Bonneville America Forums Forums Technical Forums General Tech Drive ratios

Searched the forum best as I can as I know this topic has been dealt with in the past, but my computer illiteracy has come to the fore
Ok here goes, I have the '05 SM currently with 13000km on the clock ( 8100 miles ) and have finally decided that the ratios are just not doing me any favours with the gearing. I get about 4000 rpm when cruising at 100 km/hr ( 62 mph ) and after the few snippets I have read here I am keen to go up a few teeth on the front.
I have two questions -;
firstly has anyone got the 19 tooth and if so what rpm does it sit at for 100km/hr. I have read that the 18 tooth gives approx 4000rpm at 130 km/hr so that is a good guideline for me there, just want to see how much further it drops with the extra tooth. I used to have a web link that you could punch in all these numbers to get a perfect self preference ratio.
Secondly with going up in size I will throw on a new chain, I know its best to replace everything at once but that back sprocket is like new..... should I replace that too ??
( I know the answer will be yes but I just need that push from being too tight with the $$$ and properly maintain the thing )

Hmmm....my 05' speedy reads about 68 mph at 4k. i wonder if our speedo's are off from each other by that much. I once past one of those roadside speed indicators at 52 MPH on the speedo and the indicator read exactly the same, so maybe yours is reading low?

With only 8100 miles on your bike, you shouldn't have a problem just changing the front sprocket. That's only about 1/3rd to 1/2 the chain's normal useful life.

So do the 05 Speedmasters have the 16 tooth sprocket? I thought I'd heard they went to 17 tooth, but I don't know if that's true. I think 17 tooth is just right for this bike, but others swear by the 18 tooth. With a 19 tooth, you're going to lose a lot of acceleration off the line, and I don't think it's worth that sacrifice. Just depends on your riding style.

Since we have Brent at newspeedmaster.com selling front sprockets for $20 each plus shipping, it's cheap enough to do some experimenting to find which size suits you. I certainly don't think cruising at 4,000 or even 4,500 rpm is harming the motor at all. These bikes are bulletproof by all accounts.

The book shows that all the 790 SMs are 16 front/ 42 rear. The 865 SMs are 16/43. The Americas (all 790) are different at 17/42. Interestingly, the 790 Bonnies are 17/43 (higher rear sprocket than the America) and the 865 Bonnies are 18/43. No doubt it would be best to try going up 1 on the front sprocket. At crusing speed it would drop around 500 rpms I've been told. I just bought a 18T to try on my TA. Fairly easy change keeping stock chain.

Though I have no real regrets about the 18t, I rally liked the stock 16t, too.

If I had a 17t fall into my lap, it woudl probably be the "keeper" of the bunch.

Can't go wrong with any of them, though - all 3 work fine. It's just to suit your individual riding tastes. Nobody rides the same. No ride is the same.

It is an easy change, and the stock chain should not be a concern this soon.

Love my 18...

Benny, are you riding a SM or TA? If it's a SM, is it a 790 or 865? I am curious as I plan to change to an 18T and understand you love yours, but there's a few differnt set-ups and weren't sure if we were on the same page.

To get a quick estimate of what a new sprocket will do for you, (or to you) you can use these calculations:

Speed at the same RPM:
speed/old number of teeth times the new number of teeth.
(100/16 * 18 = 112 KPH @ 4000)

RPM at the same speed:
RPM/new number of teeth times old number of teeth.
(4000/18 * 16 = 3555 @ 100 KPH)

This may not be exact, but it will be very close.

Finally changed my 17T stock to an 18T yesterday. Went riding 2-up today to check it out. It made a LOT more difference that I thought from reading all the posts. It didn't hurt the start off in first, but did take a bit of quickness off fast starts as the gears get noticeably taller. BUT, there is absolutely no tendancy to want to sift up to another gear anymore. 5th is more like an overdrive now for use above 45 mph. Before I was getting about 50 mph at 3000 rpm and now it hits 57 mph at 3000 and it about 10 mph faster at 4000. You can even downshift at 70 and take off. I used to ride in 5th a lot around town where now I find myself shifting more 4-5, 5-4, etc. I think with the added hp from the jets and exhaust, the 18T is probably right for this bike unless you just have to leave everyone off the line. With the noticeable difference at hwy speeds it should get my mileage back to where it was before all the mods. Again, I was surprised how much difference going up one tooth made. I think I'm staying here for the long haul. I would never consider to up to a 19T as you would be giving up too much. I have an overdrive now, it would be crazy to go higher unless you did nothing but hwy riding. Also, it wasn't hard to do as I followed Pat's directions I printed out. The only trouble was getting the sprocket nut tightened back to 132 nms. I ended up going by the shop manual method of putting the chain on and holding the brake while also in gear. My advice to anyone with an America and a stock 17T, you should try it - especially if you've already boosted the power a bit from stock. My guess is - you'll like it better.

I find myself screaming around in 3rd and 4th and only using 5th for the highway

Benny, haven't had enough riding time yet on the 18T, but that's what I meant by "overdrive". The revs are too low now to use 5th below 45. I kinda like the new feel AND having a highway gear.

Well finally took the plunge on Friday. Tried getting an 18 tooth locally and got messed around. So emailed Brent for shipping price and in under 1/2hr I had a reply.
Now there is a sprocket on its way from the states to my front door, that is what you call service......... I know a few businesses that could take a leaf out of Brents book and they're not all bike related

Ramsound has an 18t and I have a 19t both are easily handled, you have more displacement and a 43 tooth sprocket I believe so your bike can handle either. You will notice a serious difference in how tall your gears feel. As RamSound said 5th gear becomes overdrive.

Anyone reading this thread needs to keep in mind that there is a noticeable difference depending on exactly what you are starting out with when changing front sprockets. Stock Americas are 17/42 (2.47:1), 790 Speedys are 16/42 (2.63:1), and 865 Speedys are 16/43 (2.69:1). I changed my TA to 18/42 (2.33:1). If I decide that it is a bit high now, I'll change the rear sprocket to 18/43 (2.39:1) which is about half way between stock and where I am now. This probably should be stock for this bike. The new 865 T100s and Thruxtons come with this combo. Also, the 790 T100 and Bonnies that have changed their stock (17/43) to an 18T front now have this combo. I have yet to hear from one of those that have changed that didn't think it was the best compromise. Many America owners may get the wrong idea in that "43" bikes changing to an 18T still have a lower final drive than Americas with 18T.

I've run another tank full since changing to the 18/42 combo. Noticed a big difference in gas mileage. I had been getting about 36 mpg avg. and now getting 42-45. I no longer look for 6th gear but have a new problem - I find myself FORGETTING to shift to 5th. I was on the highway today and twice realized I was over 70 and still in fourth. I've decided to order a 43T rear sprocket tomorrow. I'm convinced it would be the best combination of keeping the quick tighter low gears and still getting a higher 5th. If the mileage splits the difference as the gear ratio, I should still get 40+ mpg. I'll post an update after changing. If I can do it soon, I'd still have time to change back before Bike Week, if I need to.

I have an 02 America and just changed to the 18 tooth front sprocket. It made a noticable difference, but I really wanted a 19 tooth for an upcoming cross country trip. (Virginia to Vegas for my daughter's dance competition in July 06) My local dealer first sold me the 19, but it was too wide (made for the Thruxton). Is there a Triumph 19 tooth made for the 525 chain? I would like the part # if there is. If not, I may buy the 19 anyway and have the sprocket machined to the width I need.

Brent sells the 19 tooth sprocket. Check it out here.

I've run both stock sprockets and Brent's sprockets for quite a few miles for each, and I can't tell any difference for noise or wear.