Greetings, Can anyone confirm that the BIS crankshaft is better (stronger?) at taking higher revs than the standard item and what would be a sensible limit to set? Thanks.

Ian.

Hi, I have just bought a nos biss crank for my 800cc air cooled 126 conversion. I read on several sites, mainly Polish, recommending the upgrade to a biss crank as it will be able to handle the additional power from the 800cc upgrade. The blitz site states the original crank can go to 6,500 revs provided the blank is tacked in position and the whole system, pistons, co rods, crank, flywheel and clutch are balanced. The site below is in Polish, the biss (bisa) crank is listed in items required for the conversion. Hope this helps.

Ralph


http://start5g.ovh.net/~maluch/tuning/silnik800.htm

Apart from the strength issue boffins in the engineering department after looking at the different cranks tell me that moving the balance weight mass closer to the crank axis will make the crank more stable at higher revs.

Hi, all I'm not an engineering boffin but looking at it logically the closer the mass is to the centre of rotation the less centrifugal forces are produced. The whole situation is made more difficult due to all the bits attached to the crank. At the end of the day we are trying to balance the system to reduce as far as possible the out of balance forces. If you can throw some advise on this subject It would be appreciated as It looks like a black art to me especially with the difficulty in balancing the 126 internals. I found this on a How to build racing engines:crankshaft selection.

Ralph

For a fixed mass (rod, piston, and attending components), a heavy crankshaft absorbs more torque and accelerates less rapidly. More torque is required to overcome its inertia or resistance to acceleration. Lighter cranks are desirable for this reason, but they may trade away durability and performance if reduced structure results in greater crank deflection, reduced stability, and the resulting effects on durability, ring seal, and in some cases, event timing. The prob*lem intensifies with stroke length; hence efforts are made to concen*trate crankshaft mass closer to the crank axis to reduce the moment of inertia or resistance to change in acceleration.

Savvy machinists keep balance weight as close to the axis as possible. When removing weight for balanc*ing purposes, they try to take weight from as far out as possible without actually drilling into the outer face of the crank throws, which can exac*erbate windage problems at high engine speeds. Many cranks have the crankpins drilled. This is generally the most effective way to lighten a crankshaft unless you get into expen*sive counterweight machining. Drill*ing the crank pins removes weight at the farthest possible point from the center of the crank, thus reducing the rotating inertia.

Ralph I read all that early this morning before I had a coffee but it kind of made sense. I know you have met the one known as Smallcox on the club126uk forum. For those who do not he has developed over a number of years a 700 cc 126 engine with turbo & intercooler which runs like a sewing machine but will put out over 100hp. Last time I went to his place he showed me a balancing machine that he had made himself after studying videos he found online about how engines were balanced before all the electronic gear came into play. Seems that he cracked it & he also mentioned that when balancing the crank he fills it with oil as when empty the balance will be out.

Toshi 975, yes I met Steve at Retro Rides a couple of years back. His car is amazing, the engineering outstanding. I believe after bending his crank on a run on the track he was looking at modifying I think a crank from a rover ?? to take the power. I don't have the knowledge or skills to modify cranks etc so I adopt solutions to fit my situation. It is my son who wants to build an 800cc 126, well it's a good excuse to allow me to dabble in the garage. In order to keep costs down we have opted to try the conversion using VW bits. As this type of conversion is carried out primarily in Poland it makes sense to use their experience and this is where the bis crank has been recommended for this conversion. I believe the counter weight is slimmer which means less maching to the barrel skirts. I am collecting the parts required to complete the conversion, the barrels and pistons are the only outstanding parts required, so hopefully we will be able to start the build early next year, this provided I take time off from completing the Subaru 1800cc transplant into my Cool 500.

Ralph

Greetings Gents, thanks for the replies - much appreciated. Ralph, you have mentioned that Steve bent his crank (126 BIS??) on the track, can you tell me at what rpm this happened???? I would like to know as some commercial upgrades (to 700cc - Athena etc) quote their output at 7570 rpm with OEM parts?? Good luck with your 800cc 500.

Ian.

Ian, I don't think it was a bis crank sorry. I will look on the 126 club site, there were some posts on some of the build areas, if I find anything I will relay to you. I retire end of March 2017 so the 800cc build has been put off until then.

Ralph