[Oldmike]

I'm after some clarification regarding changing the front sprocket. If I increase the front sprocket to 16t it will obviously effect acceleration in 1st gear as it's longer and decrease rpm in top. What I want to know is does it effect acceleration in all the gears? I'm thinking if you radically changed the sprocket size so 1st gear ended up with the same ratio as stock 2nd you'd be slower off the line but as you change up you would still match engine rpm with road speed so I can't see how acceleration other than in 1st would be affected. Anyone out there who has some insight please share. Thanks

[AzCal Retred]

Dropping overall gearing increases the HP available as the RPM in each gear increases. Net acceleration increases. Top speed suffers as you "run out" of RPM sooner. Each gear pulls for a shorter time, you run out of RPM sooner in each gear. But you cover the initial distance faster.

Increasing overall gearing slows acceleration as the HP has the leverage against it. Each gear pulls "longer" but has less "oomph".

Lower gearing is useful in the hills & mountains and if you aren't "petite". Taller gearing is useable/useful on the flats, but you'll know about every hill and headwind.

My Bullet came with an 18T aftermarket front sprocket. For me at a petite 100 Kg. and living in the foothills of the Sierras, the OEM 17T was a MUCH better choice.

An option for the Pre-Unit machines is to change the primary sprocket. This allows you to retain the 17T or larger front sprocket which is easier on the chain life.

[Oldmike]

Still can't understand how the bike would pull longer in each gear as the change in ratio between each gear is still the same.

[richard211]

Think of it as 3 reduction stages happening between the crankshaft and the rear wheel.

 The 1st stage of the reduction is between the crankshaft sprocket and the clutch basket sprocket in the primary case.

 For arguments sake consider the complete gearbox as the 2nd reduction stage.

 The 3rd reduction stage is the final drive which is from the output of the gearbox (front sprocket) to the rear wheel sprocket.

 So when the primary case sprocket ratio and gearbox ratios are unchanged they do not make any difference to the way it currently on your motorbike.

 But when you change the front sprocket to a bigger sprocket, what you are doing is changing the 3rd reduction stage. That is how the bike pulls longer in each gear, since the final drive ratio is taller.

[AzCal Retred]

F=MA, force equals mass times acceleration. Gearing is a force multiplier. If Force increases, Mass stays constant, Acceleration increases.

If the gearing is lower, the bike's engines Force is in in practical terms increased. This is how small motors move large loads, through gear reduction.

The "pulling" is how long the bike accelerates before hitting "red line". There will always be "gear overlap", a term derailleur equipped bicyclists are painfully familiar with.

If you change the overall ratio to a lower number, the resultant acceleration period spent in each gear is shorter. The total time to achieve terminal/final desired velocity is reduced. Lower gearing allows the engine to "pull harder (accelerate harder)" in each gear before hitting engine Red Line.

This is why 5 (or 6) speed transmissions are popular. If you are trying to accelerate in Traffic from dead stopped to 45 MPH, 1st thru 4th and a hand full of throttle will get you there faster that 2nd thru 5th. A 6-speed is icing on the cake. That's what the 650 Interceptor comes with standard. My 4 speed Bullet always feels like 1st is too high, but if geared down, 4th becomes too low. 

[axman88]

This is why 5 (or 6) speed transmissions are popular.

Not in a top fuel dragster.  Those guys use just 2 speeds, because time spent shifting is time spent coasting, and not accelerating.

I like my torquey engine with a low, flat torque curve, specifically because I don't need to be constantly shifting up and down through the ratios.  Wider spaced ratios would be nice though, then I could get by with just 2 gears.

[AzCal Retred]

For that matter the Honda RC115 with it's 21,500 RPM redline and 9-speed gearbox might have done as well in the 1/4 mile with a two speed...or not. 

" The "pulling" is how long the bike accelerates before hitting "red line". "
You run out of RPM faster when your gearing is lower. The idea is to keep the engine pulling as hard as possible to accelerate as fast as possible. Drag cars also rely on rear tire expansion as part of the track gearing. As long as the motor has plenty of torque available at the RPM set by the next higher ratio, you can accelerate. My ES350 "just" makes the jump on flat ground, my 500 much more so, as it has more torque overlap between gears.

"I like my torquey engine with a low, flat torque curve, specifically because I don't need to be constantly shifting up and down through the ratios."
And that's great as long as the engine power band RPM lasts long enough between gears to keep out of the "red zone". Mostly big displacement torquey engines aren't big revvers, so if the gap is very large, they'll either run out of puff or get into overspeed territory, like my ES350. There's a reason the 185 HP 11,000 RPM Hayabusa is a 4 instead of a twin. The NS500 did a very good job of imitating a V-8. The car folks racing Mazda rotaries discovered that they could spin them to 14,000 without (a lot of) fragging, so in the infield they picked up 50 feet or so every time the competition shifted and they didn't have to.

Motorcycles in the real world, engines &  transmissions equal, the overall shorter gearing machine will be quicker, the taller geared machine faster, eventually. No free lunch, just a trade off.

https://www.motortrend.com/features/top-fuel-dragsters/#:~:text=A%20Top%20Fuel%20dragster%20reaches,revolutions%20from%20light%20to%20light.
A Top Fuel dragster reaches more than 300 mph before you have completed reading this sentence. With a redline that can be as high as 9500 rpm, Top Fuel engines turn approximately 540 revolutions from light to light.

[tooseevee]

Top Fuel engines turn approximately 540 revolutions from light to light.[/font][/color]

          Please expand a bit. I don't quite get this one sentence.
          (Yes, I remember when Garlits was working his way toward 300 in the 1/4, many of us wore black arm bands for Bill Vukovich & wished we lived in Kalifornia so we could get some von Dutch pinstriping done)

[Oldmike]

Thanks for all the input. For me I'm preferring the taller gearing. Longer gears and less shifting is smoother, the bike now seems to glide up to speed. Bags of grunt left over to pull top and lower revs. I like it. I haven't noticed a drop in acceleration but I always shift between 4 and 5000 rpm.

[stinkwheel]

On bullets with 4-speed gearboxes, it also alters what speed the "gap" in the gearing between 3rd and 4th appears at (there is a very large jump in gear ratios between 3rd and 4th on an albion 4-speed). This can radically affect how useable the bike is when ridden on the road.

By gearing the bike higher, you may increase the overall cruising speed in top but run the risk of having to drop into third earlier when faced with a steep hill or headwind and then be stuck in third gear at a low speed with the engine screaming until it evens out again. Conversely, by gearing the bike lower, you make it easier to stay in top gear when under strain but lower the comfortable cruising speed.

Mind you, acceleration is all relative, and possibly less relevant  on most enfields because even if you double it, twice a small number is still a small number, rideability is more important. If it feels comfortable and the engine is running nicely at your normal cruising speed, that's much more critical than how quickly it gets there. I took my standard 350 bullet bracket racing and did a best standing quarter on the day of 19.68s with a terminal of 64mph.

[cyrusb]

Stinkwheel, that pic of you drag racing a bullet is in my collection now. Same time as a 66 vw bus I had in the 70's.
 Do you guys still run the full 1/4? We are down to 1000 feet now.

[stinkwheel]

Stinkwheel, that pic of you drag racing a bullet is in my collection now. Same time as a 66 vw bus I had in the 70's.
 Do you guys still run the full 1/4? We are down to 1000 feet now.

Mostly still 1/4 mile but they've had to drop the top-bikes (nominally street legal but pretty much anything allowed including nitrous, wheelie bars and canned fuel, no nitro though) to 1/8 mile on some venues due to lack of run-off area for braking. Most tracks are abandoned WW2 airfields and while the main strip is kept maintained the upkeep on a long enough smooth tarmac braking area to get back down from 170+mph was proving too much of an expense.

[Brian10x]

Not in a top fuel dragster.  Those guys use just 2 speeds, because time spent shifting is time spent coasting, and not accelerating.

I like my torquey engine with a low, flat torque curve, specifically because I don't need to be constantly shifting up and down through the ratios.  Wider spaced ratios would be nice though, then I could get by with just 2 gears.

That used to be the case, but I believe top fuel uses direct drive and a slipper clutch now.