I'm planning on changing the alloys on my new Titanium X from the OEM 18" ones down to 16" because I find the car on the firm side with a fair bit of road noise. While researching options online one or two people have said that I'll have to get the ECU re-programmed so that it knows the car has different wheels.
This is puzzling me because I intend to make the rolling radius the same by fitting higher profile tyres to the smaller rims and I can't figure out how the car will know any difference. Presumably the only signal being fed to the ECU is from the ABS sensors so their 'target' on the hubs will be unchanged.
I can understand why a correction would need to be made if the rolling radius changed because the speedo would be out but I'm a bit baffled about the rest.
Basically, all I'm planning on doing is putting Zetec wheels and tyres on a Titanium X!
Can someone with a bit more knowledge than me explain why the ESP/ABS needs to be recalibrated - if indeed this information I've got is correct?
Also - am I right in thinking this will make the car more comfortable?
Graeme
Edited by Mondaywoe on 20/10/2009 at 18:51
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If you're keeping the rolling radius the same, then I can't see the need for re-programming either.
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You would need the ECU calibrating IF the rolling radius was significantly different (i.e. more than 10% or so) and only then for the speedo setting.
I doubt very much that it would have any effect on the ABS, that just correlates wheel RPM against road speed and activates if the difference is outside its parameters ... the small / negligible difference that you anticipate will not affect things.
I will measure the rolling diameter of my 215 / 55 16" on my Zetec estate and let you know, so you can have an idea of what the difference is from what you have.
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A quick measurement in the dark of one of the rear tyres on my Zetec (16" 215 / 55 Michelin Pilot Premacy, not very worn at all) showed 63.5cm DIAMETER.
Will double-check in the morning ...
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Thanks Craig!
Yes 63.5cm seems just about it - maybe 64 depending on how you look at it but that's not going to put anything up or down. Even a little bit of under/over inflation would probably make more difference than that.
My suspicion is that Ford technicians don't fully understand what it's all about and do the reset slavishly when wheels are changed and I'd rather they didn't tamper with the car unless absolutely necessary! (If it ain't broke..... it maybe soon will be!)
Graeme
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I think whoever has advised you that the ECU needs reprogramming has misread the information on page 201 of the handbook where it tells you that if you change the diameter of the tyres then it may affect the speedo and to take it to the dealers for reprogramming. Someone has taken the diameter as meaning the wheel size, and not considerd the tyre profile. Reading it correctly the "diameter" of the tyres means the Outside diameter, so if that is not changed then no programming is required.
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Spoke to the Ford dealer this morning. They think it's 'probably' just needed if rolling radius has changed. They said they would have a go at re-programming if the car seemed to be behaving oddly after the change!
I've ordered a set of Ford pattern 16" alloys with 4 Falken 215/55 tyres. Should be here in 10 days or so. Hopefully all will be well.
My only worry is that everything seems fine but snags only come to light when ABS /ESP cuts in. By that time it might be too late.....!!
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The tyre size calculator in this is handy:
www.carbibles.com/tyre_bible_pg2.html
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Interestingly, the car bibles site gets the definition of rolling radius wrong! However, the calculation of the *difference* for speedo purposes is close enough for practical use.
Oilburner - how are you coming up with estimates of, not only rolling radius, but, dynamic rolling radius?
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Laziness, using an on-line calculator:
tinyurl.com/69pkmk
Even if it's not 100% accurate, it should at least be consistency inaccurate, with the pc difference between the two the key point... :)
Edited by TheOilBurner on 23/10/2009 at 14:00
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>>Laziness, using an on-line calculator:
Interesting calculator - thanks for posting the link.
I'm always a bit concerned when the formulae being used aren't listed.
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>>They said they would have a go at re-programming if the carseemed to be behaving oddly after the change!
Eeek!
Any dealer who uses the word "probably" in the context where it can be factually proven whether it is needed or not should not be let anywhere near it....
Either it needs re-programming or not. Ask them what the rolling radius is now and what it will be after the wheels are changed. If they cannot answer that question clearly and intelligibly, then you might find your car comes back with a speedo that over/under reads by some silly amount when it does not need adjusting at all!
Your new tyres are 215 55 R 16, what exactly were the old ones? Were they 225 40 R 18?
If so, your dynamic rolling radius (that is, radius when moving) will change from 309mm to 312mm. This kind of small difference ( less than 1%) requires NO re-programming at all. At 60mph your speedo will only be out by 0.5mph...
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Present tyres are 235 40 R18 and I'm switching to 215 55 R16.
Using the online calculator I make that 322.6 and 321.45 - so precious little difference.
Edited by Mondaywoe on 23/10/2009 at 21:02
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