Triumph Rat Motorcycle Forums banner
  • Hey everyone! Enter your ride HERE to be a part of this month's Bike of the Month Challenge!

1 - 10 of 10 Posts

·
Registered
Joined
·
372 Posts
Discussion Starter · #1 · (Edited)
Does anybody know what are the specifications required to balance the crank on a 1976 T140V? The shop is looking for both the small end and big end specs. This sort of thing is a first for me so any other details would be appreciated
 

·
Registered
Joined
·
1,825 Posts
I think all the cranks on these twins are balanced to 85%. Not sure on yours but my 68 T120R called for that %. There was a thread awhile back discussing the pros and cons between Dynamic and Static balancing that goes into detail about what to weigh and how to come up with the figures. Do a search in this forum. Some good information. I had mine dynamically balanced to 75% and it is really smoother. I let a friend that has several Triumphs ride it this morning and he said it is the smoothest Triumph he has ever ridden. I don't know if or how much smoother it might have been if I'd taken it to 85%.
 

·
Registered
Joined
·
1,825 Posts
Just reread some of the threads about crank balancing. The way I read it is: to arrive at the balance weight required, the rods are weighed at each end. The piston, rings, wrist pin, & circlips are weighed and that weight is added to the weight of the small end of the rod. They take 85% of this total and add it to the weight of the large end of the rod (weighed with all bearings, bolts and nuts in place). This total is the bob weight needed. This is done for the left and right sides individually and each piston, rod etc. is identified as the set it was weighed with so that they can be assembled with that group of parts and on the correct journal. For best results, the crank should have the rotor, sludge tube, and cam drive pinion with associated nuts, keys, and washers in place when balancing is done.
 

·
Registered
Joined
·
372 Posts
Discussion Starter · #4 ·
Thank you Jimmy Bush. I just got the phone with a buddy and that's pretty much the very same way he explained it - except he didn't mention about the cam drive pinion with associated nuts, keys, and washers in place. Wouldn't hurt to these extras as well along to the machine shop.

Now, if we can only be sure on the correct balance factor. I know the T120 called for something like 85% but something tells me that they used a different number for the 750's.
 

·
Registered
Joined
·
601 Posts
Now, if we can only be sure on the correct balance factor. I know the T120 called for something like 85% but something tells me that they used a different number for the 750's.
Thirdbike,

Bing bong, give that man a cigar. You are correct the 750s
according to the manual are blanced at 74%.

Pookybear
 

·
Registered
Joined
·
372 Posts
Discussion Starter · #6 ·
Yup, that was one of the numbers I had been bumbling with but I didn't want to throw anyone off.

Thanks a lot!
 

·
Registered
Joined
·
1,825 Posts
Thanks for setting things right pookybear. I didn't know for sure so said "I think they're 85%". I must've been reading 750 specs when I told the machinist what figure to use when I had my 650 done. Are "ALL" the 750's done to 74% or just the short rod (are all the 750's short rod?)? If so do you know what years that would include?
 

·
Registered
Joined
·
1,034 Posts
i've been researching this myself recently and regardless of the factory specs , from practical experience of the replies low to mid 70's seems to be the general consensus for best results .

this is from guys racing and from guys that have been building for many years .

cheers , Woody
 

·
Registered
Joined
·
162 Posts
eya gang!

something I did'nt know until recently on balancing percentages.

the up/down vibes go to foreward/backwards vibes as we change the percentages.

they don't go away so much, what they do is change direction due to the nature of the twin crankshaft.

what dynamic balancing really minimises is the "rocking" forces.

the early balancing percentages we're determined as much by the test rider reports as anything else.

the fore and aft balance extremes experiments led to cracked engine mounts occasionally.

If you're interest in the "why" and how of balancing i reccomend this article, on page 10. "Non Intuitive Firing Orders" by Kevin Cameron

http://tioc.org/1993-3PM7.pdf

This is one of the best balancing articles i've found for Triumph twins!
 

·
Registered
Joined
·
601 Posts
Everyone,

I really do not know when the 74% came into effect for the
750s if it was right off the bat with the first ones or later.

It is just a matter of fact that the test riders liked this number
the best during tests of the new 750 engines. And of course
during the rotation cycle the bob weights on the crank are there
to cancel out the stopping and starting of the piston and rod
mass at both top and bottom dead centers. However, the
forward and aft shaking comes from just the bob weights as
there is no counter acting force from the piston and rods.

The "smooth" answer to this of course was the counter rotating
shafts used by other makers. However, Triumph did not have
the money or the time for new tooling to compete. So Triumph
went for the best feel. Work pretty good, but I have to say
even though I love my ride, the engine was really outdated
by the mid 1970s as compaired to other manufactures.

A good little article showed up in Classic Bike September 1990
about the balancing of cranks and stated;

"Writing of the new unit-construction Triumph Bonneville,
developed from the separate gearbox twin, he wrote(Bert
Hopwood): 'This was achieved with a minimum of unknowns
because the basic geometry and structural factors remained
unaltered... However, once the engine was fitted into the frame,
which was also of a new design, it proved to be a real shaker.
Time was not in our favour, yet our development team...
managed to compromise with a passable degree of roughness
which we knew the public would accept.'"

Remember this was about the 650 engine, and the 750 only
made matter worse.

Also covered in the article were other causes of ruff running
that seems like an out of blance crank. First and foremost was
out of blance carburators. Also alternators are a major cause
of vibrations, with there "bucking action" as the winding are
passed by the poles. This is even made worse by the use of
a Zener diode. Now three phase system with electronic
voltage control does not have the same problems that say a
two wire system would have with a Zener diode.

Good article worth a read if you have a copy.

Also remember that there is static and dynamic balancing.
Furthermore, a really matched set of pistion, rods including
all the small parts needed to weigh the same or one develops
a rocking pair offset. Remember, that there is already a
given distance between the two rods in this style of engine,
each power stroke causes a rocking motion from left to right.
A none matched set of parts will only increase this effect
of the rocking pair.

Pookybear
 
1 - 10 of 10 Posts
Top