I had my NC serviced by Roddisons in Sheffield back in the summer. This included full 4 wheel alignment. The last time the wheel alignment was set was about 9,000 miles previously. Roddisons found that the front wheels were toeing out but about half a degree, much to my surprise because the car has never been kerbed or encountered large pot holes at speed.
This set me thinking as to what could be the cause. I read on a BMW forum that some BMW 5 Series drivers had experienced the same. The cause? Those lovely square speed bumps that are liberally scattered around our towns and cities. Sheffield has plenty, especially along my route to work. It was easily possible to negotiate these bumps at 30mph provided one was careful to line the car up precisely so that the car straddled the bump. There was barely enough suspension movement to feel as I crossed them. However, there is clearly enough impact to gradually knock the tracking out after hundreds of impacts.
I now make sure that I slow down to 20mph when crossing these bumps and, fingers crossed, my suspension will no longer be affected. It’s probably worth bearing this in mind when encountering this type of bump.
I have tried this method of negotiating the bumps but it is pretty uncomfortable unless taken at about 5-10 mph. Not a popular move during the rush hour. I doubt that the speed bumps would cause any additional wear on the insides of the tyres, its certainly not something that happened to my tyres over the four years I had the NC.
Fortunately we don’t have many around us but when I have to pass over it is with great care or I ground the car. Definately with one set of wheels never straddle. 30 mph or even 20 mph is too fast, I see many drivers take these bumps far too fast if they can, driving normal height cars, it can’t be good for the car surely.
My NC was lowered by 39mm using Eibach springs but retaining standard dampers. I did have 215 profile tyres fitted which give about 4% extra circumference which may be a factor. My car would only ground on the bumps if the tyre pressure was slightly low and SWMBO was on board. I never suffered any rim damage from speed bumps but a combination of excessive speed and low tyre pressure could easily lead to damage. I had a very nasty experience in my Smart Roadster when hitting a huge submerged pot hole at 40mph. The impact was so severe that the dashboard popped out from its mounting and the top of the front suspension unit hit the underside of the bonnet crazing the paint on the outside. Needless to say I also has a pair of substantial dents in my N/S wheel rims. Fortunately I had the winter tyres on and steel rims so a few hefty whacks with a hammer got them straight!
The moral of the story is take care on speed bumps.
On a trip out I said to the missus when I grounded on a speed bump, it never did that last time I came this way, when was that she said, when you wasn’t in I replied. She never spoke to me again on that trip and was very hurt by my comment