What are the advantages of a water-cooled V-twin over a parallel-twin?

**Steam** · 19th February 2008, 21:39

I guess on an air-cooled bike the advantages of a V-twin are obvious - the cylinders have at least 25% more surface area for cooling.

But what about a water-cooled V-twin? Are they better in any way than a water-cooled parellel twin? I can't think of any advantages.
Does it just look cool and sound good? Is it just a hangover from Harley times?
Wouldn't it be smaller, cheaper, lighter and tidier to do a parallel twin of the same volume?

**Mental Trousers** · 19th February 2008, 22:10

Water cooling means a more stable operating temperature so you can run tighter tolerances. They're a bit quieter cos of the surrounding water jacket and you can make the bike more aerodynamic because there's no need to expose the engine to the air stream. Ummm, something else but I can't remember what.

**Steam** · 19th February 2008, 22:13

Originally Posted by Mental-Trousers

Water cooling means a more stable operating temperature so you can run tighter tolerances. They're a bit quieter cos of the surrounding water jacket and you can make the bike more aerodynamic because there's no need to expose the engine to the air stream. Ummm, something else but I can't remember what.

Yes, I know that, thanks for the reply though.
But what are the advantages of a water-cooled V-twin over a water-cooled parallel-twin? Just looks?

**Mental Trousers** · 19th February 2008, 22:24

If you have a 90' v-twin and a parallel twin where both engines share a common crank journal between both cylinders then the v-twin is much better balanced and can rev higher, producing more power and less vibrations. Oh, and a v-twin can be narrower because the cylinders can overlap instead of being strictly side by side.

It gets more complicated if you use seperate crank journals for each cylinder because then you can make a parallel twin fire the same as a v-twin etc. However, you still have the pistons moving in the same plane instead of at 90' to each other etc.

**Ocean1** · 19th February 2008, 22:28

Firing pattern. Don't sound like a real big advantage but they behave quite differently. People have been trying to work out why V twins seem to make good tractable power for years.

**pete376403** · 19th February 2008, 22:28

The V design has a marketing advantage (ie it looks like either a Ducati or a Harley, both brands driven largely by "image")
Otherwise anything a V twin can do can be replicated by a parallel twin with an offset crank (eg TRX850 yamaha) and the parallel is a far more compact engine for the same displacement, simpler cam drive, and so on.
The exception would be if the engine was two stroke - there the V has advantage as there is more room for bigger transfer ports - why Suzuki went from RG to RGV

**Motu** · 20th February 2008, 17:10

Originally Posted by pete376403

Otherwise anything a V twin can do can be replicated by a parallel twin with an offset crank

And the opposite is also true.A V twin with separate crank pins (eg Honda) can replicate any other twin.The XLV750 was a 45 degree V twin,with a 90 deg firing like a Ducati ....and the RS750 that the XLV750 homogulated fired as a single crankpin V twin.

**Sully60** · 20th February 2008, 17:33

Originally Posted by Steam

I guess on an air-cooled bike the advantages of a V-twin are obvious - the cylinders have at least 25% more surface area for cooling.

By the surface area % gain you mention one would assume you are referring to a longitudinally mounted engine like Guzzi's and Beemers and CX Honda's and many more.
In a transverse V-twin, the rear cylinders cooling is affected by the front, this happens even in liquid cooled engines.
Ducati know a thing or two about the thermodynamics of transverse V-Twins.
As Pete alludes to, it's not really the most convenient layout for a motorcycle engine, but that in itself has brought about some interesting solutions.

Originally Posted by Steam

Is it just a hangover from Harley times?

That's interesting, did Harley disappear sometime?

Harley Davidson, amongst their contemporary manufacturers of the time found that grafting an extra cylinder onto their existing (to make a v-twin) singles instantly double the capacity and therefore the performance

So if by Harley times you mean when motorcyles were first starting to be produced commercially then it yes it is.

Originally Posted by Steam

Wouldn't it be smaller, cheaper, lighter and tidier to do a parallel twin of the same volume?

See post six.

**Motu** · 20th February 2008, 18:04

Originally Posted by Steam

Wouldn't it be smaller, cheaper, lighter and tidier to do a parallel twin of the same volume?

Getting down to actually answer the question asked - yes,I reckon it would be cheaper,and tidier at least,and be easier to work on.The only real difference would be width,the V twin being narrower.This could effect handling too,as the wider crank of the parallel twin would make it harder to turn quickly.....unless you used a central flywheel like the British twins.

**Mental Trousers** · 20th February 2008, 18:13

Originally Posted by Motu

Getting down to actually answer the question asked - yes,I reckon it would be cheaper,and tidier at least,and be easier to work on.The only real difference would be width,the V twin being narrower.This could effect handling too,as the wider crank of the parallel twin would make it harder to turn quickly.....unless you used a central flywheel like the British twins.

Flip side is that you either have great firing pattern but out of balance like a big single, or you have a less than ideal firing pattern and half decent balance with a rocking moment (side to side).

**Fatjim** · 20th February 2008, 18:35

V's a re much smoother, which means they don't get fancy balance shafts or pistons with all the associated loss.

I don't know of any litre parallel twins although some are getting close. So it's hard to say if there's a torque advantage in either.

V2's cranks shorter, therefor should be able to be lighter.

V2's tend to need some balancing in the jetting and cooling to make them even. E.g, the VTR's front cylinder tends to run a bit cool.

The vtr is very narrow, thats with radiators on the side as well.

**pete376403** · 20th February 2008, 20:06

*some* Vees are much smoother -90 degree (ie Ducati air cooled, maybe others) have almost perfect primary balance but because the con rods are side by side, there is still a rocking couple so there's some secondary imbalance. 45 degree Harley have a "knife and fork" big end so no side to side imbalance, but the primary balance is real bad. (watch a harley idling, if your eyes can focus on the motor

A harley crank in ducati cases...the perfectly balanced motor??

parallels have a larger rocking couple - big ends are further apart - and a central flywheel only accentuates this. Even BMW flat twins which should be perfectly balanced will still have the fore and aft offset between the two cylinders = rocking couple .

Re litre twins - isn't Triumph testing a 1700 twin.
http://www.kiwibiker.co.nz/forums/sh...ad.php?t=65750

**Steam** · 20th February 2008, 20:13

This is real interesting, thanks Guru fullas

**Motu** · 20th February 2008, 20:49

Originally Posted by pete376403

parallels have a larger rocking couple - big ends are further apart - and a central flywheel only accentuates this.

A 360 crank doesn't have a rocking couple,maybe some torque reaction on firing.These days a 360 is a good idea as balance shafts are so last century.A V twin with a staggered crank has a rocking couple - I could feel my XLV750 waddle as it went down the road,very noticable at idle too.

Really,the dividing line is capacity - a parallel twin is getting a bit porky over 800cc,where as a V twin can still be a respectable size with some huge capacities.I will be very interested to see this 1700cc Triumph twin.