question about gearing for the track

[dynaglide]

Just a general question about something I don't understand:
The usual adviceI hear for the track is to gear the bike so that it's going through the traps in 4th at redline, which for many builds out there is probably around 6200 RPM or so.
A certain gearing will keep the bike at a particular MPH if the motor is spinning at a particular RPM, right?
So, no matter how many HP the motor puts out, if you only spin it to say 6200 RPM in 4th, it will never go any faster than 113 MPH with 3.37 final gearing, right?  It might get to that speed quicker, but the gearing won't let it go any faster...
So when you add HP but don't change your gearing, your MPH will not increase in the 1/4, but your ETs will, right?
I've also heard that shorter gearing is better for the track, since it gives faster accelleration.  But if the above is true, doesn't that actually lead to slower MPH? (shorter gearing + same max RPM = less MPH).  Or, if you shift up into 5th, you lose time with the shift, which offsets any power gains you may have made...
So, how do get faster ET and MPH together? 

[FXDX95]

A serious piece or you that is biased to ET cards I would always gear to allow the car/bike to come thru the 1/4 in high gear looking for the next gear or rev limiter. That doesn't make for a good all around street performer. Needs lots of RPM.
That's where the best ET and MPH will come from but with that said you'll have to get the car/bike to hook up and use the quick acceleration right from the time you release the clutch/brake/trans brake. Much easier to put in print than on the track. It can be habit forming though.
My street FXDXT comes thru at 114/115 in 4th just hitting my 6k limit.

Al

[frito1]

If you're serious about low ET (like I used to be), you should be geared so that you are in top gear crossing the finish line and you should hit the rev limiter ~100 feet after the finish line if you stayed in it.  We did this to prevent hitting the limiter before the finish line in case of a little wheel spin.  It's sad to watch someone drive around you at the line when you're on the limiter.   

It's a pain and requires re-gearing periodically, but that's racing.

[frito1]

A certain gearing will keep the bike at a particular MPH if the motor is spinning at a particular RPM, right?

Correct

So, no matter how many HP the motor puts out, if you only spin it to say 6200 RPM in 4th, it will never go any faster than 113 MPH with 3.37 final gearing, right?  It might get to that speed quicker, but the gearing won't let it go any faster...

True

So when you add HP but don't change your gearing, your MPH will not increase in the 1/4, but your ETs will, right?

True, if you were geared correctly for the lower HP.  If you were geared to tall for the lower HP adding HP will compensate to an extent.

I've also heard that shorter gearing is better for the track, since it gives faster acceleration.  But if the above is true, doesn't that actually lead to slower MPH? (shorter gearing + same max RPM = less MPH).  Or, if you shift up into 5th, you lose time with the shift, which offsets any power gains you may have made...
So, how do get faster ET and MPH together?

Short gearing benefits a vehicle with less than an abundance of HP.  A stock vehicle for example will benefit greatly by lowering the gear ratio (numerically higher).  The problem with your question is that there are many variables to account for.  We used to say "gears make low ET's and HP makes MPH".  You are correct in thinking that gearing can limit your MPH, the trick is to find the magic number that will allow your vehicle to reach it's maximun speed and lowest ET in the given distance.  You want to run as tall a gear (numerically lower) as your current HP levels allow in order to get max speed in the given distance.  If you geared a stock bike as I suggested in my other post, to peak at max RPM 100 feet beyond the finish line in top gear and then add 10 HP you will be reaching max MPH before the finish line and thus leaving some ET on the table.  In the +10 HP scenario the ET should drop with little or no change in MPH, however if you re-gear to take advantage of the increase in HP you will gain MPH and drop the ET as well.  It's just a balancing act that never ends.  Drag racing can make you moderately wealthy, *if* you start out being extremely wealthy. 

Oh yeah, if shifting gears is costing you ET, you're shifting too slow.  Along with just practicing, there are several expensive cures for that too.

[dynaglide]

Thanks!  Great answer that explains exactly what I was looking for! 
So if I'm understanding you correctly, using all five gears in the 1/4 mile should have no effect on ET, right? (assuming correct gearing, of course)
If that's true, then is it worth running a 4-speed gear box?  I would think that doing so allows a little more of the motor's power band to be used in each of the higher gears?  Or is it better to use a narrower RPM range of just the peak power and shift more?

[PC_Hater]

Baker now do a 7 speed transmission for a very good reason. Not something I would fit on a road bike.
If you draw your torque curve vs road speed through the gears and then compare that raggedy looking graph with the nice curve you would get if you had an infinite number of gears, what you are seeing is 'gaps' where there is lost power to the rear wheel and you can't get it back unless you change the gearing.

Did I explain that well enough?
Spending some happy hours with a spreadsheet and graphs will allow you to reject many options leaving you with only two or three sensible options at most.
Try the cheapest one first!

(mainly you have to red-line through all the gears on a 4 speed or 5 speed box to get maximum acceleration)

[dynaglide]

snip:  "...Did I explain that well enough?..."

yup - tracking all.  In fact, it was my looking over my waterfall charts that started me thinking about this to begin with.