Rear Sag

[dlaing]

I have heard two things:

1) Sag should be about one third of suspension travel

2) Sag should be set on a shaft drive bike so that the shaft is in line with the transmission.

 

Now I assume that in case 2 that the sag would be one half of the suspension travel.

 

But would it make sense that if you set it to one half the shaft would spend more time below the centerline because of the bumps in the road?

And because of that, perhaps one third sag is fine?

 

Perhaps some of us are getting less power and fuel mileage because are sag is set too small.

 

I imagine if I was going for a land speed record, I'd set up the suspension for maybe an inch of travel.

 

Just out of curiousity, I wonder if anyone knows which Guzzi had or has the longest shaft.

My guess would be the Quota.

And no I am not suffering from shaft size envy <_>

But sometimes I wonder if the parallel swing arm would not be better scrapped for a longer swing arm and shaft.

I guess handling would be sacrificed for marginal gains in stability, comfort and power.

[Guest Jeff Kelland]

I agree that sag, the amount of suspension travel from topped out down to loaded ride height including rider, should be 1/4 to 1/3 suspension travel. I am not well versed in the dynamics of shaft drive bikes in regards to the relationship between the shaft and suspension travel. I do however use some software that calculates acceptable vibration limits in drivelines for trucks. It takes into account driveline angles and shaft lengths. There is a fair amount of tolerance for the angle of the u-joints before vibration becomes a problem. Just for kicks, I will try to measure the guzzi's shaft and plug in the numbers when I go back to work. I also have noted that drivelines don't necessarily like to be perfectly straight.

[al_roethlisberger]

Yep, the track/maintenance schools I've attended recommend the ~1/3 sag for all types of bikes. True the courses were "generic" so perhaps for certain types of bikes such as shaft driven bikes there may be some subtle considerations, but I've not heard any.

 

al

[dlaing]

I also have noted that drivelines don't necessarily like to be perfectly straight.

Interesting...care to elaborate?

Is that concerning vibration or power?

[Baldini]

dlaing,

 

I'm interested in the point you've raised but it is not clear to me what specifically you are concerned may cause problems, nor what those problems might be. All I can think is the varying angles of the UJ's = power loss/vibration/stress & that setting sag to prevent more extreme angles is beneficial. Is that what you're getting at - or have I missed the point?

 

Are these effects any more of a concern, in relation to suspension settings, than varying tension in a chain drive system?

 

I set sag to 30-33% cos I know no better.

 

Cheers, KB, Wales

[Guest Jeff Kelland]

The information available to me relates to vibration issues. I can inquire about power transmission with engineering and see what they say, may take a few days, stay tuned!

[dlaing]

dlaing,

 

I'm interested in the point you've raised but it is not clear to me what specifically you are concerned may cause problems, nor what those problems might be. All I can think is the varying angles of the UJ's = power loss/vibration/stress & that setting sag to prevent more extreme angles is beneficial. Is that what you're getting at - or have I missed the point?

 

Are these effects any more of a concern, in relation to suspension settings, than varying tension in a chain drive system?

 

I set sag to 30-33% cos I know no better.

 

Cheers, KB, Wales

Correct. Setting sag to prevent extreme angles would be beneficial.

To absolutely prevent extreme angles one may need shorter travel or a longer shaft.

How you define extreme angles is another issue. Perhaps setting a bike to zero sag would lose little power, but I doubt it.

I was thinking of setting the sag to minimize the angle and thus maximize the power.

Perhaps there is little difference between 30% and 50% sag in terms of power loss and vibration. I don't know.

My other question is that assuming their is significant power loss, what would be ideal sag?

When you accelerate, does the bike squat? and if so, how much?

Or does it jack under accelleration?

Do typical road bumps cause the sag to fall lower, if so, how much?

Perhaps the squat and road bumps make 33% sag an ideal setting, I don't know.

But this is what I would like to discover.

Of course all bikes are different and 35% might be ideal on one bike and 40% might be ideal on another, depending on suspension set up.

Perhaps one could put a bike on a dynometer and do a run with the bike set to different sag heights.

...or Jeff could crunch some numbers and then we might know the theoretical ideal.

PS you are lying, there is no sun in Wales!

[gthyni]

Usable power is the key...

using a less than optimal suspension setup

to get more power certainly reduce the usable power.

A local Harley rider has a tuned motor with 100+ hp wasn't

able to outrun me on the BSA with less then half of that,

cause his bike handles like sh*t

[Mike Stewart]

Rear Sag, How Much,

 

My wife says my rear doesn't sag, but the front is another story!

 

Mike

[callison]

I hear ya Mike! Have a good time at the rally - I won't be going. Drat! Ya wanna haul down the infamous CD T-Shirt? Just kidding, the logistics to get it to your house are a bit extreme for that. Think I'll save it for the next Prescott Rally. Maybe the guy who donated it the first time around will get it back.