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What is compression lock-up?

Discussion in 'New Riders and Riding Tips' started by Ckramer, Feb 23, 2008.

  1. Remember reading somewhere that if you don't downshift properly you'll get "Compression lock-up". (which is supposed to be bad?)

    Can't find anything about that term in google.

    What is that "compression lock-up"? :?
  2. It's when you downshift too quickly and the engine braking at high revs becomes so strong that the rear wheel starts to skid and slide. You can make it happen (provided you don't have a slipper clutch) by riding high in the revs in third gear, then banging it down to first and dropping the clutch.
  3. This is what I had to say on another forum some time ago. I hope it helps. :)

    ....... just so there's no confusion to new riders I'm going to explain compression lock ups.

    RPM- Revolutions per minute (the number of times your engine turns over in 60 seconds)
    Road Speed- Speed the vehicle is traveling in relation to the road. (kph as measured by your speedo)

    Think about the gears on your push bike. To travel at the same speed in different gears, you have to turn the pedals at different speeds. Your gearbox and engine on a motorbike work the same way. If you're traveling at 5000 rpm in 3rd gear and shift down to 2nd, then in order to travel at the same speed you're engine speed may need to increase to about 6500rpm. (just as you would need to turn the pedals faster on a push bike)

    If you don't match engine speed to the gear selected at a given road speed when down shifting, the road and engine will fight to make the wheel match their own velocity. Because the connection between your gearbox and rear wheel is stronger than the grip between your rear tyre and the road, the engine will win. The drive train will grab and slow the wheel faster than the bike can slow down, causing the rear tyre to skid.

    So why is it called a compression lock up? When a piston moves upwards in a cylinder it compresses a mixture of fuel and air before the spark plug ignites it. The more compression achieved, the stronger the downward force exerted on the piston when fired. Compression is what makes it hard to turn over an engine by hand and is responsible for making it difficult to push a bike forward while it's in gear. Pull in the clutch (breaking connection between rear wheel and engine) and you can push it easily. Compression is also the force working against the road when the engine and road fight to determine rear wheel speed.

    Is a compression lock up a bad thing? Well if it happens easily, it means the rings on your pistons are probably in good order. However, apart from the flat spots you put on the rear tyre, losing traction means losing control and ability to steer the bike. These outweigh any reassurance you get regarding the condition of your engine.

    Can I prevent compression lock ups? Yes. You can blip the throttle.

    What the hell does "blip" mean? As you release the clutch when down shifting, give the throttle a short and swift handful to increase the rpm. As the drive train is engaged, then immediately start to roll off again. Getting it smooth can take some practice but the idea is to let the RPM match the the road speed in the gear chosen and allow engine braking to slow the bike without locking the wheel. "Blipping" is what creates the Zum zummm zummm sound that you hear when vehicles are down shifted by a skilled driver/rider through the gears in quick succession.

    Is it ok to simply brake until the rpm drops then down shift? It won't hurt the bike, but it is not good riding practice. By doing this, you reduce your rpm to a speed where you do not have enough horsepower or torque in that gear to accelerate quickly if you need to. If you start braking hard then need to speed up because you've realised the truck behind you can't stop, you'll soon wish you had the horsepower ready to go. It's always safer to blip the throttle as you'll have power when you need it and also have better control of your bike.

    Any more tips Seany? Just one more thing. When blipping the throttle and thus using your engine braking to slow the bike in traffic, it's a good idea to lightly rest your foot on the brake to warn other traffic that you are slowing or about to brake/stop. Many car drivers have no idea how effectively bikes can stop using engine braking and might not realise your speed has dropped. Luckily, you'll have the ability to speed up quickly but it will be less stressful if you flash a brake light and let them know what you're doing.
  4. I found out when riding in the rain for the first time (pretty much ever) the other day just how easily this can happen. Approaching a roundabout and changed down into 2nd from 3rd and even though the bike wasnt going fast, the back wheel locked up and skidded briefly and started to go to the side. But thankfully I regained traction without needing to put my foot down - good the heart pumping though!

  5. I was doing little compression lockups everywhere on my father's XR600. I was used to the friction point on my XR250 and kept dropping the clutch on the 600's down shifts :newb:
  6. i intentionally perform compression lock-ups, just for fun. they are only bad when you dont do them intentionally. oh, and the engine/gearbox/chain etc. probably arent very pleased with me when i do it, but they dont realise how easily they are replaced :twisted:
  7. personal note: don't buy an mc from joel. [​IMG]
  8. Excellent post Seany!
  9. of the 20ish bikes i've owned, i've only ever sold one - an RGV250 - the rest, are either still going, or retired to the shed at the farm :LOL:

    i cant bring myself to sell them, so no risk there ;)
  10. What is this "Another forum" where such useful info getting posted?
  11. If you release the clutch slowly, you'll hear the revs try and spike up and you can prevent it happening. With experience you'll time your gear changes to prevent that position. With even more experience, you can feather the clutch out and use it as a braking measure.

    Worn/hard tyres make compression lockup easier. (All lockups actually).
  12. Another solution is not to change down gears too soon :wink: . You dont want to give the impression that blipping the throttle is the definate and fail safe solution. Blipping the throttle allows you to down shift sooner and hence keep the bike/car in the better powerband of the engine for corner exit as well as braking stability. If you get too carried away with how early you downshift it wont matter how much you blip the throttle you will still get compression lockup.

    Just for the people that may interperate like that. Although im sure most of you wont.
  13. I assume by that, you mean, if you try to change down to a gear that is beyond the rev-limit of the bike - right?
  14. Yup right on it :) You put the reason nicely and better than i could think of it.
  15. Yeah, that was my next paragraph though having read your post I think I should've add more info. You're right, if people get carried away with blipping it will lead to more problems. However, I do think it's better (done sensibly) to blip and ensure that revs are kept up high enough to not lose too much hp. Especially for learners on a 250 which have very little power at low revs. I probably should have said roll off a bit before the downshift if the revs are very high. Thanks for bringing it up. :)
  16. Something that preceeds "Backing It In" - In this case its not bad at all, but a necessity.
  17. i've just started experimenting with this "blipping" technique, have to say i'm pretty hopeless at it so far. But after reading this post maybe I have the technique wrong:

    1. clutch in
    2. gear down
    3. blip
    4. clutch out

    whereas I just read

    1. clutch in
    2. gear down
    2. clutch out + blip

    which would make sense provided I was not in at the friction point. I can see its something to get the hang of because right now it just feels clumsy and i'm very conscious of the fact i'd rather stop in time rather than get blippin carried away.
  18. my bad I see this topic has been mentioned at least 2 other times on page 1 of new riders forum... I *think* I get it now after reading some other posts - practice makes perfect.