750 twin grunt mods

  • Thread starter Thread starter morison
  • Start date Start date
Spub said:
The difference between the 1974 SF2 I had and my 1974 SFC are stark, even though they have the same engine. The extra work that went into the SFC to balance it makes it a different bike, as far as vibration goes. I'm not saying it does not vibrate, but compared to the SF2, it is Bevel Ducati like in smoothness, even with its lumpier cams. So while phasing has an effect on vibration, tolerances may have as much an effect, if not more....
Click to expand...

Spub,

The engines are based on the same design and share a few components, but that's where it ends.

The SFC cranks were actually balanced differently to the cooking bikes.  While the racing dept. set its focus on reliability and low bearing loads, they cared nought for rider comfort!  The road bikes were balanced for comfort and took slightly higher bearing loads for granted.  Theoretically, a SF engine should run smoother than a SFC.

A teacher friend (university level automotive technic) worked out ideal balancing factors for the Lav twin.  Measuring both SF and SFC crank weights/balancing confirmed his calculations.  SFCs were balanced to around 58% while road bikes were 70%, more in line with common practice on upright parallel 360? twins.

Totally agree, the SFC engine is a delight.  The way it spins makes the mind boggle.  The gearbox also helps a lot in keeping it spinning happily.  As much as I loathe the complete bike, I'd love an SFC engine.  Stick it into a nice SF2 chassis and have bags of fun! :D

piet

 
Some great points here!
Phill's comments are very interesting, especially as he's commenting on something that he has actually done!

Confused!  bet you wish you never asked  ;) :D

Julian
 
A good discussion underway here which has been gone through before on the forum.
I have both 270 crank and 360 crank SF2 twins. The 270 has an SFC crankshaft and was my roadbike at the time I re-phased it. I did the conversion to "remove" the vibration after the rave about Phil Irvings 76deg(?) British twin story. 90deg was deemed better but I don?t know of any 76deg twins to compare with. I balanced mine statically to 55% and got it extremely well trued (I forget the figure). It was always good vibration-wise compared to some SFs (I have serviced masses of them as a Lav mechanic for the importers, some vibrated even from new so badly I would have rebuilt them had they been mine). With the re-phasing it vibrated at a very different frequency which I find easier to live with. It feels to be at a lower frequency. As far as power and "rideability" is concerned there is not much difference. It sounded great before and sounds great now. It now only sees action on the track and my collection of broken bracketry proves that it does vibrate, despite actually feeling very smooth to the rider. My current roadbike (the "tippie" made from the leftover worn bits that should have been chucked out on the "tip", just like me) has a std crank at 360deg. The feel of the bike with SFC vs SF crank is totally different and without one of Piets lightening jobs you are unlikely to get that. This std motor was really pox when I acquired it but is now really good after I have set it up how I like. Std SF2 cam, ports tidied up, valve seats cut just right, 1mm squish with 10.5 compression set up on skimmed std pistons with equal weights etc. It pulls really well everywhere and is just great to ride but it doesn?t spin up anywhere like the SFC crank bike always did. The "genuine" vibration is there but easy to live with. Living in Norway I cannot let fly on the road in top gear other than short bursts where I feel unlikely to be detected; 80kmh national limit with prison for more than 30 over etc etc and they do it too!! For this normal riding the bike is fantastic and the vibes are not bad when it gets up in high revs.
For you, it would be fantastic if any forum members could give you a go on a good std, and somebody a re-phased one as they are both fantastic but totally different feel. It is not a hard job to do for someone a bit clever. It is also the same to un-do at any time in the future. I?m pretty sure the std crank can only go 88 or 92 deg because of the splines. The strength of the centre section is only a problem with heavily trimmed motors used for racing. The cam and ignition are relatively easy to change too (assume you will go electronic ign). If you stay std crank and use your efforts to set up the motor well and get the ergonomics and suspension uprated you will be more than happy.
I know of numerous triple riders who went to an SF and were very happy, they are totally different and more than capable of giving you plenty of great riding. I rode many triples and had plenty of opportunity to get a triple through my job but never really felt tempted.
 
julian said:
Some great points here!
Phill's comments are very interesting, especially as he's commenting on something that he has actually done!

Julian
Click to expand...

Perhaps we should make that a criteria which has to be met before people make a post!
 
That would remove most of us from getting the rest of you into trouble!
How else can we find out it doesn't work before we waste our own money on silly ideas?

Julian  :D
 
exactly, Julian!  ;)

Honestly:

-Do you have to fall off a bike to know it will cause pain? No, your experience of falling during the first years of life have made you an expert in pain and you can calculate that this will probably not be healthy to do.

-Does a doctor have to have had ANY disease himself to be able to heal it?

-etc. etc.

you get the point I assume...

If you always do what you have ever done, you will never develop something new which might be better than what you always did - right?
 
You must log in or register to reply here.
Back
Top