Topic: Servicing Rear Shocks?

[turkey2143]

Is there a good way to go about rebuilding the rear shocks on a 77' CB550, or is it worth it to just get new ones?

Also, do the springs wear out over time? My bike has 20k miles, and I'm having my rims powdercoated black, so I was thinking I could just throw in the springs, which have some surface rust on the chrome for kinda cheap versus buying a new pair for $60.

What would you do in this situation?

[OldSchool_IsCool]

Springs rarely loose strength unless you heat them.  The shock is not serviceable.  Replace 'em.

[MCRider]

All springs which are used cyclically, even if not beyond their design limits, will suffer fatigue from use. Suspension springs are no exception.
http://www.roymech.co.uk/Useful_Tables/Springs/Springs_fatigue.html
Need go no further than the intro.

Japanese motorcycle shock springs are notorious for becoming "sacked" with little use. That is, worn out from fatigue. That goes for the whole shock as well. The magazine road testers would often say they are shot on the showroom and should be replaced.

The better steel, the longer than can resist stress fatigue. But ultimatley they will "sack" as will valve springs, fork springs, and the like, the key being used in a cyclical manner.

Springs which are compressed to a point and held within that point which is less than their ultimate limit, will likely not suffer (as much) stress fatigue unless heated.

PS: At 20k miles on stock springs, they are junk. Additionally, the rust introduces an element of "corrosion fatigue" which just adds to the cyclic fatigue.

here's a simple Q&A on the subject. While our springs rarely break as in the Q&A, they simply become progressively weaker and enter the "sacked" condition. They may be strong enough still to allow the bike to appear all right, but when weighted they will compress more than origianlly. (sacked).
http://van.physics.illinois.edu/qa/listing.php?id=662

Another good Q&A
http://www.ehow.com/way_5761826_would-rear-coil-springs-break_.html

They keep talking about breaking a spring, which we seldom do. But weakening the spring is done almost immediatley.

[camelman]

MCRider,

I agree with the points in the last article, but I do not think they pertain to our motorcycles very often unless the motorcycle is regularly heavily loaded, driven over incredibly rough roads routinely, has very high miles on it, or is exposed to ridiculous amounts of corrosive materials.  We could go at it for ages on this topic, but from my practical experience over the last 15 years of studying and working as a mechanical engineer, I just don't think there is much need to change a spring without an obvious defect/failure being evident.  If the mileage is getting high, and the springs are heavily corroded, then I would consider swapping them out, but at 20k miles, I don't imagine they need replacing.

These springs will last much more than 20k miles as long as they are designed/manufactured correctly and not taken past their yield limits too many times.  Also, it is hard to take our springs past their yield limits due to the rubber bump-stop on the shock piston that acts as a secondary spring to keep the shock from over-compressing.

You can read more on springs at this site: http://en.wikipedia.org/wiki/Spring_(device) under the "popular mechanics" paragraph.

You might also be interested to read about low cycle fatigue, which is what most people mistakenly consider to be the failure mechanism for springs.  Our springs could fail this way, but you would remember every bump you hit on your motorcycle that pushed the springs to the point that they accumulated cycles towards their ultimate LCF limits.

So, I have to say that the springs, unless they are damaged, are okay to reuse.  If you happen to be a bad-azz rider who routinely takes the suspension to the limit, and who would notice even subtle changes in the handling of the bike, then a new set of high quality springs would make a difference.

Camelman

[MCRider]

Camelman:
Points taken. On springs in general.

But in the case of springs on a Japanese motorcycle of that era, my practical experience is that it is an area where the manufacturers cheap out from the beginning. I'll go out on a limb and say this is the general consensus of mechanics familiar with the era. You mention "designed/manufactured correctly " and I guess my point would be, they weren't.

It is not uncommon to remove the fork springs and measure them, and find they have collapsed to less than the service limit. Rarely have they been stressed from excess load, mileage or riding conditions.

It is not as common to make these measurements on the rear as its more difficult to do so, and replacing the shock body itself for loss of damping is usually the priority and leads to replacing the spring at the same time.

So I will concede your points as they relate to springs in general. But as to Japanese motorcycle springs of that era, for proper, not even superior, performance, they likely should be replaced with non-OEM, regardless of the use they have been subjected to. IMO.