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how to compression lock up?

Discussion in 'New Riders and Riding Tips' at netrider.net.au started by livingstonest, Oct 17, 2005.

  1. a few threads have mentioned compression lock up and was wondering how i can avoid it.


    when i down shift i don't blip the throttle, what i do is let it out slowly, usually breaking at the same time.

    does compression lock up usually happen when you bang down the gears quickly and the revs are too high for that gear? ie close to redline?
  2. It happens when you change to a lower gear and the engine revs are too high for that gear.
  3. i've changed down when i meant to change up (mind was in other places) haven't done that again, it's a bit of an eye opener :D

    (luckily i wasn't too high up in the revs at the time)
  4. yeh, i've only got it during sloppy clutchless 2nd-1st shift at high revs
  5. Compression lockup happens when the engine exerts too much braking force on the back wheel for the amount of traction you have.

    You're riding along at 10,000rpm in gear. You pull in the clutch and let the engine coast back to idle. You then let go of the clutch quickly.

    Two things start to happen:
    1. The engine revs are forced to rise and it resists it.
    2. The resistance starts slowing you down.

    If this happens progressively over several seconds, things should be fine. But by droppng the clutch like this, something has to give, your engine either has to instantly rise up to 10,000rpm, or your back wheel has to slow down to ~1/10th of its speed. The engine can't go from 1000rpm to 10,000rpm in a split second, so the back wheel cops a massive braking force until the engine has time to get to the right revs. When the braking force is too much for your grip, this is a compression lock up.

    So how do you avoid it? The best way is to match revs. Failing this, be gentle with the clutch, and conservative with your gear choices until you're comfortable with matching revs.

    I don't have a bike yet and just got my L's, but matching revs is something most advanced drivers do in cars. You no doubt have heard of heel-toe downshifts, this is where the driver blips the throttle with his heel while braking with his toe to stop compression lockups while downshifting for a corner. It's the exact same principle.

  6. As Justin said above, the best way of avoiding it is to blip the throttle on a downshift. It's a lot easier on a bike than in a car, as your hands are far more sensitive than your feet! You'll find there's a way you can evenly apply braking pressure and blip the throttle as you down-change - it'll take a while to get the hang of properly (read a month or so of riding everyday), but it's a worthwhile skill, IMO. Smoother braking for when you've got a pillion too.

    Some newer bikes have 'slipper' clutches, which limit the amount the of negative torque applied to the rear when you're down shifting (by letting the clutch slip a little, funnily enough). These prevent compression lockup also.
  7. ohrite, thanks alot that was excellent!

    i've never thought about it before but i guess i've always naturally avoided it. ie if i'm at high revs and pull clutch in, i wait till revs drop before i gently let clutch go.

    but i'll really try to blip throttle from now on, i'll see how that goes. i've always just breaked and let clutch go slowly till i feel it bite then let it go all the way. also i never skip gears going down unless i'm absolutely sure, ie very slow speeds.

    thanks again!
  8. You're welcome, glad the explanation made sense.
    Getting comfortable with blipping the throttle is definitely the way to go. What I did in cars to practice rev matching was to drive at low revs, then clutch in, drop down a gear or two, throttle the revs up to what felt about right, then start letting the clutch out. Repeat til smooth.
    Practicing getting the match right and smooth in a straight line at cruising speed is a lot safer and easier than trying to do it while braking hard for a corner. Once it's comfortable to do it while crusing, then I'd try to do it while braking.

    I can't see a reason why it would be different on a bike to in a car, but please, correct me if I'm wrong guys, I don't even have a bike yet,

  9. Do you mean you wait til the speed drops?

    What we want is the clutch not changing the revs. If releasing the clutch means the revs will be 9,000rpm, we want to make the revs 9,000rpm before we release the clutch.

    Unless I'm taking you the wrong way, waiting for the revs to drop before letting the clutch go is the opposite of what we're looking for.

  10. When you change to a lower gear and the engine revs are too *low* for that gear no?

  11. It's a pretty easy technique to aquire, you simply give the throttle a quick squirt when you pull the clutch in......... that's it in a nut shell.

    You already click the gear lever down automatically as you pull the clutch in with a normal gear change, simply blip the throttle at the same time. As you release you clutch youve already backed the throttle back off ...,, ( remember a blip is a very quick open and close of the throttle )

    Remember, changing to a lower gear means the engine needs to be spinning faster ( you hear this everytime you downshift - the revs go up ), blipping the throttle raises the engine speed to equal or higher than what the back wheel needs, which avoids the rear wheel forcing the engines revs to rise sharply.

    You dont need to be at racing speeds for this to happen, it can happen in many situations, Graeme "gsxr1000" was caught out a few months back on the way home from work, he was taken by surprise because he wasn't
    gunning it.......... and yes he is a fast experienced rider.

    It's only needed on downshifts by the way.
  12. It's rather...when changing into a lower gear at a road-speed too high for that gear and the revs aren't high enough to match, therefore the backwheel trying to "drive" the engine, struggling initially as the engine has too much braking but once mated through the connection engine-rearwheel is stronger than rear-tyre-to- road connection.
    Rear tyre breaks traction as a result, momentarily, until the 2 "connections" even out, the tyre-road combo winning in the end and all is back to normal.
  13. So basically compression lock up is when the rear wheel locks up??

    This happens to me alot (Daily) when i downshift, it doesn't really bother me at all but if it could damage the engine then mabye i should practice blipping the throttle.

    I have tried blipping a couple of times but when i do it and release the clutch the bike takes heaps longer to slow down (Is this normal, should i just allow more braking distance), i use the same method as ratbag described too.?

    Excuse my newbieness but i love my bike and the last thing i want to do it damage it.

  14. yeah i mean the speed. i only downshift when i'm slowing down or if i'm in too high a gear for the speed.

    so basically if i'm in 4th, i slow down so that its the right speed for third, clutch in, shift down, slowly clutch out. so i haven't been matching revs. keep forgetting to try though, old habits you know
  15. When the engine makes it skid, yeah.

    I'd be more concerned with the danger than with the bike, personally. Sure, you can handle it in a straight line, but it's a bad habit to get into and could put you off if the bike was unstable when you did it.

    You back down the gears for safety, not for braking distances. You do the majority of your braking with the front wheel, but if you're compression locking the back every time you downshift while braking, you're blowing out your braking distance because of the skidding. Engine braking is good, but it's not to make you stop fast, the fastest way to brake is to use the brakes!

    Constant compression lockups put stress on the drive train, but I don't know enough about bikes to say whether it would cause a problem in the long term. In a car though, constant compression lockups will certainly end up breaking something.

  16. Yup. The more engine-braking a given engine's got, the easier it can happen. 4-strokes are more prone to it than 2-smokes.
    Biggish V-twins more than high-revving 4-pots, big 4-stroke singles are the usually the worst in that department. (or the best, depending on how one looks at it :) )

    Then you're downshifting too early....wash-off roadspeed via the brakes first, then downshift.
    Engine damage isn't the real problem, excessive wear on chain-sprockets (drivetrain) and gearbox are more like it. Cush-drive rubbers/ output-shaft bearings and cogs in the gearbox get enormous strain through the abrupt reversal-of-load (change from pull-into-push-into pull-mode, when engine drives chain, then chain drives engine, then engine drives chain again, those changes being under load either way).
  17. I used to get a compression lock-up every day on the way to work from my old house. I'd scream down my street poking at the top end of first gear, then shift up to second for a nice quick roundabout at the end of the street. The revs would be up so high in first that when I switched to second and rolled off the throttle, I'd get a nice little squirm.

    It used to wake me up for the commando course of unpredictable drivers on the way in to the office...
  18. Sounds like your using the engine to wash off alot of your speed, instead of using your brakes.
    You will find on a nice stretch of road, simply rolling off the throttle and using the "engine braking" to lower your entry speed is ideal and there is no need to touch your brakes, while rolling back on as you exit the corner.

    Raising your overall speed on any road and trying to gun it through the twisties requires you to use your "brakes".
    Washing off speed in a controlled manner under hard braking INTO a corner requires you to keep the bike stable, MOST of a bikes stability comes from the rear wheel spinning.
    Locking the rear wheel via compression lock-up through a downshift without blipping the throttle takes away a great deal of the bikes stability, this is not a good idea while cranked over in a corner under any conditions.

    Simply blipping the throttle is good practice for any downshift at moderate speeds.
    Once in the habit and your of a mind to play "boyracer" out in the twisties, you then need to practice pulling on the brake lever with constant pressure while blipping the throttle, this requires you to let you finger/fingers slip around the lever as you blip the throttle.

    Cheers ratty