Bonneville America Forums Forums Technical Forums Performance Mods 19 tooth front: A performance evaluation.

Well I wasn't sure if this was a mod or accessory, but I opted for performance because of the obvious, it makes my bike go faster!

I went from a 17 tooth front to a 19 tooth, you all know how I like testing what I sell as often as I can. Well, I just got back from a 30 mile ride. It's 32 degrees out tonight but that didn't dissuade me from a ride.

According to my sprocket folks, 1 tooth up on the front is equal to 2 on the rear. Since I went up by 2 teeth that is the same as 4 on the back.

Starting out in first gear from stops, I could tell a little difference, but nothing that felt detrimental or anywhere close to lugging the engine.

She ran through all the gears with the same spunk as with the 17 tooth. Where I really noticed the difference? Instead of reaching for the "sixth gear", I kept forgetting to shift into 5th! Too used to what the engine "feels like" at 70. Shift into 5th at 75 is like going into over drive!

So next, I did a speed test. I ran it up on a straight stretch of four lane including a steep up hill grade which usually drains the speed out of the bike at speed, this time? I was still gaining momentum at 100 mph.

After hitting the straights again, I pinned the throttle and watched her climber faster than I have had her at before. I sailed past 104 with no problem and hit 110 with plenty of throttle and with still room to grow. After I backed off I realized that I had not yet hit 5th gear! I have no doubt that 115 or 120 are not out of reach. Biggest thing about this was the engine didn't feel like it was topped out, which it normally does at that speed.

With the relaxed engine RPM's at highway speeds, I fully expect to see an improvement in fuel economy. I'll report more on that and how she performs on the hills of Duluth (Like the San Fransisco hills) first warm day I get to make a trip up north. As of right now? I think it is a nice touch and did very well tonight.

eddy

Well, Mr. Eddy, you know how I feel about it. I love that sprocket and can't imagine going back down to a lower one. Thanks for the report.

Well Ron, back when I rode your it was in the mountains and I didn't have a real standard comparison ( and I seem to recall I kept forgetting to shift your too!) Amazing, you get used to "normal" and how such a little change can make such a positive difference. What I do recall from your bike in the mountains? It was not a problem with the tall gear even in those steep grades.

It was because of that ride on yours I was determined to give it a go on mine, guess I just took longer doing it than I had planned. Man what a waste of time!

eddy

I love my 19T. Granted I only went from a 18 to the 19...not that big jump like you did.
I was doing 80 on the fwy today...in 4th...I forgot I had one more gear

My rev limiter kicks in at 115 MPH in 4th gear with the 19 tooth sprocket up front.

How hard a mod is this to do? Is it just a matter of unbolting one sprocket and bolting another one on? (Not withstanding changing the chain, of course.)

I didn't need to make any link changes to the chain and have not had anyone report a need for a longer chain than stock length. Mine adjusted out perfect for tension (by the book).

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This thread seems to imply that you gain horsepower by changing to a larger front sprocket. That is impossible. Our bikes are not top speed limited due to final drive ratios.

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No inference to horse power gain. Just have more reach. Much like if you keep the bike in 1st gear you are only going to go so X-fast before you hit the limiter. You shift gears into second and the engine rpm's drop and you go faster because you have more "reach". No additional horse power it's just like adding that "6th" gear.

My reference to the grade was just that the bike had the power at that particular speed left (likely because the rpm's were lower than with the 17 tooth at the same speed) and not already tapped out.

I don't have a tach to be able to report the actual engine rpm's or I could likely show the difference between the two.

The bike seems to have enough horse power on it's own to handle the top end capabilities the taller sprocket affords.

The stock 112 link chain will work with a 16, 17, 18, or 19 tooth front sprocket. No need to "break" the chain to install a sprocket, front or rear. That's only necessary when replacing the chain.

Hey Joe,

I did notice a difference taking off, but it did NOT feel like it was over lugging the motor to get going. I am sure taking off on a steep hill will be different but I won't know that for sure till I get up to Duluth where there are hills in abundance for this test.

Now I only have a 790 engine, I suspect the 865's have even more reserve in the HP department.

Addendum;

I just went out and did some experimenting.

I did a number of slow start ups and the motor did not feel like it was lugging, I can tell a difference from the 17 tooth, but not such that it would personally concern me.

I did two speed run ups, I ran it up until I hit the Rev Limiter two separate times and same result

First gear hit at 50 mph both times
Second gear hit at 80 mph both times
Third gear hit at 105 both times.
I didn't go into forth as I was testing the limit in each gear and I was out of road.

Someone here knows what the limiter is set at and you can figure a comparison from there with smaller sprockets.

One thing is sure, this sprocket really acts like an extra gear at the top end. By "feel" 4th gear at 55 mph feels more like the bike did in 5th at the same speed and like you should stay in 4th until 60-65 mph it certainly makes the engine run at lower RPM's at 60 plus.

Anyone with a tach that can translate RPM/MPH with this set up can chime in.

I still have a sixteen tooth sprocket on my Speedmaster; may look at a 17 tooth when I replace chain later this year, but doubt I'll go higher than that. I don't want to lose the low-end, and the bike doesn't seem like it's straining on the highway. If I spent more time on the interstate, I might view it differently.

As far as top speed, the limiting factor for me on this bike is the seating position. Anything over about 85mph on this bike just isn't comfortable to me for very long. Wind blast on the chest and head, and feel like I'm holding on with a death-grip just to stay on the bike. But, even with the 16 tooth, the bike will easily do more...I suppose I give out before it does.

Will

From my perspective 18/42 is near optimal for all around riding conditions on a properly tuned stock internal 865 motor.

I believe the limiter hits right around 7400 rpm on the 790 engine.

I think, a consideration to riders weight may influence things as well, I am 185 lbs.

That's probably true I fluctuate between 205 and 215 lbs.

at 280 i cannont get my speedo above 110...i have opened the exhaust and removed the snorkle....

I've gotten mine up to 115 with windshield, hard bags, and my at the time weight of 230, 17 tooth sprocket. Still had throttle left, and nowhere near the rev limit. Of course, I'm thumping 70rwhp, too.

I think the speedometers are very friendly. Mine is 10% fast meaning at 55 I am about 48MPH. I believe then at 100 I am about 90 real MPH. I can really spin that speedo needle but think the reality is about 115 MPH>

Of course this is all in theory, I personally max out at about 60MPH then get scared.

My speedo seemed to get more accurate when I put the bigger front tire on.

I had the pleasure of having my speedo calibrated by the NY State Police on the way home from the NERally Sept 06.
It was 10% off.
Since then, I put on a taller front tire. Judging by the side of the road radar units, its right on now.

Now that I think of it, last week DunnSpeed and I were going down the highway, and went by a cop on the shoulder. Chris said we were doing about 80, my speedo said 70, so I think the bigger tire does make a pretty good difference. We didn't get pulled over, so we must not have been going 80.

I checked my bike against the on the side of the road radar and it was spot on last year. I think there are some variances between bikes.

My speedo reads high. Riding with a friend with an indicated of 78, the GPS over a 10 mile period showed 73. My odometer reads about 2.5% LESS miles than actually traveled.

As far as the 19 tooth discussion (original thread), you do gain a sixth gear, but you lose a first gear. The gears get taller as well. I'm happy with the 18/42 combination, but sometimes miss the quicker starts off the line riding 2-up. A 19T is more like a 17T starting off in second gear. BUT, that's another plus for chains. It's very easy to set the bike up the way you want it. I rarely travel over 80 or 85 and the revs there don't seem too high at all - especially for a bike with a limit of 7500 rpm.

I have also gone to a 19-tooth front sprocket on my stock 2008 America. I haven't done any other mods yet, will do airbox and pipes hopefully this year. I have no problems rolling out, and of course cruising is more comfortable on the freeway, "feels right" in 5th going at 65-75 traffic speeds where before I wanted to shift again. Just right. Of course now I don't know what speedo is vs. real speed and should probably get calibrated.

Is there any speedo adjustment possible?

Later - NorCal Chuck

The speedo runs off the front wheel.
Changing sprockets has no effect.

Yes it does Charlie, I can go faster so it has an effect on the Speedo!

Wiseguy

Hey Moe, hey Larry... why I ahtta!

Gosh guys, these are P-twins, not V-twins...a little RPM's more than your garden-variety V-twins are just fine, IMO.

I have a 17 tooth sprocket on my bike and go between 200-210lbs and I have yet to hit the rev limiter in 5th gear and I have tried many times. I agree that the aerodynamics and hp from our bikes is the limiting factors on top speed. I am running a 904 and I may go to an 18 tooth sprocket but no higher than that.

Hmm, I had a 17t and hit the limiter in every gear.. Triumph told me I bought the wrong bike, should get a sports LOL, I changed it for an 18t and now have not hit the limiter in top gear.. it does go off the speedo so guessing about 125?

On the old 17t I hit the limiter ONCE in top gear do I get a prize?

TrOjAn

I have recently mod'd to a 19 front. calculating the ratio it was the same as a 44.8 rear sprocket.('07 sm 18/42). I have noticed a little drag when trying to race off in 1st but nothing that will cause detriment. 70mph in 5th @ 4400rpm, stock gearing, with new on 70mph in 5th @ 3800rpm. Feels much better for riding on, I've had it out on a variety of roads and it's good. I've run it up to 100mph and not touched 5500rpm, by my calcultions it should be good for 135mph (ish).

The speedometer has to overread according to manufacturing standards, but the maximum inaccuracy allowed is <10% changing to a 120/80 tyre is an increase in wheel circumference of approx 10%, taking up the inaccuracy. The tyre shape should not be so different as to affect the handling (contact patch). Unfortunately manufacturers now are not necessarily using components which are best for the vehicle but instead allow them to pass the vehicle through manufacturer required standards. A prime example of these sacrifices are for emissions and noise requirements. (This is my current project at work.)

We need a 17t for 1st gear then jump it onto a 19t

TrOjAn