Sprocket & Revs

[Robc76]

Good afternoon everyone.

My GS is broken so I might have to ride the Himalayan 1,000 miles to the Pyrenees instead. Unfortunately that means a lot of motorway/dual carriageway riding. Since my current chain is due for replacement I have the option to change the gearing at the same time. With standard hearing I cruise at 60mph at about 4,700 rpm. Can anyone suggest which sprockets I can use to let me cruise at 70 at the same revs?

RobC

 

[OldGuy]

Check the latest version of the Himalyan FAQ's - I've included a bunch of gearing charts to help you decide which way to go and by how much.
OG

 

[Robc76]

Splendid. Thank you.

RobC

 

[DaveM]

A 16 tooth sprocket will change the gearing by 6%. So 60 mph at 4700 rpm will result in 60mph at 4434 rpm.

 

[Roy Gavin]

You are looking at around a 16 1/2 % change , one on the front and three off the back will be close, so 16/35 or 17/ 36.
Good luck finding them!
But the Hima might not have enough power to pull this on anything other than a flat road with a tail wind, and it is going to pretty useless once you get to the mountains.
It takes the same power to move at 70 mph no matter what the gearing, so the change just means the motor has to make the same power with less revs and actually increases the load on the motor, even after the minimal decrease in rotational friction.

 

[Bluestrom13]

17T / 38T will get you what you ask for. And Hitchcocks have 17T in stock.

but....

it is going to pretty useless once you get to the mountains.

EXACTLY!
And not very happy even before...16 teeth can make 5th gear redundant sometimes.
edit - added-
(charts from gearingcommander dot com)

 

[Roy Gavin]

17/38 only gives around 12% reduction in ratio, and he needs around 16% , don't believe everything you read!
But his 4700 and 60 mph is lower than some, speedos and rev counters are at best +- 2 1/2% and tires are up to 5% difference in rolling radius, both when new / worn and brand to brand, so any advice is an approximation.
And my opinion/ comes from someone who runs 14/40 so there user bias too!

 

[tom_d]

17/38 only gives around 12% reduction in ratio, and he needs around 16% , don't believe everything you read!
But his 4700 and 60 mph is lower than some, speedos and rev counters are at best +- 2 1/2% and tires are up to 5% difference in rolling radius, both when new / worn and brand to brand, so any advice is an approximation.
And my opinion/ comes from someone who runs 14/40 so there user bias too!

According to Gearing commander which does focus on these calculation, an 18 tooth front will give roughly 16% change and drop RPM's at 70 mph to about 4500. The 17 tooth front with stock rear looks to be the best match for what OP asks, but either way, 17 or 18 tooth primary, the bike is falling well behind its power curve and will certainly give a very poor experience along with not providing the desired result. For a stock engine on flat prairies where highways are unavoidable, large portions of Texas as an example, some have found a 16 tooth to be of benefit. Seems that most have found the 15 tooth very compatible with their use with some going to a 14 to get some mechanical advantage for their environment which might need less top speed.
OP's question just actually sounds like a job for a more powerful motorcycle, or a route which puts less high speed demands on the bike.

 

[Laserman]

Just run a 16T on front with the stock 38T and call it good. This is already the tallest recommended gear for a Himalayan.

You'll be able to cruise at 70MPH at roughly 5K RPM all day long unless you encounter long inclines or a big headwind!

I've been cruising for well over a year at these speeds with 15/36 gear. I just switched to 16/38 and couldn't be happier without engine mods.

 

[Robc76]

I've been thinking hard about this. I've ordered a chain and sprocket set with the standard gearing. I wanted to only alter the rear sprocket size as I was going to swap it out once I got to my destination but looking at the numbers a 15T / 36T combination would only give me around 3mph at my normal cruising revs. That's a small return for an expensive sprocket.

Thanks for all of the answers and suggestions though. I learned a few things.

RobC

 

[Laserman]

The reason I had initially gone with a 36 rear instead of a 16T front was because I didn't think I had enough chain to accommodate a larger front sprocket. In hindsight, I think I could have done it had I waited another month for the chain to "stretch" a bit. The rear sprocket costs as much as 3 or 4 front sprockets!

The front sprocket is far easier to replace than the rear sprocket (at least, if you have an impact). I highly recommend 16/38 for high speed running.

 

[Robc76]

I was only going to use the 36T rear for this one trip and only on the ride there and back. I was going to swap it out for the duration of my stay as I'll be staying well away from fast roads and be seeking out the twisties in the mountains. £72 (I think) for a rear sprocket is too crazy. I'll just ride at my normal cruising speed of 60ish and accept each leg of the journey will be slightly longer.

Changing a rear sprocket can be done easily with the tools I carry on the bike. The front is not which is the reason behind keeping the standard 15T at the front.

RobC