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high gear low revs- low gear high revs?

Discussion in 'New Riders and Riding Tips' at netrider.net.au started by watertrade, Feb 23, 2006.

  1. Hi Everyone,
    I must start by saying that I know very little about the mechanics of motorbikes. I have been riding for about 3 weeks and have a question about the functioning of gears and revs.
    I have been recently 'riding in' a new gs500 - the manual says that for the first 800 Km I should keep the Rpm below 5500. Then below 8000 rpm until 1600km.

    I have just hit the 800km mark. I have ridden at all legal speeds up to 110 keeping the rpm below 5500. Now that I am past the 800 mark I have been taking the rpm above 5500. But my question is...

    What happens with the bike in high gear and lower revs or low gear, high revs?? I'm guessing lower gear has more torque and acceleration...
    And different rpm's have more of the 'power'.

    What should a newbie know about gears and revs??

  2. Basically labouring an engine (ie putting it under load at too low an rpm) causes excessive wear - you actually want this occur during running in though (to get rid of any irregularities in moving parts) which is why it's recommended to keep revs low. Higher revs improve the efficiency of lubrication and therefore reduce wear but the downside is it increases the chance of mechanical failure. All engines have a torque and power curve - acceleration will be best when the bike is in a gear that puts the revs closest to the peak of the torque curve, you should upshift as soon as you hit the peak power rpm (revving higher achieves nothing). Of course for everyday riding you'll want to shift up to the highest gear possible - this brings the revs down and reduces fuel consumption. However just be careful not to shift up too many gears at too low a speed. If the RPM is too low then the engine will be too far down the power curve and basically won't have enough power to propel the bike (this'll also damage the engine).
    Edit: Hey, I actually found a chart of power/torque curves for a (modified) GS500 here :grin: (might help explain what I'm talking about). Seems for optimum acceleration you want to be at around 7500rpm, for general cruising probably best not to go too far below 6,000.
  3. speaking from experience, gs500 while putting round town, keep the rpm below 5grand. The engine is smoother and won't be laboring unless you're trying to go up a big hill at 80k/h in sixth. Best power is made at about 9grand on a stocker, so if you have to haul arse, rev the bastard, otherwise, be nice, keep those revs low and the throttle mostly closed and your engine and fuel economy will thank you.
  4. Actually most engine wear occurs as a result of too low an rpm, not too high - especially when cold and under load (ie moving). Seems weird but oil only works efficiently if there's minimal force between the parts (usually unavoidable) and/or they're moving quickly. I remember someone also posted up something from one of the carbie manufacturers stating that running at lower revs than what the carbs are designed/tuned for can actually increase fuel consumption. Keeping revs too low, too often, can also foul up the sparkplugs.
  5. oh boy - loads of various opinions stated on this topic, so i might as well put my 2c in

    1. rpm limits reason 1: if something does go wrong, manufacturer usually has a cheaper repair under warranty
    2. rpm limits reason 2: primarily legal. Many studies (see Hurt report) show that if you are going to bin a bike, statistically it will happen in the first 5 months of ownership. They just hope to keep you moving slow to get used to it first.
    3. high throttle at low engine speed (lugging it) doesn't increase cylinder pressure (or bmep) and help seal piston rings jd. The engine will sort this out for itself and higher bmep comes at the peak torque speed with wot - this is what it means.
    4. Tailus; because an engine is just an air pump, the less restrictions you place at either end of it the more efficient it becomes. High throttle openings are better than low ones, and the best way to get this is high gears. You have to be careful not to go too far and have the engine pinging or knocking however, esp if the bike doesn't have knock sensors like the gs. This is the opposite of what joe avg thinks and was one of the primary reasons young pilots ran out of fuel in the second world war. The engine 'felt' better at higher speeds and lower throttle openings, but uses less fuel the other way around.
    5. 'real bikes' :) have instructions that tell you to let it warm up and use the whole rev range during the 'run-in' period. They also say to avoid full throttle starts for a while, however this is mostly a legal thing imo.

    "What happens with the bike in high gear and lower revs or low gear, high revs?? I'm guessing lower gear has more torque and acceleration...
    And different rpm's have more of the 'power'. "

    On the first point you are correct - lower gears = more torque. Torque = acceleration.

    Second point can be explained very easilt by looking at dynomometer curves of an engine. You will notice from the below


    that the curve represents maximum power at each engine speed. If you close the throttle, you can get less power at each engine speed, but if you want a certain power (or torque) you need a gear and thus engine speed that will deliver what you want.

    Finally jd, engine wear is comprised of many factors, nearly all of which include keeping "minimal force between parts". Pressurised oil systems are designed to feed oil to where it is needed, however the design of moving parts in this type of environment encourages hydrodynamic lubrication, meaning that under load, the oil keeps the two parts seperated. You get into trouble at low engine speeds, particularly when the engine is cold, because the oil pump doesn't deliver sufficient pressure to keep slow moving parts seperated or because the cold gap tolerances are too large to encourage proper hydrodynamic pressure accumulation between parts.

    Points about carbs are interesting, particularly in regard to rich mixtures fouling plugs and is one of the reasons efi systems work so well. Carbs tend to have only four ranges in which they work well, all others are a compromise (pilot, idle, intermediate and max jets in carbs). efi systems can be variable right across the range of engine speeds and throttle positions.
  6. Not quite - and keep in mind I spent three years studying lubrication and mechanical wear. Whether the oil pump can deliver oil into the engine is a side issue, fact remains that parts moving at low speed can displace the oil between the surfaces (particularly under high load) and cause galling - most engine wear particles are characteristic of this kind of damage. This is a common problem in gearboxes which is why gearbox oil is different in being chemically modified to bond to the metal surfaces (using a higher viscosity also helps somewhat). Lower revs doesn't change cylinder pressure true, but it does increase the load on other components such as the crankshaft and main bearings - it also serves to work harden the surfaces of some of these parts. Lower revs will however reduce the efficiency of lubrication between the rings and the bore which will help the engine bed in by removing irregularities through increased mechanical wear - again a reason why a slighty thicker oil is used for running in so that the extent of this wear is minimised to only irregularites in the metal surfaces. Gets me back to my original point - higher revs does in fact reduce the force between moving parts (particularly the piston/bores and crankshaft/main bearing), reducing wear - however it does increase the strain on other componets (particularly the conrods) which will shorten their time to failure. So basically you need to find a balance - don't hold the engine at redline all the time but also don't chug around at low revs in high gear.
  7. thanks guys, you have given me alot to think about.
    I will certainly stop riding around at 80km in first ... :wink:
  8. I'm buying a new bike soon (soon as it hits Australia) so most of my spare time of late has been researching 'breaking in' a new bike. Basically a lot of people are of the opinion that this is a good resource to read. (opinions from people here?)

  9. Well what that guy says basically goes against all I was taught by my metallurgy lecturer (who spent most of his professional career testing piston and piston ring failures). Basically he's right about the ring needing to be worn down in order to get a good seal between it and the bore - however thrashing the engine does not make this happen faster. Many components in an engine are designed to surface harden under use - starting out easy on the engine at first allows them to "bed in" before fully hardening. Thrashing the engine will harden these parts early meaning that they will never really fit together properly (no good if you want the engine to last as long as possible). Some of the points like changing the oil and not using synthetic are good advice - but then the manufacturers tell you to do that anyway. I wouldn't trust a guy who'll only explain why the manufacturers are wrong if you sign up to his newsletter (and pay him money).