Why should you upgrade both parts of the suspension?

I have been told by more than a few different people that if you replace one part of the suspension - say the front fork - you should replace the rear shocks as well, and vice versa.

The explanation I have been given is that if you replace just one part of the suspension, it is going to change the way the front "talks" to the rear and the bike will handle differently. This makes sense since the OEM suspension is designed to work together. Is there any other, more specific reason?

If you do a fork swap and now have a cartridge fork in the front, how do you know how the rear should change? How do you spec out the rear suspension to match with the new front fork? I have a bike that I want to put an USD fork on. What do I have to do to my rear shocks and/or swingarm to compensate?

Conversely, if you replace the rear shocks, how do you know how the front fork should change? How do you spec out the front fork to match the new rear suspension? I have replaced the rear shocks in my bike and am getting a RaceTech cartridge fork emulator for the front fork. Is that enough?

I usually stiffen up the suspension on all of my bikes, but in the process there will be a few days (cough: months) where only one end is done and it makes for an interesting ride. The bike does a lot more porpoising or rear-end bucking.

There must be more than a few people you know that don't really know what they are talking about.
No need to touch the rear if you are doing the front and vice versa unless the other end is worn out too.
The only thing you should do is replace or service forks or rear shocks as pairs and not separately.
So if you change the oil or seals in one fork leg it's good practice to do the other side at the same time.
This keeps the units performing equally and balanced as pairs.
When fitting a modern front or rear to an older machine you will most likely need to either stiffen/soften or alter the preload and damping to suit the weight of the machine but this won't affect the function of the existing suspension at the other end.

You don't need to upgrade the rear end if you install an USD fork but you will want to if you are looking for performance improvements. People typically do front and rear as a package to maximize the potential as "the whole is greater than the sum of its parts". This is certainly not necessary, but neither is installing a USD fork in the first place. I think a decent set of rear shocks will give you more bang for your buck in improving the ride quality than any other mod on a otherwise stock bike.

When I see a vintage UJM with a USD fork and stock shocks/swingarm I just think it looks half assed as the stock rear end has more room for improvement that the front.

DoctoRot wrote: .................. I think a decent set of rear shocks will give you more bang for your buck in improving the ride quality than any other mod on a otherwise stock bike.

When I see a vintage UJM with a USD fork and stock shocks/swingarm I just think it looks half assed as the stock rear end has more room for improvement that the front.

Agree! This is certainly true of the 1977 KZ650 bikes. The rear shocks were marginal when new and totally shot before the odometer hit 10k miles. The front forks on mine still work fine with regular maintenance after 62k miles. Notice on the 1977 KZ650 that broke all the 750cc records at Daytona the stock rear shocks were removed and S&W shocks installed. That bike ran 133.7 mph for 10 kilometers; 127.9 mph for 100 kilometers; and 117.1 mph for 24 HOURS. It held the 750cc records for 8 years after which a Suzuki GSX-R 750 set new 750cc records. Ed

I got custom Hagons for my KZ750 LTD a while ago and I definitely thought they were an improvement over the old stock ones that I had, but I did think that the front shocks were decidedly very harsh. I am planning to get cartridge fork emulators for the stock fork I have and then MAYBE - just maybe - I'll do the whole thing and swap out the front end for the 92 GSXR front end that I have with the USD forks. At that point, I'll definitely get a new swing arm at the very least.

The only thing I really dislike about USD forks are the stupid 3 spoked wheels that are an inch or 2 smaller than the original wheels. I don't particularly like the way the KZ750 LTD looks with 2 wheels of the same diameter.