Suspension THUMP!

[grbrown]

My bike is a 1990 Evo, owned from new, except I recently fitted an S&S with new engine mounts, True-Track swinging arm solid bushes and handling kit. Everything is new, including Fournales high pressure rear air shocks (running at 155 psi), plus larger section 140/90 rear tyre (Avon Venoms both ends). The shocks are not supposed to bottom out, because of their design.

The bike handles great and we have just returned from a 2,500 mile tour to Slovakia. But despite all the new stuff, my 'old lady' still occasionally thumps when hitting a bump or pothole. We ride fully laden at around 1200 pounds and I run 44psi front and 50psi rear. The bike has always done this (as best I can remember!), so I had hoped my swish new shocks would cure it.

It really does sound as if something solid is hitting something else solid, but I can find no obvious cause of this. I have even considered fitting longer than standard rear shocks, to get a bit more suspension travel.

I know from US forums this is not uncommon, also I don't think it is model specific, so can anyone shine a light on this thump?! Any idea what causes it and if there is a solution?

[old-mo]

Probably wont help,, but here goes anyway..

On my old Softail I was getting a thump/knock when going over rough ground/roads, and sounded like suspension problems, searched and searched and just happened to try lifting the bike with the tank and got a similar sound.

Took half the tank off and the rubber grommet thingy was so old and compressed it was allowing metal to metal. Over rough roads etc.

You never know till you check.

[grbrown]

Thanks Old-Mo, but my bike has a one-piece tank bolted metal-to-metal. That thump coincides with an unpleasant sensation through the backside! Mrs especially suffers and as we now have an intercom I also suffer through both ears. There are far reaching consequences....

[SHOEI]

Could it be when your'e riding fully laden & hit a pothole ect that the sudden impact is causing the swing arm to hit the rubber bumpstops which are directly under your'e backside, have a look to see if they are still intact because from personel experience i found one had perished on mine (1989) & must of fallen off because there was only the mounting stud there & the other one was rock hard & well perished. If they have dissappeard this may explain the solid to solid your'e suggesting.
Just a thought.

SHOEI

[grbrown]

Shoei,

That's not 'just a thought', its very helpful! I have been through the workshop manual and parts books, also been round the bike on my hands and knees but cannot find these things. If your bike is an FL model it should be pretty well identical to mine. Can you be more specific just where they are? Ta!

[SHOEI]

Sorry Graham should have taken more notice of your scoot before replying. Mine is a Softail.

[grbrown]

Originally Posted by SHOEI

Sorry Graham should have taken more notice of your scoot before replying. Mine is a Softail.

Shoei,

Ah well! Had me going for a while!

I plucked up the courage to email Fournales over the weekend and they replied first thing. Their website is only in French (viva la difference - most other nations translate!), so I was hesitant in writing to them in my best English. While they give recommended pressures by model on their website, they don't mention varying from that. Seems I can pump up my shocks from 155 psi to as much as 180 psi, to compensate for additional load.

This bike has always bottomed out when fully laden, despite using max factory figures on the original factory shocks, which is a real pain, so I am not too optimistic increased pressures with the Fournales will cure it, but it may help. We are still recovering from the memories of our trip, so have yet to venture out on the bike since getting home.

Any other comments or suggestions welcome.

[daddykemp]

I have an '03 883 sportster custom that does exactly the same thing weather it's just me, the missus or one of my sprogs on board. What I think it is, is the heavy unsprung weight of the rear wheel and swing arm falling down the pothole or whatever and because of the high pressure that we use in our tyres it's like having solid tyres with no ,or very little, give in them (remember your old bike/trike when you were a nipper with solid wheels and how you felt every lump and bump?) and the resultant shock wave travels up the frame and suspension units straight into your ( or your wife's ) butt with extremely uncomfortable consequences!
By the way the rear on my sporty is one of them solid alloy things that weighs half a ton, so perhaps a lighter spoked wheel would be better?
It seem's to be a common problem on a lot of the Harley range of models.
I don't think it matters what suspension unit you put on, I don't think it will ever be completely eliminated.
But I'm sure someone on here will say different!! (respect to them if they have found the answer!)
By the way I've just been thinking whilst typing this, could it be that the suspension units don't do a good job of stopping the wheel from dropping into the pot hole? they kinda just let the wheel flop into the pothole instead of keeping it at road level? Could also be the low profile tyres we all seem to use nowadays. Just a thought!
Also keep an eye on your rear tail lights as I've had three go on me now from the shock of the resultant bump from this problem!

[grbrown]

I did indeed have a little trike, but that was a long time ago and all I recall is it was green and red! But you've touched on some interesting points. My bike has 90 profile tyres, so it is as well provided for in that department as it could be, and there should be plenty of give in the rear tyre. However I suspect that all Harleys have rear shocks with very limited travel so, as you say, they don't do a good job.

Your point that the wheel flops into a pothole is well made, as the lack of suspension travel doesn't allow the units to push the wheel down and keep the bike more level. Perhaps that thump I mentioned is actually a double thump? One as the wheel drops into the pothole, followed immediately after by the bike bottoming out as it follows the wheel down, or meets the wheel coming back out of the hole!

It will be interesting to see if anyone comes up with a fuller explanation. I have tried to prompt discussion on a US forum, but without any success! Had hoped there would be a larger pool of expertise over there. I agree we may never eliminate thump, but it would be nice to understand it better and reduce it - if only for improved marital bliss!

As for your solid wheel, that must not only be heavy, but have zero flexibility, so I agree with you there. In the early 70s, when alloy wheels first came on the market, there was an interesting article in one of the Yank bike mags. They studied accident damage of bikes that hit something head on. Spoked wheels collapse at the point of impact and, if the forks fold backwards, again where they hit the engine/frame, ending up banana shaped. Alloy wheels break their rim at the point of impact, then do the same again where they hit the engine/frame, staying round. The 'crumple zone' is dramatically less than with a wire wheel, but also loads are transferred into the frame, damaging it around the steering head, which does not occur to the same degree with a wire wheel. That all wanders a bit off the main topic here, but ephasises the contrast between wire and alloy wheels.

Since I last wrote I have exchanged further emails with Fournales. It seems the pressures I previously mentioned were only suggestions and that their units will actually accept eye-watering maximum pressures. I won't repeat them here as I don't believe they will do anything more than lock them solid! At least owners have free rein to experiment well beyond the 10.5/11 bar recommended on Fournales' website.

[Adrian]

Originally Posted by grbrown

My bike is a 1990 Evo, owned from new, except I recently fitted an S&S with new engine mounts, True-Track swinging arm solid bushes and handling kit. Everything is new, including Fournales high pressure rear air shocks (running at 155 psi), plus larger section 140/90 rear tyre (Avon Venoms both ends). The shocks are not supposed to bottom out, because of their design.

The bike handles great and we have just returned from a 2,500 mile tour to Slovakia. But despite all the new stuff, my 'old lady' still occasionally thumps when hitting a bump or pothole. We ride fully laden at around 1200 pounds and I run 44psi front and 50psi rear. The bike has always done this (as best I can remember!), so I had hoped my swish new shocks would cure it.

It really does sound as if something solid is hitting something else solid, but I can find no obvious cause of this. I have even considered fitting longer than standard rear shocks, to get a bit more suspension travel.

I know from US forums this is not uncommon, also I don't think it is model specific, so can anyone shine a light on this thump?! Any idea what causes it and if there is a solution?

Graham my Bike is a 1991 soft tail FLSTF modified with DNA Springer forks. I also had the constant bottoming out problem on the swing arm especially when loaded. No matter how much preload I applied to the comparatively un-used shocks. I purchased a legend air ride L8 system which I have run now for 2 years this sorted all the bottoming out and improved progression and ride comfort on bad road surfaces and differing loads. I have even run the bike with my brother on the back (combined all up weight of both of us being 224 Lbs !!!) plus the bike, the system worked very well. I dont want to split the thread but Has any body else had experiences with the Legend air ride systems with non softail applications ie road king ect. Ade