28 replies to this topic

[Spider]

Bit hard to reduce the harmonics - they are largely produced from the 'bang' that occurs above the piston and the size of it. So at the risk of sounding like a wise guy, the way to reduce the harmonic is to reduce the Horse Power.

 

However, what we can do is to control the effects of the harmonics and move the resonant points. In part it is done with the Harmonic Damper fitted to the Nose of the Crank, ensure that it suitably tuned for the engine (not an easy task). Something else that is often done is to lighten the bottom end, which if done right, will move (hopefully!) the harmonic resonant points outside of the engines normal and maximum rev range.

[mini13]

my onderstanding is one thing that will help with harmonics is to increas crank stiffness, thus moving the resonant frequency of the crank upwards ( and hopfully furthur away from the power band)

 

surely increasing journel diameter/ overlap will help with this?

 

obviously there are various ways to skin a cat, and to be honest the faux 5 bearing crank from swiftune/ russel engineering ( anyone else do one?) probably works better than enlarging the journels.

 

i would be looking at 1300 sized bigends rather than S sized ones on a crank like this for looong strokes though, as it will be billet anyway...

[fwdracer]

 

How can you reduce the harmonics to reduce the risk of a snapped crank?

Everything is relient on a very effective crankshaft damper (KAD, etc) and they are strong money but much cheaper than the damage a busted crank can produce. The harmonics on a standard stroke 1275cc engine can be quite destructive when this engine is revved hard (8K) and will induce a power loss and hole in the power delivery curve unless an effective damper is used.

Aside from the big bore short stroke derived S-engines (970/1071) The 998cc crank and power unit is BY FAR the sturdiest.

[Spider]

Yes and no. It's a very complex area and there's no magic bullet for it either. Just keep in mind going to bigger journals - especially big ends - adds weight usually where you want it least. Adding weight usually lowers the resonant frequency. Bigger journals is adding weight.

 

Regarding stiffness, that can help, but likewise with most materials, making them stiffer can also make them brittle®.

[ACDodd]

This is the way to control crank harmonics.

http://www.kentautod...er-front-pulley


Agreed above, stiffer crank is not always the best. Controlling the forces one of the most effective ways to increase crank fatigue strength.

I believe Keith Calver has experience in machining and building big main bearing striker a-series bottom ends, give him a shout;

http://www.calverst.com




Ac

Edited by ACDodd, 04 August 2015 - 11:55 AM.

[Jon937]

I will have a look on Keith Calvers website but if i were to lighten the crank on which parts would the material be removed as obviously the webs offset the big ends?

[ACDodd]

You need to call him, info won't be on the website I would have thought.

AC

[Spider]

A friend of mine sent me this link;-

 

http://www.ausmini.c...2&t=68263

 

Some very interesting & relevant reading there on Harmonics, Crankshafts, Nitriding, Materials, Harness and associated brittleness. Well worth a read.

[Jon937]

Looking at swiftune 5 main bearing crank I was wondering where do the 2 additional mains mount as on the block there is still only space for 3?

Just a question as far too expensive to think about

[Jon937]

Ignore that, found the answer on a different forum.

[Spider]

Looking at swiftune 5 main bearing crank I was wondering where do the 2 additional mains mount as on the block there is still only space for 3?

Just a question as far too expensive to think about

 

 

Yeah, I think on the cash / improvement ratio, these are on the low end. They were trialed that I know of first in the 70's but very soon dropped as there wasn't any worthwhile advantage found at that time. Times have changed since then and perhaps there is some gain these days. Arrow also produce them.

 

But if you think that expensive, going up a size in Mains would be equally as expensive.

Edited by Moke Spider, 07 August 2015 - 08:34 PM.

[Jon937]

So that crankshaft would be more beneficial than increasing the size of the main bearings. Would that crank work okay being stroked to 84mm instead of 81.3mm?

[Spider]

Without doing a proper test bed analysis for Torsional Harmonics, that can't be answered, but there's a few more pieces of the jig saw needed too. All the bits and that going to be hung off the crankshaft also need to be taken in to account.

 

One big advantage of this particular crank is the pins are aligned on centre with each bore, so the rods don't have an off-set. IMO one backward step with the Swiftune package is the longer rods - just my humble opinion, but they have gone the wrong way, they should be shorter, not longer. Peak Piston velocity all happens at further up the bore with longer rods, so cylinder filling is less than optimum. Also the added length in the rods will always outweigh (as in heavier) any added length (if needed) of the Piston. It appears to me they designed the rods around a production piston (from another engine) - just my guess. I have spoken with another knowledgeable member on this forum on this topic and he has a differing view (if he's willing to chime in )  Anyway, that's getting OT.

 

These would be well worth consideration;-

 

http://www.swiftune....crankshaft.aspx

 

IMO, smart, sensible design and not stupid in price.

Edited by Moke Spider, 07 August 2015 - 09:52 PM.

[Jon937]

I emailed them asking how much it would cost for their DC5.2 without rods and pistons, so that I am able to use Med's pistons and rods with fully floating pins. In the same email I asked the question as to if they could produce it with 84mm stroke and what the price difference would be. Just waiting for a reply.