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Why would a bike not shift under load?

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I'm looking for info on transmissions. Why would a newer bike (under 15hrs) shift fine except when wide open going from 3rd to 4th? (The clutch is being used.) Would also like to know how the transmission works with the forks and all. What does each fork do?

Edited by zziptie

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Don't shift under power, the transmission is not designed for that. It will break. You have to blip the throttle to create a little bit of slack.

 

The transmission is a continuous mesh, which means all the gear are always engaged, and the shift "fork" swaps from one gear to the next.

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if you are talkng about not using the clutch, then ye I wouldn't suggest that, but I shift all the time staying on the gas while using the clutch, it's better on long stretches.

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Example is wide-open and using clutch between shifts.

 

Ok, in that scenario it is often a worn clutch basket, poorly adjusted lever, or too thick a clutch oil - your clutch doesn't disengage fully/drags too much, so it's like you are trying to shift in gear.

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if you are talkng about not using the clutch, then ye I wouldn't suggest that, but I shift all the time staying on the gas while using the clutch, it's better on long stretches.

 

The topic of clutch vs no clutch has been beat to death, but here's my .02 cents - I shift with the clutch when normally riding, but if I am on a technical climb running out of steam, or need a gear NOW for whatever reason, I will definitely bang down a gear clutchless without any hesitation.

 

It might wear your tranny a bit, but a lot of guys never use it and have quite decent transmission life. It's not good but it isn't that bad either. But definitely don't try and do it while under power.

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There is NO transmission that will shift under a load! bike car or truck you have to take load off it by letting off or using clutch. Take my semi no way in hell you can even pull it out of gear with throttle on.

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You don't need to use the clutch to take the load off.  Hold the shifter up with your foot and chop the trottle quickly.  The chop will unload the tranny and the transmission will slip into the next gear.  Downshifts are a litte more difficult to do and the clutch will guarantee a successul downshift but it is absolutely not necessary for upshifts.

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I'm looking for info on transmissions. Why would a newer bike (under 15hrs) shift fine except when wide open going from 3rd to 4th? (The clutch is being used.) Would also like to know how the transmission works with the forks and all. What does each fork do?

I can see by some of the advice you are being given transmission repair will be a regular part of your life. let off pull in clutch then shift its that simple lots of riders can ride fast using that method, and so can you. shifting with out the clutch or holding the throttle pinned while pulling in the clutch and shifting can cause the shifting fork to wear and will make shifting become increasingly difficult

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Example is wide-open and using clutch between shifts.

 

One benefit of my hydraulic clutch is that I can use one or two fingers to disengage the clutch plates, I feather the clutch all the time.  This makes it easier to ride with my habits of using the clutch every single time I shift, up or down.  And I've never had tranny issues with any of the three bikes that I've pounded since 2004. 

 

But you must roll off the throttle when you pull the clutch to shift.  My brother hopped on my bike once before it was warmed up, took off revving the bike to the moon and didn't roll off the throttle to shift.  It sounded awkward and damaging to the machine.  No professionals do that, they always keep the motor in the "sweet spot" which requires chopping the throttle between shifts.

Edited by LovingOffroadPain

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The topic of clutch vs no clutch has been beat to death, but here's my .02 cents - I shift with the clutch when normally riding, but if I am on a technical climb running out of steam, or need a gear NOW for whatever reason, I will definitely bang down a gear clutchless without any hesitation.

 

It might wear your tranny a bit, but a lot of guys never use it and have quite decent transmission life. It's not good but it isn't that bad either. But definitely don't try and do it while under power.

Right, this isn't about "clutch or clutchless" Everybody will have their .02 cents. I was reading what somebody posted, and not knowing if he was using the clutch or not. 

 

 I have a much harder time shifting while staying on the RMP on my 96' RM250, but I am ok if I get off the gas real quick.

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So you guys are saying "some bikes wont shift while wide open with a blip of the clutch", "with these bikes you have to quickly ease off the throttle to shift" and that's okay? This would be news to me. I have ridden many bikes and never come across having to leave off the throttle.

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So you guys are saying "some bikes wont shift while wide open with a blip of the clutch", "with these bikes you have to quickly ease off the throttle to shift" and that's okay? This would be news to me. I have ridden many bikes and never come across having to leave off the throttle.

 tbh I do think there is a problem, you should never have to get off the gas to be able to shift with the clutch. what is it, and the year(unless I missed it somewhere) my 96' is old, you probably could do it before, or it could be the way I am doing it, but I can't shift without mis-shifting, unless I get off the gas. 

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For a bike under 15 hours to not want to shift under load from 3rd to 4th with clutch indicates a problem.  To answer your question about the forks, they slide the gears from one position to another.  They are moved by a drum that has grooves cut into it.  The force needed to move a gear from one position to another should be minimal, even under load.  The reason is, even with a lot of vertical force on the dogs, they should easily slide laterally away from the gear they are engaged with.  If they don't, they are either damaged or not properly lubricated.  

 

MXA has pointed out in every recent test that the Kawi's don't like to shift under load, which means the test riders needed to use the clutch or back off the throttle more than they would like (their expectation is that they would never have to do that).  The controversy about whether you have to use the clutch or not is not controversial.  You don't.  Below is a 2011 KXF250 transmission that has been abused by rarely using the clutch to shift while rolling.  It looks great and works perfectly.  Your issue is either a damaged gear or drum, or it could be you just need to try a different oil.  The oil would be easy to test.  It would be helpful to know what bike we are talking about, too.  

 

I promised some others I would post a current picture of an abused transmission after my next tear-down.  Here it is.  

 

CLEANKXFTRANS090213_zps949094f3.jpg

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Here is a pic of an assembled transmission.  I outlined the two forks.  Each fork slides over a guide post.  At the base of each fork is a tab that goes into the drum in the middle.  The drum has grooves machined into it.  When you shift, you are turning that drum.  As the drum turns, it moves both forks from side to side (in the picture it would be up and down) to disengage one pair of gears and engage another.  In the picture above, you can see that some gears have tabs (dogs) that fit into recesses in other gears.  Notice how much room there is for the tabs of one gear to slide into the recesses of another?  That's one reason you don't need a clutch to shift a dirt bike.  The other reason is all of the gears are always spinning all the time at close enough to the same speed that you would have to work very hard to cause damage.  

 

In your case, either the dogs are not sliding out of their partner gear smoothly, or the gear itself is not sliding easily on the grooves cut into the shaft.  It could be a hang up on the drum I suppose.  Anyway, it seems your bike may be new enough to take back to the dealer for evaluation.  Have you considered that?

 

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Hey thanks for the post. I bought the bike from a guy that works at the dealer, so i cant just take it back. The bike was pretty much new it's a yz85 and his kid wouldnt ride it. I keep fresh oil in it. My son was having trouble shifting like i explained so i jumped on and tried to do a holeshot and it didnt go from 3rd to 4th. Its shifts smooth when you are not wide-open though. Im just tryjng to understand transmissions to figure out what's wrong.

-im using 10w50 would a 90w oil help?

-do some transmission shift smootnher with more hours?

-which shift fork would engage 4th gear?

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A friend of mind, who is a amsoil distributor, bought some of this 4 stroke oil, that is good for tranny, and transmissions. he switched from his old oil, to this oil in his street bike, and he said he noticed right away that it made it shift better. I can't say this personally, I didn't notice my ktm shifting hard, or bad without it, but I use it.

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it didnt go from 3rd to 4th. Its shifts smooth when you are not wide-open though.

A lot of different conversations going on here, but I'm going to focus on your problem directly.

If a bike shifts fine when not under load, but won't shift when revved with the clutch out, the clutch is the problem, not the transmission.

The clutch is supposed to disengage the engine, creating an identical situation to "not under load".

Simple as that.

BTW super tech 10-40 conventional oil from Walmart is one of the best clutch oils I've tried, and cheap as dirt.

Edited by mudguy

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Lots of folks swear by Rotella T http://www.walmart.com/ip/14958327?wmlspartner=wlpa&selectedSellerId=3&adid=22222222227009452554&wl0=&wl1=g&wl2=t&wl3=13689595630&wl4=&wl5=pla&wl6=34445874910&veh=sem

You asked for an explanation of how the transmission works so I got into more detail. I doubt the clutch is the problem only because you indicated 3rd to 4th is the only sticking point. I have had many dirt bikes and all of them shifted under load just fine. Yours should, too.

There is no question some oils perform better for some people. You started with some brand of 10/50 and that is not correlating with the results you want. Before you tear into the motor, just try another. I suggested the Rotella because lots of people swear by it and it is cheap. Others with your machine like BelRay Gear Saver. It is one variable that is easy to control. Let us know if it works.

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