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Old March 25th, 2017, 05:02 AM   #1
Schweedo
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Won't redline in 6th gear

Hi everybody, this is going to be my first full season of riding so I'm still new to this. I'm not sure if this is normal or not but I can't get my bike to redline in 6th gear, it only gets up to 11.5-12k. I did replace my cdi with one from a 88-94 to fix my tach problem and that also advanced the timing but that's the only mod. I'm aware that the old cdi has a higher rev limit but I'm wondering why I can't even get to 13k, it will get there easy in the other gears.
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Old March 25th, 2017, 06:17 AM   #2
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It doesn't have the power in top gear to overcome aero drag and keep revving.

Each gear has less torque multiplication than the last, and at a certain point there is just not enough power/torque to keep accelerating as drag increases.

If you didn't care about cruising RPMs, you could gear it "down" until it would reach the max revs in top gear. That would make it accelerate quicker all around, but would have it screaming (more than it already is) anytime you were on the hwy.

Some people go the other way and gear "up" to give a lower RPM cruise, but you do sacrifice acceleration and may actually have a higher top speed in 5th gear(at max revs) that 6th gear.
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Old March 25th, 2017, 06:46 AM   #3
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Jay, you really don't sacrifice much acceleration with taller gearing, other than off the line, since you can still keep the engine in the power band. The main difference is you'll shift at a higher road speed in any particular gear, and as Chris has noted, you won't necessarily reach redline or possibly even top speed in 6th. One way to look at is you won't have as much acceleration in any particular gear, but you make up for it by being able to stay in that gear longer than with stock gearing. Similarly, you don't really gain acceleration with lower gearing, other than off the line, and you'll shift at lower speeds in each gear.

Top gear roll-ons are a different story, of course.
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Old March 25th, 2017, 07:01 AM   #4
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Quote:
Originally Posted by Triple Jim View Post
Jay, you really don't sacrifice much acceleration with taller gearing, other than off the line, since you can still keep the engine in the power band. The main difference is you'll shift at a higher road speed in any particular gear, and as Chris has noted, you won't necessarily reach redline or possibly even top speed in 6th. One way to look at is you won't have as much acceleration in any particular gear, but you make up for it by being able to stay in that gear longer than with stock gearing. Similarly, you don't really gain acceleration with lower gearing, other than off the line, and you'll shift at lower speeds in each gear.

Top gear roll-ons are a different story, of course.
tripplejim , could you tell me the different story, i;ll tell you what i did and why, tell me if im wrong, on our longest straight i could not achieve max speed, so i geared down in hopes of having more roll on out of the corners.
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Old March 25th, 2017, 07:16 AM   #5
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tripplejim , could you tell me the different story, i;ll tell you what i did and why, tell me if im wrong, on our longest straight i could not achieve max speed, so i geared down in hopes of having more roll on out of the corners.
Oh, Schweedo was talking about road riding, and being able to reach redline in 6th gear. Clearly track riding has much different gearing requirements. My limited track experience made it obvious to me that one tooth difference one way or the other on the front sprocket can make a huge difference in where shift points occur relative to turns, etc..
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Old March 25th, 2017, 07:31 AM   #6
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The change in spark timing does not match the original valve timing.
As a result, the delivered torque at 12K rpm is reduced.
Lower gears can make the engine spin faster because the resisting load of drag is much lower than it is at top speed.
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Old March 25th, 2017, 09:16 AM   #7
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Quote:
Originally Posted by Triple Jim View Post
Jay, you really don't sacrifice much acceleration with taller gearing, other than off the line, since you can still keep the engine in the power band. The main difference is you'll shift at a higher road speed in any particular gear, and as Chris has noted, you won't necessarily reach redline or possibly even top speed in 6th. One way to look at is you won't have as much acceleration in any particular gear, but you make up for it by being able to stay in that gear longer than with stock gearing. Similarly, you don't really gain acceleration with lower gearing, other than off the line, and you'll shift at lower speeds in each gear.

Top gear roll-ons are a different story, of course.
I don't know Jim - by gearing "down" you are increasing torque multiplication in every gear. That translates into more acceleration and quicker revving due to the increased multiplication.

You will need to shift sooner and more often than you do with taller gearing, but you will gain acceleration. It's allowing the engine pull less load because of the benefit of torque multiplication.

That's how I see it, but I'd be interested in hearing other opinions.
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Old March 25th, 2017, 10:37 AM   #8
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Think of it in a simpler way: Maximum acceleration happens when you produce the most horsepower from the engine and send it to the wheels. As long as you're near the HP peak, you're getting maximum acceleration.

Yes, you're right that in any particular gear, you acceleration will be greater with lower gearing, but I'll still be in 1st after you've had to shift to 2nd, so at that time my overall gearing is lower, so I'll accelerate more than you. Then I shift and we're both in 2nd and you're gaining ground, then you shift to 3rd, I'm still in 2nd, I gain ground, etc..
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Old March 25th, 2017, 10:47 AM   #9
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so acceleration out of a corner is awash, i would be better served picking my gearing, for my shift points coming in or going out of a corner. am i correct in this assumption ?
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Old March 25th, 2017, 11:07 AM   #10
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so acceleration out of a corner is awash, i would be better served picking my gearing, for my shift points coming in or going out of a corner. am i correct in this assumption ?
Exit.... always favor on the throttle vs on the brakes. Good pads are cheap.
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Old March 25th, 2017, 01:02 PM   #11
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Thanks for all the replies guys, I was just curious and making sure it was nothing to be concerned about. I figured it probably had something to do with the timing change but I couldn't remember if it ever actually hit redline with the original cdi either. Maybe I will put the original back in and see what effect it has. I did feel like I gained some power in the mid range after I put the 88-94 cdi in.
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Old March 25th, 2017, 02:12 PM   #12
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Quote:
Originally Posted by Triple Jim View Post
Think of it in a simpler way: Maximum acceleration happens when you produce the most horsepower from the engine and send it to the wheels. As long as you're near the HP peak, you're getting maximum acceleration.

Yes, you're right that in any particular gear, you acceleration will be greater with lower gearing, but I'll still be in 1st after you've had to shift to 2nd, so at that time my overall gearing is lower, so I'll accelerate more than you. Then I shift and we're both in 2nd and you're gaining ground, then you shift to 3rd, I'm still in 2nd, I gain ground, etc..
Well, there's a bit more going on than that -

http://www.sportrider.com/shift-points#page-4

I still say lowering the gear ratio will result in more overall "Driving Force at the Rear Wheel".

Here's the formula -

**Driving force (lbs.)=T x G x P x S x R
T=torque (ft.-lbs.)
G=gear ratio
P=primary reduction ratio
S=final drive ratio
R=radius of rear tire (ft.)
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Old March 25th, 2017, 02:59 PM   #13
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Yes, now integrate the driving force over distance for both sprocket sets, for all the gears, assuming you shift at redline or some other reasonable RPM each time. The difference will mainly be from zero mph to the first shift point.
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Old March 25th, 2017, 06:17 PM   #14
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Cool

Quote:
Originally Posted by Schweedo View Post
.......I did feel like I gained some power in the mid range after I put the 88-94 cdi in.
It is normally a trade-off rather than a miraculous gain.
Same with sprocket's combinations.
What you gain at certain rpm's, you lose at some other.

"Golden Rule of Mechanics: Whenever you use ramps, levers or belt-pulleys, screw-winches or or mechanical equipment to lift an item, the rule says: "The less force you need, the more distance you need"."

Playing mainly with the timing (and throttle diameter for carburated bikes), the manufacturer tries achieving a middle compromise, good for low, middle and high rpm's, but not great for any particular range.

Old bikes had engines with flat performance, reason for which three combination of gears were enough.
The engine of the Ninja was designed to be a peak-performance type, good for 9K to 12K rpm's and mediocre for anything out of that range.
For that reason we need to use six gears to keep the engine spinning at its best for any speed of the bike.

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Old March 25th, 2017, 08:33 PM   #15
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Yes, now integrate the driving force over distance for both sprocket sets, for all the gears, assuming you shift at redline or some other reasonable RPM each time. The difference will mainly be from zero mph to the first shift point.
...and why exactly do you think you need to integrate??
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Old March 25th, 2017, 09:32 PM   #16
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As I understand it, Jay is wondering what the result of different final drive ratios really makes in overall acceleration through all the gears. He knows that in any particular gear, a lower final drive ratio makes for more acceleration than a higher one, and I'm pointing out that when you're going through many gears, the most important thing is to keep the engine near peak HP.
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Old March 26th, 2017, 07:07 AM   #17
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Kinda specific to drag racing, but some interesting comments about gearing selection from racer/tuner/instructor Ryan Schnitz -

http://ryanschnitz.com/school_advice.php
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Old March 26th, 2017, 09:25 AM   #18
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As I understand it, Jay is wondering what the result of different final drive ratios really makes in overall acceleration through all the gears.
I understand it differently than you do.

First question posed was why he couldn't top out his highest gear on a straight away. Answer is because once you go past peak HP, the output drops off dramatically, see a reputable dyno graph from Area P below:

Assuming we have unlimited tarmac and the correct final drive, maximum speed will be attained when all your drag forces and road load equals the tractive force of the vehicle at peak HP in the highest gear. After adding power, this is what Racer X has spent tons of time getting right for his land speed record runs. More likely than not, the factory gearing is not optimized for attaining top speed because this is not the purpose of the bike, so peak HP engine speed will not correlate with the maximum possible road speed. Some 250's are able to hit the limiter in 6th from the factory, some bikes are not. Jetting, altitude, road grade, wind, etc. all contribute to this.

Second question posed was how to select final drive gearing and whether it was more important to plan final drive based upon shift points for corner entry or acceleration out of the corner. I feel that is completely a personal preference item which can be determined by time in the saddle and experimentation.



Then you brought up the topic of multiple gear accelerations. Despite not being what he asked about (at least not directly unless I just suck at reading ) it's a great topic that deserves some more digging!
Quote:
Originally Posted by Triple Jim View Post
He knows that in any particular gear, a lower final drive ratio makes for more acceleration than a higher one, and I'm pointing out that when you're going through many gears, the most important thing is to keep the engine near peak HP.
Just so we're all talking the same language, let's use consistent convention. Longer gearing has a lower ratio number. Shorter gearing has a higher number. We're talking about a gearing reduction for the final drive, so a higher number designates a greater reduction. For example, an optional 4.10 rear end in a Wrangler is a larger reduction (shorter gearing) for more torque at the wheels compared with a 3.90 rear end

Some good reading here on shift points; it's not 100% as simple as you say it is. It's all about torque at the wheel for a given road speed (related to engine speed by the overall drive ratio) compared with the torque at the wheel for the next gear at that same road. When they are equal, the optimal shift point has been reached. The idea for optimizing acceleration for a multi-gear acceleration run is to always select the transmission gear that results in the highest wheel torque for the given speed. This is why the ideal shift point for 1st to 2nd is not the same as the ideal shift point from 2nd to 3rd is not the same as the ideal shift point from 3rd to 4th, and so on and so forth. As the difference between transmission gear ratios get closer, so does the difference in RPM from the current gear to the next gear, and therefore so does the difference in wheel torque as you change gears. This is why we have unique ideal shift points for each shift. It's not as simple as always keeping the engine at the speed that results in peak HP output.

That said, for a power limited vehicle like our baby ninja, maximum instantaneous acceleration will be experienced at peak HP in the lowest gear. This is not always the case for all vehicles, as you also have to consider the cases of being traction limited or (if it's a moto) being wheelie limited.

Quote:
How is this the correct method? For any point in time, the highest acceleration is acheived with the highest torque at the wheels. Gears multiply torque, so generally, the lowest gear provides the highest acceleration. The problem is, you can't go very fast in 1st gear, so you need to upshift to go faster, even though it means lower torque output. As long as the output torque in 1st gear is higher than output torque in 2nd for a given road speed, you should stay in 1st. Of course, you will soon run out of rpms and be forced to upshift.

The same holds true for 2nd. As long as the output torque in 2nd is greater than what you would get from 3rd for any given road speed, you should continue to accelerate in 2nd. Depending on your car's torque curve and gearing, there may be a point where, for a given road speed, you can get the same torque in 3rd as you are getting in 2nd. This is where you would want to shift, as the torque curve is falling, and because of the multplying effect of gears, the output torque in second is falling faster than it would be in 3rd. In this case, more torque is acheived in 3rd gear, so you should use it.
http://glennmessersmith.com/shiftpt.html

This agrees with all automotive text I've used. This is why fancy dual clutch transmissions in cars are king of acceleration; they can go immediately from one gear to the next, always optimizing the gear selection for wheel torque with significantly reduced time penalties associated with gear changes. A sport bike with sequential gear box and a quick shifter that cuts spark is very good as well.

For more reading (and the equations to calculate everything we've talked about in this thread) grab a copy of Fundamentals of Vehicle Dynamics by Gillespie. It's old, but good. Chapter 2 walks you through acceleration and how to calculate maximum possible acceleration. As long as we're not talking about cornering, most of it applies to bikes.





Anecdotally speaking, I got my fastest measured 0-60 times with 14/47 final drive gearing but there were 2 shifts in there. I used clutchless shifting to reduce shift times, but I've always wondered whether 15/45 or 15/43 gearing is faster because they will touch 60 mph at red line in 2nd. Also, I never tucked the mirrors back. As far as aero is concerned, the mirrors are the single biggest piece of equipment on the bike that can be easily removed. Often times, automotive manufacturers will remove mirrors to play with their fuel economy testing numbers. There's been big pushback from the EPA to stop the use of this tactic because road users are required to keep their mirrors on.
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Old March 28th, 2017, 06:39 AM   #19
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Since I swapped out to 15/42 I've found that 6th has nothing power wise, it didn't really have much with the 14/43 though, it's brilliant for sitting on the hwy in 6th doing 100kph at 7000rpm, but, if I need to overtake I need to shift down.

The 15/42 is good for blasting around from 1st to 5th though, 6th is exclusively for commuting.

My bike is just a daily hack though, it still pulls slowly in 6th but only from a shift from redline in 5th, but, you could have a coffee and smoke while waiting to gain 1000rpm.

It's no 600 or 1000 but it's not bad for a little 4 stroke 250.
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Old March 28th, 2017, 07:10 AM   #20
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The 15/42 is good for blasting around from 1st to 5th though, 6th is exclusively for commuting.
15/42 puts your 5th gear where your original 6th was, effectively giving you a "7th gear", so it makes sense that you don't use your current 6th as often as with stock gearing.

Choneofakind, as I said, my post about integrating was in response to Jay's posts (JKV45). I think the original poster, Schweedo, had gotten a couple good answers by then.
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Old March 28th, 2017, 09:26 AM   #21
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My question has been more than answered, some good info in here though. Thanks guys!
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Old March 28th, 2017, 10:47 AM   #22
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