M50b25 Kampuran paino.

[Fatmonkey]

Mielenkiintoista. Kevennystähän tuossa ei haettu, vaan sitä että limput yhtä kaukana keskipisteestä ja kehänopeus siten sama. Eli jos yhdessä limpussa r = 6mm pidempi sen inertia suurempi kuin lähempänä keskipistettä olevan.
Luonnollisesti kaikki koneistetut osat tasapainoitetaan uudestaan.

[jannmann]

Tuskin on niin tarkkaan tehtaallakaan punnittu.

Kyllähän noin on kautta aikojen tehty ja hyvin pelaa.

Ainakin S-sarjan akselit on hyvin puntitettu tehtaalla.
Veikkaan, että noi tavallisetkin on hyvissä arvoissa.
Tolla knife edgingillä vaan pilataan täysin veivin ja männän vastapainot.

Eli kautta aikojan noita on tehnyt kaverit jotka ei ymmärrä asiaa.

Epäilemättä se siellä pyörii, ei siinä mitään.

[Jasu]

Kampura piloilla jos ei tasapainoita joka limppua uudestaan...

Tuskin on niin tarkkaan tehtaallakaan punnittu.

Kyllähän noin on kautta aikojen tehty ja hyvin pelaa.

[Dunky]

Kampura piloilla jos ei tasapainoita joka limppua uudestaan...

[jannmann]

Mitähän ton tasapainolle tapahtui jos tavaraa ajettiin pois millitolkulla.

Se pilattiin. BMW:llä ei ole yhtään kilpa-akselia joko olis noin kevennetty.

Tossa yksi juttu liittyen tollaseen "keventämiseen". Siinä käydään hyvin läpi ongelmat mitä tollanen sorvaaminen aiheuttaa.
Tosin Englanniksi.



Hi Dave, Ill try to keep it easy. If not ask one of your dutch friends to translate...

Knife-edging the crank like in your foto isnt such a good idea. But, first we need to also know what RODS and PISTONS you are running, can you say?


The person who did this job must not understand very much about crankshafts. I would never modify a factory crank like this! If you design a brand new crank & rotating assembly from scratch, you might justify it, but its still a poor design.

Here are some reasons why its not a good idea:

1) the crank counter balance weights are almost shaved off. This means you have very little counter balance to compensate for the rotating weight and the reciprocating weight.

2) the result is increased dynamic imbalance which creates more vibrations and larger stresses. The amount of vibration increases with rpm squared.

3) this means your ultimate maximum safe rev limit will be lowered.

4) you run the risk of breaking the block, e.g. the main journal webbing.

5) you run the risk of breaking the crank.

6) the greater imbalance creates a larger bending couple in the crankshaft. This can cause higher main & rod bearing wear, and its severity is increasing with rpm.

7) I cant tell in your picture if the crank pins (where the rods connect) have holes drilled thru or not. Having holes in the crank pins is a good idea. If this crank does not have these, its further evidence that someone did not understand what they were doing when removing mass from the counter weights.

8) it looks like the overlap has been changed and the fillets look different, this reduces strength. Perhaps not a lot, but who really knows in this case?

9) the usual benefits claimed for this kind of knife crank is lower inertia, less aero drag on the crank, less windage loss due to oil mist creating drag. However, there are other crank counter weight profiles which are "aerodynamic" and we have an OIL WINDAGE TRAY which is designed to run a specific distance from the crank and reduce windage loss. As far as lower inertia goes, thats less relevant if the rotating assembly is not optimized. We can lower inertia other ways.


The reason I asked you what rods and pistons you have is this is important if you want to have an optimized rotating assembly. If you are close to stock weights, this crank is definately not for you (well, I dont think its good for any case, due to poor execution, but the lighter the pistons and rods, the more viable it becomes).

rotating mass = mass of crank pin + rod big end + rod bolts + rod bearings
reciprocating mass = mass of piston + piston rings + wrist pin + clips + rod small end

In your case you need to ask yourself if the crank balances 100% of the rotating mass.

Secondly, you need to know what percentage of the reciprocating mass is balanced. It can not be 100% even on a good design. There is 50% balance (=happy medium), under-balance, and over-balance. Under-balance creates larger vibrational forces ("shaking forces") in the vertical plane. Over-balance creates larger shaking forces in the horizontal plane. On race motors you might deviate from a medium balance design, if the crank is strong enough for it and it harmonizes with its particular pistons, rods and block.

Third, to understand how imbalance is created, its important to realize that the pistons travel at different speeds in the upper half of the cylinder (faster) compared to the lower half of cylinder (slower). Because of this fact, you get dynamic imbalance at the fundamental frequency (speed) and multiples there of (first and second order is what most are concerned with). These imbalances rise with rpm squared. So the stress inside the crank, in the block and in the main & rod bearings will also increase as a direct result. The crank wont rev as high safely as compared to a crank with a better design. If the crank is strong, it may not crack, but your block may suffer a failure.


The picture below shows a factory BMW MS 2.5l race crank compared to a standard Sport Evo 2.5l crank. The MS crank weighs roughly 16.4 kg. Notice where they removed weight on the counter balances and the crank throws. In comparison to the crank above, they removed very little on the counter balances.

One other thing: these 4 cylinder cranks (and other properly designed ones) are *inherently* balanced so fine balancing is done on the crank minus any other parts attached to it (the process of removing small amounts of weight by drilling holes into the counterweights). The flywheel & clutch assy are to be balanced separate from the crank (and usually come that way). Dont let a shop try to tell you you need to balance it all together, because you dont need that at all. Besides, what happens if you later change your flywheel and the crank is now somehow modified? ...

What often happens is that they just check it, but charge you for checking and balancing, its just a money maker to add on the invoice.

[petrolhead]

Mitähän ton tasapainolle tapahtui jos tavaraa ajettiin pois millitolkulla.

[FroZ]

Hola.
Pyörittelin m50 kampuraa pöydällä ja huomasin etteivät limput ole saman "korkuisia" keskenään. Kampi kainaloon, koneistamolle, sorviin kiinni, kellotus ja sorvausta niin pitkään että limppujen ulkokehä on yhtä kaukana akselin keskipisteestä. 6mm piti ajaa pois. Samalla pieni viiste ulkokehälle.
Unohtu punnita ennen työn aloittamista. Onko jollain tietoa mitä vakio m50b25 kampi painaa tai kampea jota voisi käyttää vaa'alla?

Täällä jonkinlaista näkemystä painosta -> http://bmwfans.info/parts/catalog/E3...earing_shells/

[Fatmonkey]

Hola.
Pyörittelin m50 kampuraa pöydällä ja huomasin etteivät limput ole saman "korkuisia" keskenään. Kampi kainaloon, koneistamolle, sorviin kiinni, kellotus ja sorvausta niin pitkään että limppujen ulkokehä on yhtä kaukana akselin keskipisteestä. 6mm piti ajaa pois. Samalla pieni viiste ulkokehälle.
Unohtu punnita ennen työn aloittamista. Onko jollain tietoa mitä vakio m50b25 kampi painaa tai kampea jota voisi käyttää vaa'alla?