V7 shock - why so short travel?

[GonzoB]

I bought a 07 Breva 750 recently and the top of my standard shocks look like this:



The current V7s are the same. There is clearly space to make the shock body at least 25mm longer, and thus give ~30mm extra wheel travel. The 750s have 93mm of wheel travel, which is low compared to the many other bikes in the class with mono-shock linkages. I see why MG might want to keep the "look" of the twin shocks, but why not give them more travel? There's plenty of wheel clearance. Surely it's not a cost issue?

Gonzo

[jacksonracingcomau]

Can’t answer the why question
But I’ve fitted a pair of ohlins with 100mm of shock travel to my 80’s SB (essentially same ) with no downsides, recently took apart to regrease splines after 2 rear tyres, splines new then, still perfect.
Someone did post of issue with 650TT shock length and splines so perhaps factory are staying ultra safe, leaving any mods to the aftermarket ?
Interesting if ohlins, wilbers etc etc offer more still offer more travel as bolt in option, shocks I have were listed for lario and mk 3 le mans ( with different spring rates) in 1986.

[Frenchfrog]

The longer shocks on the Enduro models ( TT and Ntx) certainly didn't help the UJ  as the angles were getting too extrema...for the Dakar factory specials the whole rear end was modified to take the big bloc set up.

[Roebling3]

Plus1.
It's that simple and likely always will be?
An electric drive motor in the hub?   R3~

[Darren Williams]

"An electric drive motor in the hub?"

Lot of un-sprung weight to deal with.   

[Roebling3]

Pls forgive me. I don't know how to add the proper cartoon. R3~

[GonzoB]

So... if I understand correctly... by letting the wheel to travel further towards the seat than the original shocks allow will risk damaging the universal joint? Really? Has it ever happened?

Gonzo

[huub]

i used to run a hotted up V65 TT off road , the rear suspension was its Achilles heel  ,
just fitting longer shocks reduced the UJ life to a couple of weekends.
As mentioned above, the serious off roaders use the rear drive of a bigblock, so they have two UJ, basically doubling the angle available.
if that means yous V7 Uj will explode if you add one inch of suspension travel? i dont know,
 i had huge suspension travel at the time,

[Frenchfrog]

Well I've recently seen photos of one small bloc  that had the uj blow-up and take out the rear of the gearbox ......the rider was lucky not to be hurt.There is obviously a number of possible causes so checking the driveline should really be a routine maintenance point...and then a few preventative measures like keeping the shock length reasonable and fitting a steel tube to the gearbox output tunnel may be smart moves too.

[huub]

i have even had the swingarm taken out by a failing UJ.
if you fit a steel sleeve in the gearbox, a failed UJ will not take out the gearbox cover ,

recommended!  fitting the sleeve  is a cheap insurance if you plan to run the bike hard
the steel sleeve is available at most guzzi parts specialists, (or easy to make yourself if you have access to a lathe)

[Frenchfrog]

I have not been able to track one down Huub...and don't have a suitable tube to make one out of at the moment.

[Tom]

Cheaper for mass production and "adequate" for most applications.

[huub]

size:
57 mm long,
outside diameter 64 mm tapering down to 61,5 on the other side
inside diameter 57 mm

i had a look, apparently TLM ran out of them
https://shop.tlm.nl/nl/modificatie-kruiskoppeling-bus-v35-enz-192008000011-moto-guzzi,

but i usually make them from a piece op scaffolding pipe
fit is not critical just glue them into place with whatever you have (i use silicone kit)
if the UJ seizes , the bush will turn inside the gearbox casing, protecting the casing

[Frenchfrog]

Thanks Huub !

[Off @ 90]

Good idea to check the cardan drive shaft splines front and rear for grease .Especially on a newish bike as the factory seem to be a bit shy of using it. On my Stornello one spline (rear ) was dry and swing arm pin rusted slightly only 2 years old 4000 Km. I would check/ grease splines at least every tire change . Shaft drive does not mean zero maintenance.
Duncan

[GonzoB]

Good idea to check the cardan drive shaft splines front and rear for grease .Especially on a newish bike as the factory seem to be a bit shy of using it. On my Stornello one spline (rear ) was dry and swing arm pin rusted slightly only 2 years old 4000 Km. I would check/ grease splines at least every tire change . Shaft drive does not mean zero maintenance.
Duncan

Thanks Off. You must have read my mind.

My Breva is 12 years old but had done only 2000km when I bought it two months ago. It's now got 5K and I've replaced the tyres, all the fluids (except the fork oil), greased the steering bearings and swing arm pivots, and took it upon myself to lube the shaft splines yesterday. "This will be easy", said I. "Just take off the wheel and unbolt the hypoid gear bit". Yes, well, three hours later I was re-assembling the whole swing arm. The splines had bugger-all grease. The steering bearings I did previously were the same. Is grease expensive in Italy??

All good now.

Gonzo

[GonzoB]

OK, thanks for the comments about the drive shaft. This inspired me to go and take some measurements off my bike.

Picture this:



The red line shows the centreline of the drive spline as it comes out of the gearbox.
The yellow line shows the centreline of the driveshaft, meeting the red line at the swingarm pivot.
I transferred the measurements to a graphics package and worked out angle -A under various conditions

At full (standard) shock extension (390mm)  A= -11 degrees
At full compression, A= almost ZERO (actually less than plus 1 degree)
At a standard rider sag of about 30mm, A= -7 degrees

So, on a standard bike, most of the time the swingarm is bent down 7 or less degrees, and on big bumps it goes to zero.

Now, one of the reasons I am investigating this is that I want to put shorter shocks on the bike to reduce the seat height, and that will mean that angle -A will become more positive. I calculate this:

With a 20mm shorter shock:

At full extension,  -A = -7.5 degrees
At full compression, -A = +3.5 degrees

I think that should be well within the capabilities of the cardan joint.

Gonzo