Technical Rear axle bottoming out

[Timofimo]

Hi all

First time poster here. I've recently become the proud new owner of a beautiful Fiat Barchetta from 1997. It's only got one problem. Small bumps cause a quite uncomfortable thud inside the car. It feels like it's mainly coming from the rear axle. I inspected the bottom of the car and found, well.... nothing. The bushes, shocks, springs and joints are all in good condition. No play and no noises anywhere. The only thing I noticed which seemed strange to me is that the bump stops seem to almost touch the underside of the car just sitting on my driveway. But after looking online this seems to be normal. (https://www.fiatforum.com/threads/m...nti-roll-bar-calipers-paint-some-more.331463/)

I'm looking for some advice as to what my next move should be. Since nothing is broken there's no obvious solution. But I'd still like a fix.

Any help would be appreciated

 

[bugsymike]

Not specific to your model, but have you checked the ride height as it is quite possible the springs have sagged a little over the years.
Another point is many cars have progressive bump stops which start to make slight contact and then more as load increase.
Also have you tried testing the shock absorbers, they make look in good order but when removed and tested you often find they have lost that damping effect.

 

[Timofimo]

Not specific to your model, but have you checked the ride height as it is quite possible the springs have sagged a little over the years.
Another point is many cars have progressive bump stops which start to make slight contact and then more as load increase.
Also have you tried testing the shock absorbers, they make look in good order but when removed and tested you often find they have lost that damping effect.

I checked the ride height. The distance from the ground to the wheel arch is 63.7cm on both sides. I'm not sure if that's low or not, but at least both sides are even.

I found online that the Fiat Punto 176 on which the Barchetta is based has 2 options for the bump stop. With sportier versions using the same as my Barchetta and the less sporty version using a much shorter bump stop. I'm considering trying the other bump stops. But I'm not sure what that will do to the handling or if it's even safe.

I haven't removed the shocks to test them but I did give the car a good jiggle. The car returns to it's resting position right away like it should. Again, maybe the shocks are a bit tired but they've definitely not failed.

 

[bugsymike]

I checked the ride height. The distance from the ground to the wheel arch is 63.7cm on both sides. I'm not sure if that's low or not, but at least both sides are even.

I found online that the Fiat Punto 176 on which the Barchetta is based has 2 options for the bump stop. With sportier versions using the same as my Barchetta and the less sporty version using a much shorter bump stop. I'm considering trying the other bump stops. But I'm not sure what that will do to the handling or if it's even safe.

I haven't removed the shocks to test them but I did give the car a good jiggle. The car returns to it's resting position right away like it should. Again, maybe the shocks are a bit tired but they've definitely not failed.

Perhaps another owner of your model in the Barchetta section can confirm the ride height.
As a rough guide, standing away from the vehicle does it appear lower at the rear?
Another clue is sometime on suspension designs, as the weight goes on to the suspension or in your possible case as the suspension settles/weakens over the years, when looking from the rear of the car the wheels can appear to be leaning in at the top (Negative camber) over a period of use this can cause the rear tyres to wear more on the inside of the tread.
Even with progressive bump stops you shouldn't hear the suspension bottom out as that is the idea to be "progressive".
When under vehicle are there rubbing marks where the bump stop has been hitting the suspension arm and causing the knock? As if not the sound can be from elsewhere.
Have you checked any rear antiroll bar mountings?
Looking at your second photo I cannot see bad signs of bump stop contact, however the general condition of the road springs does make me wonder why the shock absorbers look fairly new but the springs look quite old to me. If the original shock absorbers had failed and needed replacement it is not unusual for them to have weakened and even broken in some cases the road springs.
Many years ago (1975) as an impoverished young motor mechanic I had a 1964 MGB Roadster and knew the front shock absorbers were worn out but put off replacement to my detriment as it ended up with broken front springs as well so my parts bill increased!

 

[AnthonyH]

Axle bushes will be shot. Probably the shocks too.

 

[Timofimo]

Perhaps another owner of your model in the Barchetta section can confirm the ride height.
As a rough guide, standing away from the vehicle does it appear lower at the rear?
Another clue is sometime on suspension designs, as the weight goes on to the suspension or in your possible case as the suspension settles/weakens over the years, when looking from the rear of the car the wheels can appear to be leaning in at the top (Negative camber) over a period of use this can cause the rear tyres to wear more on the inside of the tread.
Even with progressive bump stops you shouldn't hear the suspension bottom out as that is the idea to be "progressive".
When under vehicle are there rubbing marks where the bump stop has been hitting the suspension arm and causing the knock? As if not the sound can be from elsewhere.
Have you checked any rear antiroll bar mountings?
Looking at your second photo I cannot see bad signs of bump stop contact, however the general condition of the road springs does make me wonder why the shock absorbers look fairly new but the springs look quite old to me. If the original shock absorbers had failed and needed replacement it is not unusual for them to have weakened and even broken in some cases the road springs.
Many years ago (1975) as an impoverished young motor mechanic I had a 1964 MGB Roadster and knew the front shock absorbers were worn out but put off replacement to my detriment as it ended up with broken front springs as well so my parts bill increased!

Yeah I checked the car. Front and back seem to be at the same height. It's also not leaning left to right.

I wouldn't say I hear the car bottom out as much as I feel the car bottom out. I can feel that the bump is transferred into the body.

The shocks probably look newer because I cleaned them a bit. I was trying to find a date or something to figure out when they were manufactured but I don't see any date on it. Only a part number and a "do not disassemble" warning.

The anti-roll bar mountings look okay. A bit tired like the rest. Some visible cracks in the bushings but they're still all there and there's no play in the mounts.

I agree that the springs look quite bad. I looked online for some replacements but I'm paralyzed because of the amount of options they have. I don't understand the differences in shapes and prices. I also don't know how they compare to coilovers. It makes my head hurt.

 

[bugsymike]

Yeah I checked the car. Front and back seem to be at the same height. It's also not leaning left to right.

I wouldn't say I hear the car bottom out as much as I feel the car bottom out. I can feel that the bump is transferred into the body.

The shocks probably look newer because I cleaned them a bit. I was trying to find a date or something to figure out when they were manufactured but I don't see any date on it. Only a part number and a "do not disassemble" warning.

The anti-roll bar mountings look okay. A bit tired like the rest. Some visible cracks in the bushings but they're still all there and there's no play in the mounts.

I agree that the springs look quite bad. I looked online for some replacements but I'm paralyzed because of the amount of options they have. I don't understand the differences in shapes and prices. I also don't know how they compare to coilovers. It makes my head hurt.

There should be a guide on Fiat Forum using Eper etc. to select the correct springs for your model.
Personally I wouldn't touch "coil over conversions " with a barge poll, as I have seen to many cheap rubbish fittings that have had to be costly reverted to original spec.
Re the car "It's also not leaning left to right." as you say , what I meant is when looking from the back of the car , say about 20 feet away the back both rear wheels would appear to be leaning inwards at the top, there is a tool to measure this for steering geometry and for performance cars a little Negative Camber can improve cornering speeds. However I was more concerned with it maybe being a sign of worn springs/sagging suspension.

 

[Timofimo]

I used my phone to get a look under the car while I'm making it bounce. I'm not na expert on this but it does seem to me like there's an unusual amount of give from the springs. Like I'm not a big guy by any means but there's a lot of movement just from me sitting in it. Just by pushing down on the rear I can make the bump stop toch the suspension arm.

Also looked at the tires and the toe and I think you're right. The tires look a lot worse on the inside and the toe is quite aggressive compared to the front.

So I think the springs are definitely in need of replacement. Thanks for helping me figure it out

 

[bugsymike]

I used my phone to get a look under the car while I'm making it bounce. I'm not na expert on this but it does seem to me like there's an unusual amount of give from the springs. Like I'm not a big guy by any means but there's a lot of movement just from me sitting in it. Just by pushing down on the rear I can make the bump stop toch the suspension arm.

Also looked at the tires and the toe and I think you're right. The tires look a lot worse on the inside and the toe is quite aggressive compared to the front.

So I think the springs are definitely in need of replacement. Thanks for helping me figure it out

Hopefully the Eper on Fiat Forum will identify the correct parts for your model.
If you get the chance undo the lower mounting bolts on the shock absorbers if you can do it safely, then push them into the fully closed then fully open position, you should feel a steady firm resistance, but travel in both directions should be even if the shock absorbers are working correctly.
I have worked on some cars which customer had not noticed a fault with, but it was possible to open and close the shock absorbers fully in both directions with no effort at all as they were totally worn out.
Another point , if the car is sitting low at the rear it actually throws the front steering geometry out causing early front tyre wear.
The suspension does look a little rusty so the bolts to undo may need some oiling to free them off.
Just listened to the video, to me it sounds like the shock absorbers are noisy/empty of fluid, making a sort of "whooshy " noise, so it will be interesting with the lower mounting bolt to see if they are very easy to move up and down.

 

[AnthonyH]

The shocks are supposed to resist the upward motion of the car, but they don't. The whoosy sound is air coming in and out of the shocks, when its supposed to be contained inside it? So the shocks and springs need replacing i'd say.

 

[Timofimo]

Hopefully the Eper on Fiat Forum will identify the correct parts for your model.
If you get the chance undo the lower mounting bolts on the shock absorbers if you can do it safely, then push them into the fully closed then fully open position, you should feel a steady firm resistance, but travel in both directions should be even if the shock absorbers are working correctly.
I have worked on some cars which customer had not noticed a fault with, but it was possible to open and close the shock absorbers fully in both directions with no effort at all as they were totally worn out.
Another point , if the car is sitting low at the rear it actually throws the front steering geometry out causing early front tyre wear.
The suspension does look a little rusty so the bolts to undo may need some oiling to free them off.
Just listened to the video, to me it sounds like the shock absorbers are noisy/empty of fluid, making a sort of "whooshy " noise, so it will be interesting with the lower mounting bolt to see if they are very easy to move up and down.

I tried manually operating the shocks. There's a fair amount of resistance still. Although considering the compromised position I was in, not enough to keep the car stable I think. I don't really have a good reference for this. Both sides felt the same.

 

[bugsymike]

I tried manually operating the shocks. There's a fair amount of resistance still. Although considering the compromised position I was in, not enough to keep the car stable I think. I don't really have a good reference for this. Both sides felt the same.

Usually if worn out it is fairly obvious, so maybe just rear springs a bit soft.
Pushing and pulling the shock absorber it should be fairly firm hard pressure, but with no sudden easy bit as in where air in shock absorber or oil/gas low.
I have had brand new shock absorbers that had been laid on their sides so when first worked there was a easy bit before it started to damp the movement, although a few "pumps" and they start to work correctly.
You can soon see if they are working, just try bouncing the car on the road with them disconnected.
On that old MGB I mentioned earlier, after driving across a bumpy car park at a hill climb, I managed to lock the car and start to walk away whilst it was still bouncing.