mph, leaving 5th as a true cruising gear. The run back from the shop also included a couple of roundabouts and I got held at two red lights. Pulling away from a standstill is no different at all, probably thanks to the low-end torque the Twin possesses.

They also said that I need new tyres, so incoming are a pair of Road Attack 3s.

 

Bang on, it's exactly the same principle as a push bike: go smaller at the back or bigger at the front if you want longer gears/higher top end but for more effort and vice versa. 42/17 isn't a huge change but will reduce his top speed by a couple of mph and make it accelerate out far too quickly. Unless he's using the bike for zipping around town or racing on tight circuits then it's an unneeded change in my (very) humble opinion. Might as well get himself a 300cc Vespa...

For the ST, changing 1 tooth at the front is roughly the same as changing 2 at the back. Not sure if this applies to other bikes but if you are so inclined you can get fully geeky at the below link. Took me a little while to get my head around but is really informative. Apparently I should top out at 118mph instead of 112...shame it's just started raining as there's a decent run home to test that out!

https://www.gearingcommander.com/

 

I'd highly recommend this change if you're finding yourself pushing out of the comfortable rev range. I've put the sprocket ID numbers below for JT sprockets who Chippenham Motorcycles use. They also seem to come up in all the Haynes manuals too and were really helpful when I spoke with them on the phone.

Front:

OEM replacement: JTF1185RB (the RB means it has a rubber cushion). 17 tooth only, which is OEM standard.

Non-OEM: JTF1186RB, comes in 16 or 17 tooth options. (Go smaller for more acceleration, less top speed).


Rear:

OEM replacement: JTR2020. 41 tooth only, OEM standard

Non-OEM: JTR2014. 37, 39, 42, 46, 47, 50. (Go bigger for more acceleration, less top speed). I went for the 39 tooth.

For the non-OEM sprockets you will need a 525 gauge chain which is considerably more chunky. I'm not too fussed about the slight additional weight gain in comparison to a significant gain in comfort.