A class speed hump scrapes problem SOLUTION

[v0y4ger]

Hi all,

I searched the web from top to bottom trying to find a solution to this problem. But no one seemed to know.
So... A180 CDI 2006, scraped bus speed humps, was hellish coming off kerbs making a scrape sound or thumping the underside of the car. Mercedes themselves didn't seem to know what to do about it.
I toyed with the idea of fitting taller shocks all round but didn't want to mess with the stability.
Then... A friendly mechanic who is heavily into rallying suggested fitting larger tyres to the front axle.

So we've gone from 195/55/R16 all round to 205/60/R16 on the front.

Problem solved

2 cans of emergency tyre foam in the boot give me a little confidence in case I run into trouble (yes this is legal, Renault cc's don't carry spare tyres in favour of emergency tyre foam either).

6 months of running haven't flagged any problems other than slightly lighter steering.

 

[turbopete]

but didn't want to mess with the stability.

but by changing the tyres you will, technically have done exactly that! one of my escorts had 185/65x14 tyres instead of 185/60x14 and it was TERRIBLE to drive. it was like driving a jelly, especially in the twists. correct sized tyres cured that though

So we've gone from 195/55/R16 all round to 205/60/R16 on the front.

Problem solved

you just need to worry about whether or not your speedo reads within the legal parameters for %age error etc.

on a rally car the suspension is tuned to the type of surface and tyre sizes fitted. they always use wider, lower profile tyres on tarmac stages/rallies than they do on gravel/forest type stages/rallies

 

[Doug1234]

Do ring your insurance company and just inform them , I would not expect them to charge any extra but if you have an accident then its the 'get out clause' they will use.

You do really need to ensure all four tyres are same size though.

 

[patxi]

According to http://www.etyres.co.uk/tyre-size-calculator your new front tyres have 5% greater circumference than the old ones. This means you will have a 5% error on speed (reading 70mph is actually 73.7mph). Fuel consumption calculations will be similarly affected as will the odometer, travelling 19k miles will indicate 20k.

The calculator recommends a maximum deviation in tyre size of 2.5%, you are double that. Personally, I would change back to the original size and stop driving down kerbs.

 

[anyweb]

i wouldn't count on those two tins of stuff, if this happens (as did on my sambos A 150 yesterday) you'll need a new tire or a spare.

 

[PovertySpec]

you will have a 5% error on speed (reading 70mph is actually 73.7mph)

So, given that speedos generally under read by a margin (or over report) at least the speedo ought to be more accurate now

 

[Naraic]

I'm surprised the Cruise control still works with such a big difference between the front and rear.

 

[v0y4ger]

In reply

Hello,

Wow I didn't expect so many replies.

Allow me to clear a few things up though

Handling is great as far as I can tell, stability unaffected for normal use. Bearing in mind this is a road car and I'm not rallying it tearing around racetracks. I can think of many previous cars I've owned that haven't felt as sound.

Speedo vs sat nav
Before 70mph was 68 on the nav
After 70mph is still 68mph on the nav
With the Law allowing for a small error I reckon I'm safe.

So far, no flashes from cameras or points on the licence and yes, if it's 30 I do 30 and so on.

Good point about the insurance, will phone them in the morning.

It passed the MOT last month with the tyres on there.

Fuel consumption
Bear in mind that I used to detour around roads that I knew had speed humps because of the terrible scraping noises. So now being able to drive where I like is a bonus and not detouring saves me far more fuel than any tyre differences.

So in summary, I did it, I'm happy with the results and it solved a major issue that frankly was ruining my enjoyment of the car.