Technical Tracking

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Technical Tracking

DaveMcT

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Wheel tracking and axle alignment

My 100HP has always pulled slightly to the left and I'd not got around to getting it properly adjusted at a garage with alignment equipment. The car is now running straight so what's changed?

I have just had my rear axle off the car for refinishing and now the job is complete, I've found the steering tracking has improved. Nothing fundamental has changed other than the axle does not have any dowel alignments to the chassis so it looks like the alignment has changed.

So when you have the tracking checked (which I will still be doing by the way) be sure to have the mechanics check the rear alignment to body centreline as well as the steering arms. Problems are usually caused by poor adjustment at the steering arms but it looks like the back axle also has a role to play.
 
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A3jeroen

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Had that done to my old Stilo MW. The 4 bolts on each side have some room to wiggle the axle straight (or crooked). Panda has a similar rearaxledesign.

gr J
 
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DaveMcT

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Had that done to my old Stilo MW. The 4 bolts on each side have some room to wiggle the axle straight (or crooked). Panda has a similar rear axle design.

gr J


That has to be it, but the wriggle room is really quite limited.

When I did the 1.2 axle, there was no issue - it slotted in easily and handling/steering were not affected.

I struggled to get the 100HP axle back into place. I think the wheel arch skirts get in the way. I had to line it up and get one bolt loosely into one side then jack the other side until it would accept a bolt. The other bolts then went in easily.
 

jrkitching

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Re: Wheel tracking and axle alignment

My 100HP has always pulled slightly to the left and I'd not got around to getting it properly adjusted at a garage with alignment equipment. The car is now running straight so what's changed?

As you've already worked out by now, the alignment of the rear beam has changed and the rear wheels are now tracking more nearly in line with the centreline of the car. This is the first thing to get right when doing a 4 wheel alignment on any car, but there's no provision made for adjustment, apart from any slack which there may be in the bolt holes. It's also important that the rear wheels run parallel to each other; again, there's no provision for adjustment if they don't. Be aware that if the car's ever taken a hefty sideways slide into a kerb at the rear (or even hit a bad pothole) this could have distorted the beam.

When refitting the beam, you should do all you can to get it aligned with the centreline of the car; ideally with a laser, but you could probably improvise something if you've the mechanical nous to take on the job in the first place.

It's an inherent weakness of the twist beam design and seems to be particularly bad on the Panda, as the factory tolerances are quite generous.
 
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DaveMcT

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The Panda back axle is simply bolted into place - there are no dowels or fitted bolts to set the correct alignment. There is just a lot of slack between the bolts and bolt holes. This "can" be adjusted but there is no datum point under the car so It would need expert equipment to get it 100% right. Even then what do you choose as a correct body shell centreline?

It's only partially correct to say there is no individual adjustment for each wheel's castor/camber/toe angles. The hub spindle carriers are bolted to the swing arms with four M10 high tensile studs which have at least 8mm of spare thread. These could be adjusted with shims but again you would need special equipment as there is no baseline. They would also be far easier to adjust if just three studs had been used. With four studs it become much more complicated to make changes.

Fiat's twist beam axle is a very mature design. It's been about for years and by now has evolved into (what should be) an effective but also very cheap to produce item. While not perfect it's unusual to see a Panda (or any other Fiat with similar axle) crabbing down the road. In the days of the old Austin Mini, that was a common sight. Usually caused by body or subframe corrosion but could be bad fitting as well.

The most recent development was fitting an anti roll bar (ARB) into the 50mm wider track axle used on the Fiat 500. Everything else is identical). That will have improved lateral stiffness but has probably not added much impact resistance as the ARB is curved and will simply curve a bit more if thumped hard enough. The swing arm tubes are flattened at the front (pivot) points so will bend more easily on impact that a straight tube joint would do. The Panda beam wont add much lateral strength on impact as the U shape beam will just flare out a bit.

However, all designs will include a failure point. It's doubtful it was designed this way, but the flattened tubes may well bend before the car floor pan bends. It's obviously better to lose a sacrificial part than potentially scrap the whole body shell.
Sadly, while the manufacturing costs must be as low as it's possible to be, Fiat still charge silly money for a new axle. So dont belt your Panda back end into the nearest tall kerb stone.

In terms of functional stiffness, I think the Fiat beam has pretty good balance. Race cars strive for utmost stiffness in the body and chassis parts so the suspension control can be dialled in to suit the track etc. On the road, we have to consider ride comfort and we don't want a car with twitchy handling or one that become harsh mid corner on a bumpy road. Especially on a car with basic suspension parts like the Panda.

This is illustrated by motorbike frame design. Back in the glory days of British bikes we had the excellent handling Norton "Featherbed" frame that everyone strived to reach. Back then, suspension was pretty crude but the Norton was still very good.

Then in the 1970s with mass produced Japanese bikes, the frames could not keep up with engine power. The Suzuki GT750 water cooled was known as the Flexi Flyer and Kawasaki actually sold bikes because they could only be ridden by experts. Their frames were way below what was needed to meet the engine's needs.
The Japanese were not happy to continue that way and as their bikes became more sporty the frames became stiffer. Some Suzuki designs actually echoed the old Norton frame though it would be unfair to say it was copied. They then ran into a problem where frames were too stiff. Once it became possible to make an uber stiff frame it was realised that some lateral torsion and twisting was needed. Not a lot, but important all the same. Road impact forces over bums are always vertical. When a bike is leaning into a bend and hits a bump the impact forces are only partially contained by the suspension which can be 45 degrees or more from vertical. If the frame is too stiff the bike will bounce off the bump and lose tyre grip. If the frame is too flexible the bike will squirm and weave. Both are very bad. To deal with this, some frame flex had to be designed back in.

In a car, the effect is far less pronounced but nobody wants the handling to become harsh or twitchy when cornering so some lateral spring especially at the back is good to have. I wonder if this is why Fiat build the Panda swing arms with narrow ends where they weld to the pivot point or whether that was just factory convenience.
 
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DaveMcT

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I will be getting my car professionally aligned though it has become less urgent now the back end is not affecting the steering. I will then have to look at a way of marking the correct alignment for the axle chassis mounts. Maybe drill a bolt hole right though on each side to act as a dowel but more likely some paint marks will have to do the job.
 
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