sudden bottoming of stock fork springs 1985

[mtc]

i took em out to put .85 racetech, last year after i got the bike, never really rode the stock one other than riding it home, the left fork was leaking bad, ruined the left front brakes, a little wallowy but no bottoming.( seal mate works great)

didn't ride the racetech much either, and they were/are to stiff for my light azz weight.

so during this covid 19 lock-down , i got some time to switch it back to stock try to sell the racetech's and recover some much needed money.

so i put in 135mm measure from the top, springs out, fully compressed and worked the tubes up and down to distribute the new 15wt Motul fluid

the stock one went back it,

wow, no brake dive (anti-dive deleted last year) and handle great for me and gave me much needed confidence.
done; i love yamaha, ridin was good

put around 200 miles on them freeway,  over 2 weeks, with 80mph sweepers and switchbacks, short but fun run on the deserted 110 pasadena freeway out here in lock-down Los Angeles.


suddenly yesterday, getting off the Hollywood freeway, onto a bad bumpy strip of city street, while on the brakes the front started bottoming, i dialed up the preload 1 notch and damping all the way, same thing

rode home that evening, wtf

does anyone with any suspension sense, or and engineer type want to take a stab at what the heck happened?

[FJ_Hooligan]

Maybe the 15wt oil didn't allow fast enough rebound of the fork and it "packed down" on a series of bumps and just ran out of travel. 

You may have made the problem even worse by going with maximum (compression) damping.  Back that off some and try again.

How much spacer did you use and how much did it preload the spring?

[mtc]

Maybe the 15wt oil didn't allow fast enough rebound of the fork and it "packed down" on a series of bumps and just ran out of travel.  

You may have made the problem even worse by going with maximum (compression) damping.  Back that off some and try again.

How much spacer did you use and how much did it preload the spring?

it rode fine for 2 weeks, if it packed down would it not have happened almost immediately?

how Weak are the stock springs as to not me able to handle a 1 step increase in viscosity?
i left the bike alone for 5 hours and once i got back on. the bottoming was all too apparent.


and wouldn't the warm weather and the riding marginally thin the fluid causing less dampening?

i put in the stock spacer, i'd say maybe an inch?

i thought maybe i broke a spring, so i took them out, and in went back in the racetechs , but this time with 10w40 motor oil, i said f it, i aint paying $15 for fork-you oil when yammie says motor oil $3.99 auto zone

cured, albiet a little stiff, still the mystery of bottoming


one last thing, when i put the stock ones in i had to fight to get the cap on, when  i took them out , they didn't seem to push back as much. weaker?

[balky1]

You said you had removed anti-dive units? Put some block-off plates? If yes, those plates need to have oil passages.
I think that without emulators you should only be using progressive springs. Stocks are very weak.

[mtc]

You said you had removed anti-dive units? Put some block-off plates? If yes, those plates need to have oil passages.
I think that without emulators you should only be using progressive springs. Stocks are very weak.

i just disconnected the hoses and threw bolts where the lines went,, those are ugly looking things, but they are out of the loop, i did take the hat off today and flipped it upside down as per fj monkey's suggestion and man do they look pretty now....

[Pat Conlon]

Those .644 kg/mm stock fork springs were weak the day they left Iwata....
Now, 35 years later, they are truly sacked.

[mtc]

Those .644 kg/mm stock fork springs were weak the day they left Iwata....
Now, 35 years later, they are truly sacked.

and the winner is pat, scrap metal time

[mtc]

Those .644 kg/mm stock fork springs were weak the day they left Iwata....
Now, 35 years later, they are truly sacked.

and the winner is pat, scrap metal time

i want to add that the bikes has ~43,000 miles