Heavier rear springs

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I also have a kz750LTD and had the exact same issue with the stock shocks bottoming out with a passenger no matter what the setting was. I switched to progressive suspension’s 12 series shocks and their 105/150 rate heavy duty shocks and that is an issue of the past. Yes they are a bit stiff for a single rider(180lbs) but as soon as the wife gets on(140lbs), we can go anywhere without any worries about scraping or bottoming out. My 4-1 exhaust hangs about 1-2 inches lower than the factory one so my issue was scraping over speed bumps. We actually recently rode to Disneyland in Orlando, Florida on the bike and had absolutely no issues over any of the never ending speed bumps. 

Thank you for an excellent response. I did contact progressive yesterday. Waiting for a call back. Thanks again 

Just to throw another great U.S. made suspension manufacturer into the mix:  Worx Performance Shocks.  They custom tailored the shocks for my KZ 650.  If they have not made a shock for your particular model, they will take your measurements from your bike; swingarm to lower shock mount.  lower shock mount to upper shock mount, etc. 
Here is their link. >>>  Street Bike Shocks (worxshocks.com)

PLUS, don't forget your fork springs.  If you do not put in heavier rate fork springs, you will find yourself with less control on braking such as fork dive when riding two up.  Also, my stock springs were so soft that I would get head shake and loss of control at speeds above 90 mph.   RaceTech makes some great fork springs as well as Progressive.  They will also provide to your weight/riding style.  Here is the Racetech website >>>     Race Tech    

Here is my KZ with Worx Performance Shocks and Race Tech fork springs.  It handles great two up and holds lines in corners very well.  

 
 

Wow. Thank you for that info. Didn't know I needed to upgrade the front too. I just added a lil air to them and it solved the bottoming out up front 

wrote:

Wow. Thank you for that info. Didn't know I needed to upgrade the front too. I just added a lil air to them and it solved the bottoming out up front 

Trying to understand your reply above.  Did you add "air" to your front forks?  Or did you add air to your tire?  
I believe your forks have springs and fork oil in them.  There are seals to keep the oil in place and dirt out.  I do not believe they are a type of air fork suspension. 
 

Definitely air shocks up front.  Added 10psi to each side over the weekend and it's good now. 
I just had the fork seals and oil changed in the front. 

wrote:

Definitely air shocks up front.  Added 10psi to each side over the weekend and it's good now. 
I just had the fork seals and oil changed in the front. 

Still a bit confused.  Do you have air shocks in back, the rear twin shocks under the seat to the swing arm?  You mentioned fork seals and oil changed in the front.  I don't see how you have air in the forks?  
Just trying to help.  

Definitely air in the front. The only way I knew was when I read my owners manual. 
No air on the twin shocks out back. Just 2 adjustments. One for spring stiffness and one for rebound. Neither seems to really help...
adding air to the front did help! 
And yes it was in the shop for a few things, one was the leaking front fork seals, and fresh shock oil. 

wrote:

Definitely air in the front. The only way I knew was when I read my owners manual. 
No air on the twin shocks out back. Just 2 adjustments. One for spring stiffness and one for rebound. Neither seems to really help...
adding air to the front did help! 
And yes it was in the shop for a few things, one was the leaking front fork seals, and fresh shock oil. 

I must apologize.  I was locked in on the KZ750 twin that was produced from 1976-1978.  This later 1983 750 LTD version did have a pneumatic front fork.  I know next to nothing about those forks so I will defer to others.  

Regarding the front air forks - I loved the idea, the flexibility and how easy it is to customise the bike to the rider, and stuck with them for years. But to be honest, putting in a thicker oil and slightly longer spacers and removing the air pressure was one of the best upgrades I've done. Adding air pressure makes the seal tighten around the stanchion, which makes it grab. Increasing the air pressure only makes it worse. Dropping the pressure makes the suspension so much smoother.

wrote:

Regarding the front air forks - I loved the idea, the flexibility and how easy it is to customise the bike to the rider, and stuck with them for years. But to be honest, putting in a thicker oil and slightly longer spacers and removing the air pressure was one of the best upgrades I've done. Adding air pressure makes the seal tighten around the stanchion, which makes it grab. Increasing the air pressure only makes it worse. Dropping the pressure makes the suspension so much smoother.

What size spacers did you use on the front?  I put a1982 GPZ750 front end on my sons KZ650 and while the fork seals don't leak oil, they slowly let air out and it's a pain in the butt, I want to add slightly heavier oil and put some preload spacers in to stop the dive in the front, it's too soft......