knife edged crank
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02-01-2004, 04:14 PM
#9
Your statement that “there is really no downside to this type of modification", posted here: https://rennlist.com/forums/showthre...hreadid=111799 is very misleading.
There are many misconceptions about lightened cranks- people often believe that a lighter crank, if balanced, is the same in theory as a light flywheel. Either lets the motor rev more quickly- this makes it harder to ease the car away from a stoplight and sometimes confuses a stock engine management system. On the other hand for racers the benefits of a more responsive engine are huge, and the performance benefits more than outweigh the performance downsides. There is more to it than that, though…
Why is a crank so heavy to start out with? All those counterweights placed opposite the journals can practically be removed and the crank would still be balanced, because the weights of each pair of rod journals, if they are opposite one another, basically cancel one another out. So what are they doing there? Imagine if there was only one cylinder, and thus only one section of the crank. Without those weights directly opposite the journal (and the rod attached to it) the crank would be completely unbalanced. The stress on the bearings, etc would be massive. Now when you add a second cylinder to the crank you can use the second journal/ rod set as you counterweight, but it requires all the stress built up by the two individually unbalanced systems to be channeled down the length of the crank, past the main bearings, to cancel one another. At high RPMs these stresses become enormous, and cranks actually flex, particularly at harmonics, causing the engine to blow prematurely. Some forms of racing, depending on race duration, rev range, engine type, etc can get away with less balancing than others (ie nascar removes some weights totally). What counts, critically, is not simply the behavior of the crank by itself, but how it behaves with the bearings, rods and pistons attached. The normal automotive aftermarket is generally not equipped to predict these results…
A good practical example of this going bad is the Rover K series engine used in Europe in the Lotus Elise and Caterham 7, etc. Both companies were sourcing high hp versions of these motors from a number of builders around the UK to put into their hottest cars, but then the motors started to blow up- enough so that they started looking for a different type of motor. Come to find out that the tuners were either lightening the cranks or replacing them entirely with aftermarket versions that were “stronger”. In changing the factory balancing, however, measured tests proved they were unwittingly putting almost 20x more stress on the main bearings. Boom. A properly blueprinted stock engine actually had less the 1/50th of the stress, and a much higher rev and power limit.
Moral- be really careful messing with this stuff. Sure, you can remove a little weight and you won’t screw things up too much, especially if you are using lighter rods and thus have less to balance. For a sprint motor that’s going to be rebuilt often it might still make sense to take more off. But in general stay away- the factory engineers know what they are doing, and are not dumb. That thing is heavy for a reason! I’d suggest you look to the flywheel (get a good one) before you think about pulling weight out of the crank.
Pete
There are many misconceptions about lightened cranks- people often believe that a lighter crank, if balanced, is the same in theory as a light flywheel. Either lets the motor rev more quickly- this makes it harder to ease the car away from a stoplight and sometimes confuses a stock engine management system. On the other hand for racers the benefits of a more responsive engine are huge, and the performance benefits more than outweigh the performance downsides. There is more to it than that, though…
Why is a crank so heavy to start out with? All those counterweights placed opposite the journals can practically be removed and the crank would still be balanced, because the weights of each pair of rod journals, if they are opposite one another, basically cancel one another out. So what are they doing there? Imagine if there was only one cylinder, and thus only one section of the crank. Without those weights directly opposite the journal (and the rod attached to it) the crank would be completely unbalanced. The stress on the bearings, etc would be massive. Now when you add a second cylinder to the crank you can use the second journal/ rod set as you counterweight, but it requires all the stress built up by the two individually unbalanced systems to be channeled down the length of the crank, past the main bearings, to cancel one another. At high RPMs these stresses become enormous, and cranks actually flex, particularly at harmonics, causing the engine to blow prematurely. Some forms of racing, depending on race duration, rev range, engine type, etc can get away with less balancing than others (ie nascar removes some weights totally). What counts, critically, is not simply the behavior of the crank by itself, but how it behaves with the bearings, rods and pistons attached. The normal automotive aftermarket is generally not equipped to predict these results…
A good practical example of this going bad is the Rover K series engine used in Europe in the Lotus Elise and Caterham 7, etc. Both companies were sourcing high hp versions of these motors from a number of builders around the UK to put into their hottest cars, but then the motors started to blow up- enough so that they started looking for a different type of motor. Come to find out that the tuners were either lightening the cranks or replacing them entirely with aftermarket versions that were “stronger”. In changing the factory balancing, however, measured tests proved they were unwittingly putting almost 20x more stress on the main bearings. Boom. A properly blueprinted stock engine actually had less the 1/50th of the stress, and a much higher rev and power limit.
Moral- be really careful messing with this stuff. Sure, you can remove a little weight and you won’t screw things up too much, especially if you are using lighter rods and thus have less to balance. For a sprint motor that’s going to be rebuilt often it might still make sense to take more off. But in general stay away- the factory engineers know what they are doing, and are not dumb. That thing is heavy for a reason! I’d suggest you look to the flywheel (get a good one) before you think about pulling weight out of the crank.
Pete
02-01-2004, 06:34 PM
#10
Those are good points, tho I don't know to what extent they aplly to this guy's crank.
One note -- As computer power became cheaper, PAG was able to lighten their cranks a lot by using finite element modelling for the later cars -- see 993 & 964 info below.
993 crankshaft (Frere, p. 93), but has no vibration dampers, saving 732 g. (p. 96)
964 crankshaft (M64 engine) was 2.2 kg lighter than in its predecessor (the 3.2?) - Frere, p. 86
One note -- As computer power became cheaper, PAG was able to lighten their cranks a lot by using finite element modelling for the later cars -- see 993 & 964 info below.
993 crankshaft (Frere, p. 93), but has no vibration dampers, saving 732 g. (p. 96)
964 crankshaft (M64 engine) was 2.2 kg lighter than in its predecessor (the 3.2?) - Frere, p. 86
02-01-2004, 06:52 PM
#11
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That is true to some degree. But for this prticular case, the engine is getting lighter rods and pistons as well, so what would you suggest engine builders do when everyone who is building race motors runs lighter and stronger reciprocating asseblies. Now you are outside of the original engineers design perameters, especially if the counterweights were designed for the orignal setup that may be changed. I know what you ar saying, and it is true, that to do his properly requires knowledge, but would not perform this mod on a motor if I didnt do some research and testing on it
If you look around at the way most people will "knife edge" a crank the typical one or two ways that I have seen, first there is the pointed leading and trailing edge of the tips of the counterweigt, and then there is the half cut full lenght counterweigt that is done by mounting the crank in a lathe and clipping only the sides of the weight that is on th main, since if it were done on both sides in a lathe, the tool bit would hit the rod journal. The first way is null. The second method will actually offset the centerline weight concentration of the counterweight to the peaked side, and will cause the cheeks of the crank to bend in at high RPM.
The way that I have done it is to clip both sides evenly so that the inertia is centralized on the weight opposing the rod throw. The hole punched in the center of the countrweight is closer to the centerline of the crank, so the total weight is reduced, but there still is enough meat out at the end to counter the rod, so the interia and mass is still present. Sure, one could just go and clip the whole weight off and gain much more in terms of lighteneing, but the counterweight is needed to stabilize the crank, and it is still there on this crank.
Once done, I balance the crank on a balancer that reads both kinetic and dynamic planes. This motor is going to be used for drag,road race, rally, and possibly a speed record, so it needs versatility, and good response. A light flywheel would not just do the trick. There is much more than this going into and being done to this engine, its has all been figured beforehand.
As far as engine longevity, in physics, mass goes up by the square of the RPM. This would mean that the weight of a pack of matches lets say,about 6-7 grams becomes the equvalent of tons at RPM. So removing 3.5 lbs from the crank alone, and having it balanced in the engine will not only improve acceleration, free up power, it will makeis last much longer since there is that much less force trying to pull things apart, bend the block,loadthe bearings, or push the crank out of the block. THink of the old trick on the 911s of shuffle pinning the cae halves, they walk due to the foces at high RPM partially because of the load seen with having a 35 lbs crank, with rods and pistons and so on flying around at 7k RPM and the poor case is trying to get ripped apart.
If you look around at the way most people will "knife edge" a crank the typical one or two ways that I have seen, first there is the pointed leading and trailing edge of the tips of the counterweigt, and then there is the half cut full lenght counterweigt that is done by mounting the crank in a lathe and clipping only the sides of the weight that is on th main, since if it were done on both sides in a lathe, the tool bit would hit the rod journal. The first way is null. The second method will actually offset the centerline weight concentration of the counterweight to the peaked side, and will cause the cheeks of the crank to bend in at high RPM.
The way that I have done it is to clip both sides evenly so that the inertia is centralized on the weight opposing the rod throw. The hole punched in the center of the countrweight is closer to the centerline of the crank, so the total weight is reduced, but there still is enough meat out at the end to counter the rod, so the interia and mass is still present. Sure, one could just go and clip the whole weight off and gain much more in terms of lighteneing, but the counterweight is needed to stabilize the crank, and it is still there on this crank.
Once done, I balance the crank on a balancer that reads both kinetic and dynamic planes. This motor is going to be used for drag,road race, rally, and possibly a speed record, so it needs versatility, and good response. A light flywheel would not just do the trick. There is much more than this going into and being done to this engine, its has all been figured beforehand.
As far as engine longevity, in physics, mass goes up by the square of the RPM. This would mean that the weight of a pack of matches lets say,about 6-7 grams becomes the equvalent of tons at RPM. So removing 3.5 lbs from the crank alone, and having it balanced in the engine will not only improve acceleration, free up power, it will makeis last much longer since there is that much less force trying to pull things apart, bend the block,loadthe bearings, or push the crank out of the block. THink of the old trick on the 911s of shuffle pinning the cae halves, they walk due to the foces at high RPM partially because of the load seen with having a 35 lbs crank, with rods and pistons and so on flying around at 7k RPM and the poor case is trying to get ripped apart.
02-01-2004, 10:04 PM
#12
You’re making arguments similar to all the tuners that were blowing up those Rover K series motors. Sure, they rebuilt their motors using “lighter, stronger” components as well. The fact is the weight savings made on the crankshaft bearing ends of the rods (where it’s most critical in terms of balancing) is virtually always minimal. Removing mass from the crank is almost always overdone, because unless you have FEA modeling, etc to figure out the forces you’re virtually working blind. “Gut feel” and “it’s worked before” get plenty of people into trouble- Lotus isn’t going to source its motors from anyone without a good reputation and track record. These guys are experienced and capable of making 230+ hp from a NA 1.8L street motor, but didn’t have the analysis ability to understand the internal and dynamic/ system stresses. Unless you are really know what you’re doing with this stuff you’re likely to do more harm than good.
A couple other points:
“As far as engine longevity, in physics, mass goes up by the square of the RPM.”
In physics mass is fixed- it doesn’t change with acceleration. You are referring to weight, which does.
“So removing 3.5 lbs from the crank alone, and having it balanced in the engine… will makeis last much longer since there is that much less force trying to pull things apart, bend the block,loadthe bearings, or push the crank out of the block.”
The issue here is one of balance, not mass. The only place a dynamically balanced, heavier component (crank, flywheel, top, whatever) increases stresses is internally, that is stress inside the part itself trying to rip itself apart as it spins faster. Making a top heavier isn’t placing any extra sideways load on the point (bearing), for instance. You do increase the stress inside the spinning component (the crank counterweights are trying harder to break off and go in the opposite direction as the journal) but you are not in any danger of ripping the crank apart at any RPM you are going to think about.
Where stresses become a big problem is when you are not dynamically balanced with the other components attached to the system (the rods, pistons, etc are attached to your top too). Get this off just a little bit, or design your top in such a way that it’s not balanced and strong enough to resist internal flexing and the side forces on your top’s point get extreme regardless of the fact that your top got lighter. The question, from a longevity point of view, is did you make the dynamic balance and internal part stress/ deflection better or worse? I don’t know, and to be blunt unless you’ve done the required dynamic analysis neither do you. Thus I don’t like your claims. Yea, the motor will rev faster and more responsive. Yea, it will make more power on an inertia type dyno (though NOT on a constant load type, and thus the actual HP numbers won’t change by the strict definition- this doesn’t mean the car won’t accelerate faster). But IMHO you are most likely reducing the reliability of the motor overall, particularly at high revs, unless you have done the dynamic modeling with all the components attached. Anyone thinking of doing the lighter crank thing needs to be aware that this is the case with any weight reduction- unless you really know what you are doing you’re likely taking some reduction in reliability for your performance gains. Of course you can argue is that this is acceptable, because how often are you seeing main bearing failures? And the methods you’re using (dynamically balancing the crank by itself) are the same as most other tuners. I still believe you are likely hurting reliability. Perhaps you could convince me otherwise if you detail the methods you used to insure the system (crank with pistons, rods, etc) is balanced and/or that the bearing loads are minimized.
A couple other points:
“As far as engine longevity, in physics, mass goes up by the square of the RPM.”
In physics mass is fixed- it doesn’t change with acceleration. You are referring to weight, which does.
“So removing 3.5 lbs from the crank alone, and having it balanced in the engine… will makeis last much longer since there is that much less force trying to pull things apart, bend the block,loadthe bearings, or push the crank out of the block.”
The issue here is one of balance, not mass. The only place a dynamically balanced, heavier component (crank, flywheel, top, whatever) increases stresses is internally, that is stress inside the part itself trying to rip itself apart as it spins faster. Making a top heavier isn’t placing any extra sideways load on the point (bearing), for instance. You do increase the stress inside the spinning component (the crank counterweights are trying harder to break off and go in the opposite direction as the journal) but you are not in any danger of ripping the crank apart at any RPM you are going to think about.
Where stresses become a big problem is when you are not dynamically balanced with the other components attached to the system (the rods, pistons, etc are attached to your top too). Get this off just a little bit, or design your top in such a way that it’s not balanced and strong enough to resist internal flexing and the side forces on your top’s point get extreme regardless of the fact that your top got lighter. The question, from a longevity point of view, is did you make the dynamic balance and internal part stress/ deflection better or worse? I don’t know, and to be blunt unless you’ve done the required dynamic analysis neither do you. Thus I don’t like your claims. Yea, the motor will rev faster and more responsive. Yea, it will make more power on an inertia type dyno (though NOT on a constant load type, and thus the actual HP numbers won’t change by the strict definition- this doesn’t mean the car won’t accelerate faster). But IMHO you are most likely reducing the reliability of the motor overall, particularly at high revs, unless you have done the dynamic modeling with all the components attached. Anyone thinking of doing the lighter crank thing needs to be aware that this is the case with any weight reduction- unless you really know what you are doing you’re likely taking some reduction in reliability for your performance gains. Of course you can argue is that this is acceptable, because how often are you seeing main bearing failures? And the methods you’re using (dynamically balancing the crank by itself) are the same as most other tuners. I still believe you are likely hurting reliability. Perhaps you could convince me otherwise if you detail the methods you used to insure the system (crank with pistons, rods, etc) is balanced and/or that the bearing loads are minimized.
02-03-2004, 01:24 PM
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First off, this is not a Rover K series motor. Sencondly, my mistake, mass stays constant, I worded it improperly. i am not some tuner in here making wild claims either. And while you and I could go back and forth all day in terms of approaches, benefits, merits and such, I will be brief. You are coming from the scientific, FEA, and dyno approach. It seems as though you want me to supply you with FEA data, CAD modeling, dyno graphs, and other information to justify my standpoint. When in reality I do not build motors to race dynos, a dyno doesn't win races, neither does a computer program, or a flow bench for that matter. Now I am not saying that they are not beneficial to developement to a degree, but at the end of the day, it is the engine that has to perform. So if you expect me to retract my statement, or weld the crank back up, or say OK, I give, you are right and I am wrong, I should not have done this, well that will not happen. For you, it may be fine to leave good enough alone, but we will push the envelope, and not carelessly at that either. Like I said, this mod was tested, and calcualted, and would not have been done if it were not proven.
If the manufacturers build products that were designed and tested with all of the most sophisticated equipment available today, then why is it that when I take a new engine apart to build for a race, the tolerances are all over the place? Why is it that this crank, before the mod, was out of balance when I checked it on the machine? This crank came from a motor that had 1000 miles on it, it wasnt even broken in. I could cite hundreds of examples of things such as this for any motor, you would be amazed. So as an engine builder, do you suggest that the next time I take something apart, and find it to be wrong, off balance, misaligned, or in need of improvement, that i should just leave it alone and tell the customer that this must be the way that the manufacturer wanted it- for lack of a better word: WRONG. Don't mistake my response as me being mad, you do make a valid point to a degree, and i appreciate it, and I too have seen instances where there are inherernt problems with certain parts of a engine, modified or not. But at the end of the day, if the motor runs stronger, performs better, lasts longer, etc., then what is the point of arguing about it?
If the manufacturers build products that were designed and tested with all of the most sophisticated equipment available today, then why is it that when I take a new engine apart to build for a race, the tolerances are all over the place? Why is it that this crank, before the mod, was out of balance when I checked it on the machine? This crank came from a motor that had 1000 miles on it, it wasnt even broken in. I could cite hundreds of examples of things such as this for any motor, you would be amazed. So as an engine builder, do you suggest that the next time I take something apart, and find it to be wrong, off balance, misaligned, or in need of improvement, that i should just leave it alone and tell the customer that this must be the way that the manufacturer wanted it- for lack of a better word: WRONG. Don't mistake my response as me being mad, you do make a valid point to a degree, and i appreciate it, and I too have seen instances where there are inherernt problems with certain parts of a engine, modified or not. But at the end of the day, if the motor runs stronger, performs better, lasts longer, etc., then what is the point of arguing about it?
02-03-2004, 03:43 PM
#14
I think his intent is to warn others, Garibaldi. You may get away with the mods with no problem. But there is a risk in doing this without fully studying it first. You've taken that risk -- tho as you say, you did calculations. Of course, it is not clear what those were, or how closely they capture the real effects. At minimum, I urge people to balance with compensation for all the reciprocating parts -- not just the crank itself. It's not clear if you did that, or how you did it. If you want to post that, fine. If not, also fine. Best wishes and good luck.
02-04-2004, 02:05 PM
#15
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I have personally lightened a few cranks from 944 n/a's and have had no problems at all.
I think I have taken 11-13 off one and about 18 off from another both engines run and perform fine.
My thinking was that there is no more harm done by doing this than there is to put light weight pistons and rods in.
For me it was cheaper to lighten the crank then to buy pistons and rods.Kind of the hillbilly approach...But then again If it blows up I will know the answer I guess.
I will be in the process of taking the crank out of the lighter engine in the next few weeks and will check to see if there is anything noticably wrong.
One car is track only the other with the heavier crank still drives it on the street.we thought we would have stop light start problems but in all reality have not noticed much of a problem.other than in the lighter of the two the car has very good low end not and just shoot our of any corner with ease.We run these together and have the same driving experience and have noticed a big difference from one car to the other
I think I have taken 11-13 off one and about 18 off from another both engines run and perform fine.
My thinking was that there is no more harm done by doing this than there is to put light weight pistons and rods in.
For me it was cheaper to lighten the crank then to buy pistons and rods.Kind of the hillbilly approach...But then again If it blows up I will know the answer I guess.
I will be in the process of taking the crank out of the lighter engine in the next few weeks and will check to see if there is anything noticably wrong.
One car is track only the other with the heavier crank still drives it on the street.we thought we would have stop light start problems but in all reality have not noticed much of a problem.other than in the lighter of the two the car has very good low end not and just shoot our of any corner with ease.We run these together and have the same driving experience and have noticed a big difference from one car to the other