Technical Axle Tramp/ Wheel Slip

[RDC42]

Any 500x owners with the 19 inch wheel option suffer from front wheel slipping at full lock or axle tramp when the front wheels lose traction.

It’s very easy to lose traction with the firefly turbo engine if the turbo kicks in at low speed when pulling away so the tramping is uncomfortable.

I’m suspecting my wheels might be out of alignment or it’s just a design fault that the suspension can’t really handle the bigger wheels and maybe stiffer tyres.

I did have wheel skipping on my old Subaru in cold weather but I haven’t experienced axle tramp since my old rear wheel drive Ford Escort.

 

[s130]

I occasionally get this on my 500X (2015) with it's 18 inch wheels. As you say can be uncomfortable but I tend to wonder what this is doing to the drive shaft/CV joints!

One of the problems with these bigger wider, lower profile wheels is partly due to their required higher tyre pressures due to reduced sidewall height. Typically 2.4 bar or more. This affects tyre contact area.

e.g. the 500X 1.4 max. permissible weight is 1875kg with 1050 kg and 900kg axle weights.

With a tyre pressure of 2.4bar = 35psi
1050kg = 2310lbs
Tyre contacts area for two wheels = 66 square inches
ONLY 33 square inches per front wheel. i.e. 6 inch by 6 inch patch

In theory on static full lock an NO weight transfer then we still have a 6x6 contact area.

With older higher profile and narrower tyres a typical pressure is 28 psi. Now our contact area has increased to 84 square inches or 42 square inches per wheel compared to 33.

Now when on lock and moving away there will be weight transfer away from the inside wheel thus reducing the contact area and lowering the grip.

If you look at an F1 Racing car on slicks with those really wide wheels then to get them to maximum grip they have to run with really low tyre pressures hence all the tyre wall wobbling when bouncing of curbs an turning as the same time. See: https://the-race.com/formula-1/why-f1-teams-want-to-run-lowest-tyre-pressures-they-can/ (23 psi and 20 psi)

 

[RDC42]

I occasionally get this on my 500X (2015) with it's 18 inch wheels. As you say can be uncomfortable but I tend to wonder what this is doing to the drive shaft/CV joints!

One of the problems with these bigger wider, lower profile wheels is partly due to their required higher tyre pressures due to reduced sidewall height. Typically 2.4 bar or more. This affects tyre contact area.

e.g. the 500X 1.4 max. permissible weight is 1875kg with 1050 kg and 900kg axle weights.

With a tyre pressure of 2.4bar = 35psi
1050kg = 2310lbs
Tyre contacts area for two wheels = 66 square inches
ONLY 33 square inches per front wheel. i.e. 6 inch by 6 inch patch

In theory on static full lock an NO weight transfer then we still have a 6x6 contact area.

With older higher profile and narrower tyres a typical pressure is 28 psi. Now our contact area has increased to 84 square inches or 42 square inches per wheel compared to 33.

Now when on lock and moving away there will be weight transfer away from the inside wheel thus reducing the contact area and lowering the grip.

If you look at an F1 Racing car on slicks with those really wide wheels then to get them to maximum grip they have to run with really low tyre pressures hence all the tyre wall wobbling when bouncing of curbs an turning as the same time. See: https://the-race.com/formula-1/why-f1-teams-want-to-run-lowest-tyre-pressures-they-can/ (23 psi and 20 psi)

Wow. Think that covers it comprehensively, thanks. I was thinking it might be a possibility with those tyres.
My old Legacy had standard 17 inch wheels and it never did that but my Subaru Impreza had 18 inch wheels with aftermarket very wide and very low profile tyres and I think that behaviour is normal for those type of 4wd cars with those wheels. I just never experienced it in a front wheel drive car before but I never had wheels that size either.
I’ll keep an eye on the tyres and make sure they aren’t wearing unevenly. I know if I go to one of those tyre places they’ll mess with the tracking even if it doesn’t need it and they’re guaranteed to leave the steering wheel off centre so I prefer to avoid them.

 

[portland_bill]

Check the bushes on the front suspension arms, especially the rear ones. They have a short life. They can let go and appear normal to a visual, static inspection, but will show excess movement when pushed.

Just replaced the lower arms on the Panda, the slight clutch judder has disappeared with the old arms.

 

[RDC42]

Check the bushes on the front suspension arms, especially the rear ones. They have a short life. They can let go and appear normal to a visual, static inspection, but will show excess movement when pushed.

Just replaced the lower arms on the Panda, the slight clutch judder has disappeared with the old arms.

Don’t think that’s it, but it’s worth a check all the same. Car only has 27k kms. I have had suspension failures early on my new Alfas that had to be replaced under warranty. My 147 was particularly bad, for that, it had some links replaced front and rear after less than 20k miles. Even though my current car has low miles, it was a dealer demo so it could have been trashed by a few test pilot types.

 

[s130]

I've noticed on some Mercedes & BMWs then when on or near full lock the inside wheel really cants over at a really steep angle. This must certainly reduce the contact patch and/or move it to a edge. Often looks like the suspension is damaged

 

[chris3234]

I've noticed on some Mercedes & BMWs then when on or near full lock the inside wheel really cants over at a really steep angle. This must certainly reduce the contact patch and/or move it to a edge. Often looks like the suspension is damaged

Noticed that many times as well looks quite broken when on full lock

 

[RDC42]

I've noticed on some Mercedes & BMWs then when on or near full lock the inside wheel really cants over at a really steep angle. This must certainly reduce the contact patch and/or move it to a edge. Often looks like the suspension is damaged

Yes. I’ve seen that too. I used to have a bmw 316 and it used to look like that with the wheels locked, but it never skipped on lock. Possibly because it was on skinny tyres.

 

[s130]

I think there must be a reason for the heavy cant/camber on full lock. I'm wondering if it relates to wide wheel track, longer wheel base, smaller turning circle and the Ackermann angle. Canting/cambering the wheel over might change the tyre slip angle/properties??

https://www.racecar-engineering.com/articles/tech-explained-ackermann-steering-geometry/

 

[RDC42]

I think there must be a reason for the heavy cant/camber on full lock. I'm wondering if it relates to wide wheel track, longer wheel base, smaller turning circle and the Ackermann angle. Canting/cambering the wheel over might change the tyre slip angle/properties??

https://www.racecar-engineering.com/articles/tech-explained-ackermann-steering-geometry/

Yes. Or to give it a decent turning circle they allowed the wheels to lock a bit too far or at a funny angle. Turning circle isn’t great as it is, so it would probably be unusable if they stopped them a bit earlier. Think it’s just something I’ll learn to live with because I really like those 19inch wheels. At least I have experience of it from my old Impreza.
Axle tramp seems to be mainly on concrete roads, I was testing it on wet tarmac today and it didn’t tramp. Luckily we have few of those old concrete roads left. Also maybe when the tyres start to wear in a bit, they may not bite as hard into the road when turning-they still look almost brand new.

 

[Ralf S.]

Mine has done it since new, andi have no reason to believe the suspension is not set up completely "straight"... so I don't think it's worn or mis-laigned suspension... it's more likely the Ackerman angle in the suspension design.

Mine usually only does it when reversing out of my drive - I have to go full lock to turn it enough (I live along a fairly narrow road), just after I've checked that the nose is out and I'm starting to accelerate backwards some more. I think it's only happened whilst moving forwards once (wet road and on full lock the other way, having just reversed out of my drive).

I've also got the 19" optional standard alloys... and apart from it's a bit unusual (my old Stilo was on 60-section tyres) I never worried too much about it.


Ralf S.

 

[RDC42]

Mine has done it since new, andi have no reason to believe the suspension is not set up completely "straight"... so I don't think it's worn or mis-laigned suspension... it's more likely the Ackerman angle in the suspension design.

Mine usually only does it when reversing out of my drive - I have to go full lock to turn it enough (I live along a fairly narrow road), just after I've checked that the nose is out and I'm starting to accelerate backwards some more. I think it's only happened whilst moving forwards once (wet road and on full lock the other way, having just reversed out of my drive).

I've also got the 19" optional standard alloys... and apart from it's a bit unusual (my old Stilo was on 60-section tyres) I never worried too much about it.


Ralf S.

Thanks. It’s probably just the way it’s designed then. If someone else is experiencing it with a similar set up, I don’t think I’ll go to the expense of checking the wheel alignment, I’ll just keep watching the tyres wear just in case.
Happy New Year.

 

[RichardH13]

I think its the Ackerman Angle

My 500x Sport on 19"s does it all the time. Especially when the tyres are cold and its a bit damp

My Tipo occasionally did it, but that had 16" wheels so there was flexibility in the tyres

I borrowed an Alfa Giulia, it did it

Not really concerned

 

[portland_bill]

Ackerman determined that the steering arms should point towards the centre of the back axle for perfect steering angles.
Later that was found to be great for corners, especially tight slow ones, but different angles would give better handling at faster speeds, so the wheels do not follow exactly. Different manufacturers choose different angles, but most will have some forced side slip at slow speeds and tight angles.

 

[RDC42]

Update. 1 year later. I had to replace my tyres this week. Car originally had Dunlop Sportmax RT 225/40/19's.

I couldn't get a set before Christmas, the size of this tyre was very hard to find as well, and My fronts had dangerously low thread.

I was lucky enough to find a tyre place, after a few days searching, and he recommended to get a full set of Pirelli P Zero Run flats which actually cost less than the Dunlop's, eur 220 each fitted vs eur 236 per tyre plus fitting for the Dunlops.

500km in so far and the wheel skipping at full lock is completely gone, ride is vastly improved, stability is much better, car isn't wallowing around on bumpy back roads and the car is much more peaceful and comfortable, I'm not constantly correcting the steering on bad roads.

I haven't checked for tramping on wheel spins because the tyres are still too new, but even taking off briskly on a damp road, it hasn't broke traction as quickly as it would on the Dunlops (when they were much younger, may I add).

Fiat recommend those Dunlops for the 500x Sport but apart from excellent wear (60,000kms), they must have been the worst tyres I've ever had on any car.

I'd recommend trying the Pirelli tyres as a solution, I think they wear out much faster than the Dunlops, but the benefits outweigh this. Plus they're run flat so I won't have to mess around with compressors and cans on the side of the road if I get a puncture, I can just drive on with a flat tyre and they can be repaired as long as any nails aren't too close to the sidewalls.

Anyway, that's my update and Merry Christmas and a very happy new year to everyone.

 

[portland_bill]

With runflats, there should be some recommendation about max speed, and max distance when running flat. I think also, that when run flat, they are destroyed, whereas the goo can be cleaned and some might be repaired.

A few years ago, a neighbour carried on for several days with one flat tyre, continued his daily commute, until it started to break up. I'm sure you understand, but many don't.

 

[RDC42]

With runflats, there should be some recommendation about max speed, and max distance when running flat. I think also, that when run flat, they are destroyed, whereas the goo can be cleaned and some might be repaired.

A few years ago, a neighbour carried on for several days with one flat tyre, continued his daily commute, until it started to break up. I'm sure you understand, but many don't.

50 miles and no more than 50mph is the limit for these and they should remain serviceable. And you're right, some would just keep on driving as normal and destroy them very easily.

 

[RDC42]

50 miles and no more than 50mph is the limit for these and they should remain serviceable. And you're right, some would just keep on driving as normal and destroy them very easily.

And I would say it would be rare for a tyre place to say the tyre was repairable, even if the puncture was in a good place. I reckon any puncture means one or two new tyres. But I might be lucky.

I found a spare wheel kit on Amazon for a 17" wheel, Jack, brace and tightening wheel for eur 230. This size spare will do as a spare for a 19 inch wheel (I used to have a 16" spare for an 18" on my old Impreza and that worked fine on a few occasions over 11 years).
This might be a good investment because I don't need the extra boot space.

If it was safe and dry I could actually just change the wheel right away, maybe save a tyre too if I got a puncture and use the run flat only where it would be too dangerous to stop and change it.

I have breakdown cover with my insurance that covers punctures so I don't think I would actually ever run on flat, but it's nice to have the option and maybe the spare wheel kit might be a good back up to have as well.

 

[portland_bill]

Have you found these guys?

Fiat space saver spare wheels - online shop uk

www.thewheelshop.co.uk

 

[RDC42]

Have you found these guys?

Fiat space saver spare wheels - online shop uk

www.thewheelshop.co.uk

Yes, thanks. That's them on Amazon. If I go direct to the wheel shop, Shipped to Ireland its going to cost about eur 200 but I have to do customs and pay for it myself, this could be about eur 70 on top so more expensive.
For once, going with Amazon is cheaper but they're currently out of stock on Amazon and from the supplier direct. Amazon cover the customs so it saves a bit sometimes.
I set an alert with both sites for when it does come back into stock.