Rebound slap issue

[poorbiker]

69 FL, start by stating I am no hydraulic front end guy, that said here is the issue. Sent aftermarket tubes, stock length and lowers to Bill's and had new bushings installed. Using Progressives springs and the stock dampers and Belray 15w. 7 ozs. Spacers cut at 3.5" washer in-between. Compression is really good however on rebound they slap. Took them all apart to find no issues. Reinstalled, same issue. I have read the dampers are crap but better to ask those who know than to make guesses. Thnx in advance for ideas suggestions...

[Rubone]

That Belray is way too thin. Early H-D forks (pre '77) like heavier oils, up to 30W, which will slow down or even stop the clunk.

[poorbiker]

That Belray is way too thin. Early H-D forks (pre '77) like heavier oils, up to 30W, which will slow down or even stop the clunk.

Thnx, just read a good article on panhead/flathead site, said same thing.. Will try 20w first I think then 30w if necessary.... Thnx Rubone.......

[duffeycycles]

The '49-E'77 fork has a long plug that does not fit in the spring.Progressive normally needs abt 3/4" to 1" preload for other forks,the 3-1/2" spacer is more than 1" preload & is to clear the plug and makes them clunk on full extension.Later forks do not use this breather plug

[poorbiker]

The '49-E'77 fork has a long plug that does not fit in the spring.Progressive normally needs abt 3/4" to 1" preload for other forks,the 3-1/2" spacer is more than 1" preload & is to clear the plug and makes them clunk on full extension.Later forks do not use this breather plug

Not running the baffle valves that fasten to end plugs per progressive, one they don't fit into the PVC, two, progressive said I don't them with their setup but if I used a different spacer one that it's ID would allow clearance I could if I wanted. Now admittedly I'm not a hydraulic fork guy, just going by what I can find out. I understand the function of the baffles but they were adamant about not needing them. As far as preload they had no other info other than the 3.5" spacer....

[duffeycycles]

If you read the progressive install instructions,it says if your fork is not listed,you should have 3/4" to 1" preload.The 3-1/2" preload spacer is if you take off the tin part & leave the bolt,you need 3-1/2" to clear the bolt head before the washer on top of the spring.[way more than 1" preload]If you remove the bolt and plug with short allen screw,you don't need that 3-1/2" spacer and can use a preload as the other forks use--3/4" to 1"...this will reduce the spring pressure to the same as all other setups.You should have abt 1/4" above the tube with 3/4" of threaded plug = 1" preload=less clunk on full extension...You can also get later style seals from VTwin#14-0046
http://www.vtwinmfg.com/webapp/wcs/s...102_3293672_-1

[poorbiker]

I went back online and it seems I overlooked that. Seems strange dude at progressive didn't relay that tidbit of info. He knew I was leaving baffles out and I stated I was using an allen to plug threaded hole.
Will give that a try before increasing wt of oil though many have said that 15 wt is too light..thnx for info..

[duffeycycles]

I got the same info from them,seemed clueless, & this is my thought on the problem.This would still be more spring pressure than stock spring setup and progressive wound as you go down

[motorhead1]

I think Duffeycycles is on the mark with this one. Compression and rebound rates differ and with the added spring pressure and thin oil, rebound slap can be counted on. You didn't state if the dampers were new, only stock. We have had good results with new dampers, 25 to 30 wt oil, and added an extra 1.5 ounces to the dry capacity which covers the top end of the damper and helps further dampen the rebound clunk. I hope this helps.

[duffeycycles]

V Twin dampers often have a problem with the spring ends going in too far and scoring the damper.If using these make sure the spring does not extend thru/past...grind small amount off end of spring or they will self destruct..I see this on 50 percent of their dampers the other 50 are OK

[larry]

I've noticed that the original dampers use two different gauges of wire for the upper and lower springs. The aftermarket use the heavier gauge for both. I wonder if that can make a difference in dampening.

[poorbiker]

Update,
Took 1/4" off each spacer, drained and added Belray 30wt fork oil and test drove. No more rebound slap and compression on bumps still feels good, not harsh as I expected with the 30wt....thnx for all the advice.

[duffeycycles]

Good deal,this is one of the topics I have been wanting to share.That is still more than 1" preload??

[duffeycycles]

Just did this job,the threads are just over 1/2" long + 1/2" above the tube with the prelaod spacer gives a 2-5/8" spacer to achieve a 1" compression.That is 7/8" shorter than the 3-1/2" needed when you leave the breather studs in the top caps.The heavy oil is probably better than the light oil because of the increase over the stock spring tension...similarly when I put heavy springs in my sports car the stock shocks were too weak causing bounce,added koni racing shocks and they could hold back the heavier bounce of the heavy springs