I just replaced my 3x8 front derailleur with a Shimano Claris 2403 8-speed triple. I also have a trigger shifter, Shimano Acera shifter. Everything works fine but to get it to shift into a higher gear it takes an extraordinary amount of thumb power both in terms of strength and distance to push the trigger shifter. However, once it clicks it shifts into the higher gear. Does anyoneknow what could be causing this problem? Were these shifters built with a super strong spring or poor leverage that it would require such power? Did I mess something up?

Do you have the shift wire routed correctly? Many Shimano FDs require you to route the wire around the little tab adjacent to the clamp screw, then under the screw head; if you go directly to the screw it shortens the lever arm and significantly increases the effort required.

I believe the Acera shifter is a MTB shifter and the Claris is a road derailleur. They are not compatible - different cable pull.

+1 And I'll add that the road and MtB cranks can have different ring c-c dimensions further complicating things sometimes. Andy

Taking a lot of force to shift suggests kinked housing or cable. Or a burr on the end of the housing scraping against the cable.

With the cable disconnected from the derailleur, examine everything, moving the cable by hand and feel for excess friction. Also, check the bb guide, and where the cable exits the shifter for mis-routing , pinching or rubbing.

Check the cable housing. If the routing is too convoluted, friction can delay shifting. It's quite common for bikes to be assembled without trimming the housing to a length allows for the least length of housing needed route the cable properly. Too much housing increases friction both by the excess length creating more friction and having to go through complicated curves and loops on the way to the derailleur. Less housing is better, but only to the extent that you can stll turn the bars without binding.

Thank you for your responses. I have the shift wire routed correctly. I made sure by looking at a lot of pictures. I did make this mistake once on another bike and the LBS straightened me out. I disconnected the cable and checked everything out including the BB guide. The cable is new and the housing is okay. Shifting with the cable held in my hand worked fine.

The derailleur is adjusted perfectly on the stand but I did manage to drop the chain on a ride. I changed the shifter to a more vertical position so it would be easier to shift (pushing down on it seems easier). I can ride the bike fine. So that brings me to the MTB shifter/road derailleur. Thanks for the explanation but I must admit that I don’t understand the dynamics. I trust you’re right and I’ll see what my LBS can do for me. The options for front derailleur 3x8 with a down pull are getting rather limited. I looked at the online specs and many say nothing about whether or not it is for a road bike or MTB including the description of the one I bought. Thanks again. I’m off to my LBS. Keep on biking.