Tyre Profiles
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Category: Technical & Model Specific Forums
Forum Name: General Motors
Forum Discription: This forum will deal with technical issues for the cars not dealt with in the other forums. These don't need to be BMWs!
URL: http://www.bavarian-board.co.uk/forum_posts.asp?TID=16286
Printed Date: 21-September-2024 at 00:00
Topic: Tyre Profiles
Posted By: micky_h
Subject: Tyre Profiles
Date Posted: 04-April-2005 at 14:13
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Tried to do a quick search on this. Noticed that on my E36 M3 and my Dad's E39 525tds that the rear tyres are a lower profile than the fronts even though the the diameters are the same.
Does anybody know why BMW did this? |
Replies:
Posted By: Del64
Date Posted: 04-April-2005 at 14:24
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Simple the width is greater so the profile is lower to result in the same rolling radius.
on an e39 as an example the standard tyre is a 225/60 x 15 this is the same as a 225/55 x 16 a 235/45 x 17 but the rear would be a 255/40 x 17 it gave the cars a more muscular appearance and enhanced traction. If you are being totally pedantic then the true rear size would be a 265/40 x 17 tyre. It is the same principle with your M3 |
Posted By: micky_h
Date Posted: 04-April-2005 at 14:33
| I relised that if the wheel diameter increases that the profile would drop.But saying the M3 has 17s all round I would have thought the profiles would stay the same front to rear.Didn't know the width made a difference too. |
Posted By: Garfield
Date Posted: 04-April-2005 at 15:19
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Width makes no difference to profile .... the E39 on the front can handle up to 235's, the rears up to 255 comfortably without rolling the arches. The offset can be between 15 and 25 on a PCD of 5*120 and counter bore of 74.1mm (beware only E39 wheels fit E39's because of this despite what many claim as all other BMW wheels have a 72.6mm counter bore except the mini which is 56.1) an E34 or E60 5 series rim will NOT fit an E39 without machining the wheel. The E46 M3 uses a PCD of 5*120, counter bore of 72.6 and offset of 35 to 45. Based on the original 15" and 60 profile as fitted to the 5 series E39 the diameter overall is 627mm, if you fit a 16" with 55 profile the diameter overall is 631.9 (an increas of 0.78%), if you then fit a 17" rim with a 45 profile the diameter is 616.3mm (a decrease on the 15" rim of 1.71%) - I don't agree with fitting of the 17" and 40 profile rubber because it gives a diameter of 595.8 (a 4.98% reduction over the 15" resulting in an almost 5mph under read of the speedo at 70 mph) totally unacceptable and shows that those who do this don't know what they are doing (usually those selling replica M sport rims), 40 profile are OK on 18" giving a diameter of 621.2mm (a decrease of 0.98% over the 15" size). All data courtesy of Rochford tyres ...... Sorry if I bored anyone ...... |
Posted By: micky_h
Date Posted: 04-April-2005 at 15:32
| Stadnard fit on the back of my E36 M3 is 245 40/17s while the fronts are 225 45/17s.Just wanted to know why BMW made this difference to the profiles thats all? Could understand if the rears were 18s. |
Posted By: Garfield
Date Posted: 04-April-2005 at 15:37
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The rear size will be sized to make sure the rolling radius / diameter fall within 2.5% of designed diameter and make sure the speedo reads correctly (or as near as possible). I have yet to hear a convincing reason why they fit larger profiles to the front though - I think it may have something to do with the suspension geometry / camber and tread contact area but have no firm confirmation of this from a BMW source .... |
Posted By: Brucey
Date Posted: 04-April-2005 at 16:18
yes it does. eg 255/55 is a bigger diameter tyre than 245/55. trust me..... cheers
------------- ~~~~~~~ Brucey ~~~~~~ |
Garfield wrote:Posted By: Garfield
Date Posted: 04-April-2005 at 16:22
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the 40 / 45 / 60 is expressed as a percentage of the WIDTH which is 205 / 225 / 255 etc etc http://www.chris-longhurst.com/carbibles/tyre_bible.html - http://www.chris-longhurst.com/carbibles/tyre_bible.html so partly correct, what is important anyhow is diameter ... 1 beer too many methinks .... |
Posted By: micky_h
Date Posted: 04-April-2005 at 16:34
Aaaaaah Ha ,cheers  I always thought it was the height of the sidewall in mm,ie 40=40mm.So it works out that a xxx/40 the height is 40 percent of the witdh of the tyre.xxx/45 is 45 percent etc etc.
You learn something new everyday |
Posted By: Peter Fenwick
Date Posted: 05-April-2005 at 07:47
Surely the reason is to keep the rolling radius of the front wheels the same, or almost the same, as that of the rears. The rear tyres are wider than the fronts to give the car a more aggresive stance but mainly to provide more grip to the driven wheels. If you look at the tyre specs for any sports car, from Lotus elise to Ferrari 360 they always have wider tyres on the back with a lower profile. ------------- Entering an age of Austerity and now driving a Focus Diesel. |
Posted By: Garfield
Date Posted: 05-April-2005 at 09:26
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I did the maths having found out a few things and the 265/40 has the same rolling radius as the 235/45 (or as near as makes no difference), with the 255/40 I have seen fitted to many cars this isn't the case (23mm less than the 45 profile). But it does make sense now you mention it that it is to match rolling radius now that I cleared up my misunderstanding what 40 / 45 etc means. |
Posted By: Del64
Date Posted: 05-April-2005 at 14:20
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I found out about the rolling radius thing and that was why I mentioned that a 235/45 and a 265/40 x 17 is the same diameter, in the final extension a 235/40 and 265/35 x 18 is about as low as you would want a two tonne saloon to go on Britains crap tarmac and still retain circular rims at the end of the day.
It is note worthy that BMW are the ones who spec a 255/40 x 17 rear with a 235/45 x 17 front, that is why they are the cheaper tyre to buy over the 265 which only has a few makes and types where the 255 is everywhere. For anyone still confused take the rim diameter multiply by 25.4, then the width of the tyre multiplied by the total of the profile so say 265 x 80% add to the rim diamemter and Bobs your aunty by working out the oem rolling diameter you can work out all the upgrades and downgrades that will not throw the speedo out.(and for those who got it already,,,SORRY! not intending you to be taught how to suck eggs) |
Posted By: Philip
Date Posted: 10-April-2005 at 17:07
great post Garfield. you say "counter bore". surely you mean "centre bore"? unless I've got it wrong of course here is a link for comparing various tyre sizes http://miata.net/garage/tirecalc.html - http://miata.net/garage/tirecalc.html ------------- Philip '86 E23 735iA SE - Polaris - The Silver Surfer '85 E23 735iA SE - Cosmos Blue - VJ '86 E28 528iA SE - Dia Black - Helga '86 E23 728iA SE - Polaris - The CHAV mobile |
Posted By: neileg
Date Posted: 13-April-2005 at 07:46
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If the rolling radius is greatly different front to back, it's going to cause the ABS system problems. They are designed to sense differences in rotational speed which is what different a rolling radius would produce. ------------- Cheers, Neil |
Posted By: Garfield
Date Posted: 13-April-2005 at 16:38
| Never sure what to call it but it is the bit that fits over the hub so centre bore is no prob for me. |