Best Gear Ratio

Depends on what you're trying to achieve. 17/37 is also on my T120. I've read a number of posts on various forums about lowering by going to 39 or even 41 rear. Me personally, I am generally OK with the stock ratio. My T120 isn't a sport bike, and I like the very relaxed cruising rpms.
I also have a T100 which has 17/41 ratio. I am thinking of a higher ratio, maybe an 18 in front.

 

@BobberT and your priority is????

Acceleration.
Cruising.
About town.
Motorway/open miles.

 

I've read of folks going to 39 tooth on rear on T120 with good results. I would imagine this would really be a personal preference and that some experimentation may be required.

 

That unfortunately, to quote an English idiom, sounds like "to have your cake and eat it!"

You can't have both!

Gears are ratios so if you lower it the gears become closer, obviously the opposite is true likewise.

If you lower it then you'll pep up the acceleration but lose out in cruising overland both in revs/noise and economy so it depends on where your personal priority is.
The manufacturers have a wide audience to satisfy and so consequently standard will be a compromise so it's up to you to decide one way or the other or, maybe after further considerations, just stay as is?

 

I took one tooth off my ‘19 T120 Bonneville gearbox sprocket as it is ridiculously high as standard. I think the Bobber is already lower geared, as is the 1200 Scrambler, hence my lack of concern in doing it. Triumph do a sprocket anyway so that helps, if a bit pricey.

I went from never being in the right gear to being spot on for UK speed limits and brought 6th down to being a usable gear rather than a virtual overdrive that I never got to use much.

Pull away and response was hugely improved and, I would hazard, may even have a positive effect on longer distance fuel consumption.

 

Any check lights should not be affected by you modifying final drive ratios. It is pretty common for owners to lower the gearbox sprocket by one tooth from standard.

Larger rear or smaller front means greater acceleration. Approx 1 tooth front is the equivalent to 3 teeth rear, do the maths to clarify.

Interestingly, if you run your bike up on paddock stands the abs light may show up as the front wheel is stationary and it is seeing the difference so the ecu will cap the power/speed but will reset once status quo resumes.

 

One tooth down on gearbox won’t need a chain change but any smaller may not be so good mechanically.
Three teeth up on rear sprocket could well mean a chain change.
I was amazed at the different sprocket sizes and ratios on the various 1200 Bonnie variants.
The ‘ordinary’ Bonneville is definitely geared for loping, not overtaking. Mine feels transformed as I would have sold it otherwise. I am no speed demon but, in the busy south east, quick overtaking is more important than speed.

 

As your bike is a 2021 it may have Speedo Sensor on the final drive shaft in the gearbox, if so any sprocket changes Will require an electronic adjustment in the bike's ECU to keep the Speedo within the acceptable 10% error margins...

 

I would expect the wheel speed sensors to be used for the speedometer if it has ABS.

IMHO, a 10% error is not acceptable these days. At 55mph, the speedometer would display anywhere between roughly 50 and 60. That would've been expected back in the days of mechanical speedometer gearbox drives.

Haven't checked the accuracy of my Street Cup but my non-ABS Yamaha reads off the drive pulley and it displays 1 mph higher than actual speed at all speeds. No percentage errors so no difficult calculations or guesstimates required to know the exact speed. I know that's not always the case with newer bikes.

 

I believe the allowable 10% error is skewed to over reading only so you are never going faster than it shows. (Makes sense if the idea is to stop people speeding).

My car shows a speed consistently 10% over actual speed going by my satnav and roadside speed displays. My Triumph bike shows a speed that correct to within 1MPH usually, though can get a couple MPH over, I assume as tyres wear.

 

Your post rekindled a memory from my past.

Many years ago I was lucky to get a one on one walk around Mintex, as in the brake people. This was back in the 1980's and as a skint apprentice with an 1100 Escort I was in awe at the tooling, vehicles and facilities they had.
One garage had their test mule vehicles in which included a Sierra XR4i, anyone remember those?
This mint cars dashboard had been totally trashed with holes and wires all over. Readouts were fastened here and there telling you everything from pedal pressure to temperatures of the rotors (discs) etc.

Anyone remember Tapley meters?
Excellent piece of equipment in their day and probably still used in certain circles today i.e. permanent 4 wheel drive brake test with only a single axle mot unit available.

One anomaly that stuck in my mind @brown mouse was the sticker near the speedo with actual speeds on it i.e. 70 = 73 etc. as the speedo UNDER READ all the values how very strange and I'm not aware of any such discrepancy on a production vehicle since although my self built LR 90 project did have a 32 mph under reading when first built

Questions like "Best gear ratio" always make me smile as the obvious answer is "for what?"

No such thing as a free lunch etc and the choice is focussed for a purpose or best, in your mind, compromise.

 

Not sure how the stock Bobber gearing relates to the stock T120 gearing but here is a post I did recently about my experience and process of sprocket selection:

https://www.triumphrat.net/threads/my-t120-39t-sprocket-experience.996653/#post-2004323900

 

ABS sensor is for brakes ! And only brakes!
There is either a gear driven Speedo or a possible wheel electronic option (none ABS) or the final drive out of the gearbox has an electronic option .....

 

Not in the circuit diagram for my 1200 Twin. However, the ECU could synthesis the gearbox output speed by using the gear selection and crank position sensors. Though this wouldn't work if you pulled in clutch and freewheeled.

The wheel speed sensors connect to the ABS unit, so the only way they could been used for the speedo is if the ABS was putting the wheel speed onto the CAN bus or something like that. The ABS does have a two wire connection to ECU. (Edit, looking again, that'll be the CAN bus as the same two wires also go to immobiliser, instrument panel, and OBD2 connector)

The method used for my speedo could be determined by:

A. Whilst riding, pull in clutch and freewheel whilst engine idles, and see if speedo still reads OK. If it does, must be using ABS wheel speed sensor.

B. Unbolt both Hall sensors for ABS wheel speed ring, and tape them out the way. Assuming bike will still run, go for a ride and if speedo works, it must be using the gear and crank position sensors.

If speedo works in both cases A and B, then ECU is using both methods for determining road speed. Either one as a fallback for other, or some kind of algorithm for combining them.

Weather isn't getting above freezing for at least a week, so won't be tempted to do this experiment for while (Though I can't help thinking it'd be a bit perverse if the wheel speed sensors weren't been used to calculate road speed).

 

Many bikes will lose the speedometer function if you remove the ABS. It's just one of them that they read of off.