Gearing a Big CC Motor Street Tire GS for 1/4 Mile?

[TxGSrider]

I didn't know if this was the section to put this question or if it should be in "Racing", because Im asking about street driven bike on street tires going a 1/4 mile....

Anyhoo...

If someone was building a big cc street GS (1260-1325cc), but still running regular width street tires, standard length swingarm, no straps. Would it be better (faster) to gear it to reach redline at the end of the 1/4 mile in 4th gear or 5th gear? I mean I don't see how it would hook up in 1st gear anyhow with the extra torque and Im thinking gearing it for 4th gear would make it easier to hook up and make it better for longer rides.

Any experience/opinions?

 

[rapidray]

Stock is 15/42. I would rather have 15/45 or so in that case. My small tire bar bike is 15/48, to give you a perspective.
Ray.

 

[TxGSrider]

Stock is 15/42. I would rather have 15/45 or so in that case. My small tire bar bike is 15/48, to give you a perspective.
Ray.

So you think that gearing it down a little (higher ratio) would be better on a high torque engine with a (140) street tire? Interesting. I sure trust your opinion/experience Ray, but it sounds a little counterintuitive. Thanks!

 

[posplayr]

So you think that gearing it down a little (higher ratio) would be better on a high torque engine with a (140) street tire? Interesting. I sure trust your opinion/experience Ray, but it sounds a little counterintuitive. Thanks!

I have an over-simplification that is just shy of your over-simplification .

The amount of torque you have has nothing to do with optimum gearing. For example there is a well known principle ( in motor control engineering at least) the the optimum gearing ratio for maximum acceleration is the gear ratio that matches spinning moments of inertia of the motor to the spinning moments of inertia load (idealized for an electric motor, max torque is constant.)

For a motorcycle you would have to figure out the equivalent rotating mass of the engine reflected to the load (or viss versa). Of course doing a quarter mile is more complicated. Torque varies with RPMl. You don't have a fixed gear situation, you can change gears allowing you to keep the motor at maximum HP output assuming shifting is not hurting your ET (4th v.s .5th gear at the traps).

Here might be a way to look at it.

You shift gears to mainly stay in the maximum HP range which maximizes acceleration. The strategy is to switch gears so you move from the upper end of this RPM range in a lower gear to the lower range of RPM in another higher gear(RPM drops when you upshift).

Depending upon how fast you can accelerate you may or may not have time to go through all the gears in the 1/4 mile. How fast you accelerate off the line is based on tractive effort (how much traction you have because anything else the wheel will spin no matter the torque).

many factors, you could probably do a simulation assuming a nominal motor HPv.s. RPM model and vary the rear gear ratio till the ET minimized.
If you had a data logger you could compare RPM,Speed and times to your model and make adjustments and then change the gear ratio in the model for a better estimate.

Anyhow this is how an engineering exercise would go through the process.

 

[TxGSrider]

I have an over-simplification that is just shy of your over-simplification .

The amount of torque you have has nothing to do with optimum gearing. For example there is a well known principle ( in motor control engineering at least) the the optimum gearing ratio for maximum acceleration is the gear ratio that matches spinning moments of inertia of the motor to the spinning moments of inertia load (idealized for an electric motor, max torque is constant.)

For a motorcycle you would have to figure out the equivalent rotating mass of the engine reflected to the load (or viss versa). Of course doing a quarter mile is more complicated. Torque varies with RPMl. You don't have a fixed gear situation, you can change gears allowing you to keep the motor at maximum HP output assuming shifting is not hurting your ET (4th v.s .5th gear at the traps).

Here might be a way to look at it.

You shift gears to mainly stay in the maximum HP range which maximizes acceleration. The strategy is to switch gears so you move from the upper end of this RPM range in a lower gear to the lower range of RPM in another higher gear(RPM drops when you upshift).

Depending upon how fast you can accelerate you may or may not have time to go through all the gears in the 1/4 mile. How fast you accelerate off the line is based on tractive effort (how much traction you have because anything else the wheel will spin no matter the torque).

many factors, you could probably do a simulation assuming a nominal motor HPv.s. RPM model and vary the rear gear ratio till the ET minimized.
If you had a data logger you could compare RPM,Speed and times to your model and make adjustments and then change the gear ratio in the model for a better estimate.

Anyhow this is how an engineering exercise would go through the process.

Thanks Posplayr but I believe you're assuming a perfect traction, perfect control (hard to steer with a wheelie) scenario, and to a lesser extent, a zero lag time for shifting (something that I am nowhere near- lol).


A good and practical example is this: I have a pretty fast ATV (Polaris Outlaw, cam, header, bored carb, etc) and when me and my buddies go "drag race" in the dirt, I accelerate much faster by launching in 2nd gear (it doesn't spin as much or as long and the front end tends to stay down). The gear reduction isn't as much and is better for launching. Now probably if I stretched the quad and put slicks on it and raced on concrete, launching in 1st would be allot faster as I could take advantage of the better gear reduction.


This is really more of what I was thinking of.

 

[Guest]

15/45 or 15/48 is great on a bike with wheelie bars and probably an air shifter. However my experiences on street type bikes is the lower gearing makes them wheelie prone and harder to be consistent. With a big bore street tire bike I would try stock gearing or taller may help keep front on the ground and if you are foot shifting fewer shifts is better. Unless you have a slider clutch then you could be more aggressive with gearing. But that's just my opinion I could be wrong

 

[posplayr]

Thanks Posplayr but I believe you're assuming a perfect traction, perfect control (hard to steer with a wheelie) scenario, and to a lesser extent, a zero lag time for shifting (something that I am nowhere near- lol).


A good and practical example is this: I have a pretty fast ATV (Polaris Outlaw, cam, header, bored carb, etc) and when me and my buddies go "drag race" in the dirt, I accelerate much faster by launching in 2nd gear (it doesn't spin as much or as long and the front end tends to stay down). The gear reduction isn't as much and is better for launching. Now probably if I stretched the quad and put slicks on it and raced on concrete, launching in 1st would be allot faster as I could take advantage of the better gear reduction.


This is really more of what I was thinking of.

Sorry for making it so complicated, so to clarify I will simply say that my reference to "tractive effort" is a direct reference to the situation of wheel slip and not perfect traction (I'm not even going to ask what you think is perfect).

In my analysis I specifically pointed out that it would only apply when not "TE limited"(such as when you are in 4th gear crossing the 1/4 mile).

In your dirt hill climb example, you are clearly spinning wheels and therefore are TE limited. In a TE limited situation best traction usually occurs at a fixed percentage of spin. Too much spin and TE falls off, not enough spin and not enough of a paddle wheel effect. Your gearing change works because it is to easier to control wheel spin in 1st gear, and 2nd is easier becuase there is not as much torque available at the throttle.

So your fast ATV example is completely different to BB GS on pavement, unless you planning on a 1/4 drag race in the rain.

It is clear from all of the experienced recommendations that you should set gearing in opposite to your example which is a direct consequence of the bikes NOT being TE Limited on a whole. This was the crux of your dilemma in the first place.

 

[posplayr]

15/45 or 15/48 is great on a bike with wheelie bars and probably an air shifter. However my experiences on street type bikes is the lower gearing makes them wheelie prone and harder to be consistent. With a big bore street tire bike I would try stock gearing or taller may help keep front on the ground and if you are foot shifting fewer shifts is better. Unless you have a slider clutch then you could be more aggressive with gearing. But that's just my opinion I could be wrong 

You are not wrong, it is all about traction control so you get as maximum traction without flipping the bike where traction is limited on top of the gas tank.

 

[TxGSrider]

15/45 or 15/48 is great on a bike with wheelie bars and probably an air shifter. However my experiences on street type bikes is the lower gearing makes them wheelie prone and harder to be consistent. With a big bore street tire bike I would try stock gearing or taller may help keep front on the ground and if you are foot shifting fewer shifts is better. Unless you have a slider clutch then you could be more aggressive with gearing. But that's just my opinion I could be wrong 

Yes, that was what I was thinking too. It may not do me much good to have the additional power to the ground if I just spin off the line and when I do get traction, I can't control the bike due to wheeling.

 

[rapidray]

Then LEARN how to ride. I've been drag racing & street racing big GSs for over 30 years & experience is what makes it all happen. If you don't have experience drag racing, & LAUNCHING, a GS with torque, get a 550 or 750 to learn on so you don't hurt yourself. 15/45 works perfect for me on a street 1100. No wheelie issues or tire spin unless I want it to happen.
Ray.

 

[Guest]

I agree with you Ray. My suggestion were about maybe making it easier to learn what you already know how to do. How quick or fast a street bike runs is about 95% rider

 

[TxGSrider]

Then LEARN how to ride. I've been drag racing & street racing big GSs for over 30 years & experience is what makes it all happen. If you don't have experience drag racing, & LAUNCHING, a GS with torque, get a 550 or 750 to learn on so you don't hurt yourself. 15/45 works perfect for me on a street 1100. No wheelie issues or tire spin unless I want it to happen.
Ray.

Thanks Ray, I will listen to your advice. I may also be contacting you about a heavy duty clutch basket for an 1150 before long.