Changing tyre profiles

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Alan W
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Changing tyre profiles

Post by Alan W » Tue 05 Apr, 2016 12:21

If l was to use higher sidewall tyres on the rear would this give a softer ride / better stance to the car?
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bertiejaffa
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Re: Changing tyre profiles

Post by bertiejaffa » Tue 05 Apr, 2016 19:27

As you will have noticed I don't know much about tyres but I know much more than I did a few days ago thanks to people on here.

However, what I have read on here etc is that people who have drastically increased the rim size and compensated it with a lower profile have complained about the stiffness of the ride and have in some cases gone back the the original set up. Another thing to consider is the size of the overall wheel / tyre unit and the impact on the speedo - if you increase or decrease the overall circumference then your car will be going either faster or slower than the speedo thinks for 1 whole revolution of the wheel
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Alan W
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Re: Changing tyre profiles

Post by Alan W » Thu 07 Apr, 2016 11:43

Bertie you are now officially our resident tyre expert!!!

My reason for asking was that I have seen a really lovely set of offset 17" alloys with polished rims that have really taken my eye.. 8" wide fronts and 9" wide rears.

From what I can see, with 17" rims fitted, on the front you have to use a lower profile tyres to maintain the diameter otherwise the tyres will (might) foul on the bottom of the front strut spring carrier. However, this doesn't look like it will be an issue on the rears so I thought, to partly get around the firmer ride I could fit higher profile tyres to just the rear. I realise that this would put the speedo out a touch but I am more concerned about the stance and the balance of the car if the rear is slightly higher than the front?
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siwilson
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Re: Changing tyre profiles

Post by siwilson » Thu 07 Apr, 2016 12:09

Smaller wheels and a taller profile do improve comfort, but at the expense of handling.

Have a look at the calculator at the bottom of this page. You can use this to see what combinations of tyre and wheel size do the the rolling circumference as this is what impacts the accuracy of your speedometer. Be careful though because too much difference between front and rear could play havoc with your DSC/ABS etc. Also, moving away from the standard width font or rear will impact oversteer/understeer when you're pushing it.

Small differences are OK though. Even the stock sizes for an ///M don't match. The rear (245) is 20mm or 1% smaller than the font (225). A 255 would have been a better match and fits well on the car, but I suspect BMW decided against it due to the greater stagger upsetting handling.
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bertiejaffa
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Re: Changing tyre profiles

Post by bertiejaffa » Thu 07 Apr, 2016 13:23

Alan W wrote:Bertie you are now officially our resident tyre expert!!!?
well I have a few spare ones personally :lol:
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c_w
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Re: Changing tyre profiles

Post by c_w » Fri 08 Apr, 2016 12:28

Alan W wrote:Bertie you are now officially our resident tyre expert!!!

My reason for asking was that I have seen a really lovely set of offset 17" alloys with polished rims that have really taken my eye.. 8" wide fronts and 9" wide rears.

From what I can see, with 17" rims fitted, on the front you have to use a lower profile tyres to maintain the diameter otherwise the tyres will (might) foul on the bottom of the front strut spring carrier. However, this doesn't look like it will be an issue on the rears so I thought, to partly get around the firmer ride I could fit higher profile tyres to just the rear. I realise that this would put the speedo out a touch but I am more concerned about the stance and the balance of the car if the rear is slightly higher than the front?
The standard tyre sizes for the 17" wheel aren't particulalry low profile these days and offer a decent ride (sidewall stiffness varies massively, so something like a Uniroyal Rainsport with flimsy sidewalls will ride much nicer than a stiff Bridgestone for example).

OE sizes will fit those fine. Personally on those widths I would fit 235/40 front and 245/40 rear.

However a 225/45 will fit the 8" perfectly, as would 245/40 or 255/40 on the rear. If you're worried about ride, then 225/45 and 255/40 would be a perfect match.

mrscalex
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Re: Changing tyre profiles

Post by mrscalex » Fri 08 Apr, 2016 13:39

I believe the process of calculating the correct size is known as maintaining the rolling radius (or circumference). It's an age old problem that I remember 30 years ago with my old Mk I & II Escorts.

I would imagine technically it's an MOT fail if you put the wrong size on and it changes the gearing of the speedo such that it's no longer accurate. But how they would ever find that out I don't know.

I'm going to look at a Z3 for sale tonight with 19" wheels. I'll be interested to see if the correct profile tyres have been fitted - or rubber bands as they would probably need to be. Big wheels and loud exhausts aren't my thing but it's local so hey ho. If I bought it the first thing I'd do is to remove the wheels as a) I like it like it was designed out the factory and b) even the owner admits the tyres are rubbing on the inner arch on full lock.
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1999 Z3 1.9 Boston Green (Dimitri) - My brother's car owned from new and restored by me
2001 Z3 2.2 Topaz Blue (Trudy) - My car rebuilt from a write-off
2002 Z3 2.2 Titan Silver (Okus) - Project car waiting for a new engine
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Robert T
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Re: Changing tyre profiles

Post by Robert T » Fri 08 Apr, 2016 15:00

The speedo is not checked as part of the MOT, but there is a requirement for it to read within +/- 10%. When I swapped the gearbox in the Sprite for a 5-speed, the speedo was about 20% out, and so I used to use my satnav as a speedo. It was going to cost quite a lot to recalibrate the mechanical speedo, and as the trip counter never worked, I bit the bullet and bought an electronic speedo, which meant I could get rid of the nasty cable and as a bonus, I could re-calibrate it in the car. This means I can fit whatever wheels and tyres I want, but I have gone for a fairly standard combination.

The zed is a bit of a different kettle of fish. AFAIK the speedo cannot be recalibrated, and as it is driven off a slotted disc on the differential, it is tied directly to the size of the rear wheels. I don't think exact sizes are that critical, as it is worth bearing in mind that new tyres have about 7mm of tread and the legal minimum tread-depth is 1.4mm, so that means that the tyre will shrink by 11mm in diameter during the course of its life. Taking a 225/45 x17" tyre as an example, the diameter of this is (17 x 25.4) + 2 x (45% of 225) = 634.3mm, that is a 1.7% difference in diameter over the lifetime of the tyre. If you fit 10mm wider tyres (235/45s over 225/45s for example), then the sidewalls are only 4.5 mm deeper, which is only a 9mm difference on the diameter. What you don't want is a massive difference in diameter between the front and rear wheels, as that will confuse the ASC/DSC, nor do you want to deviate too far from the OE sizes, as that will put the speedo a long way out.

Cheers R.
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stevov
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Re: Changing tyre profiles

Post by stevov » Fri 08 Apr, 2016 16:21

I run staggered 68's on mine with 225/45 /17 on the front and 255/40/17 on the rear. You need to keep the rolling circumferences front to back and left to right similar to avoid unbalancing the abs and asc. I would not advise more than 3% front to rear and even less left to right. Also keep in mind that tyre sidewalls can be standard or xl. The xl being stiffer. The previously mentioned rainsport is one that comes in both a standard and xl version so be careful ordering which ones you go for.

mrscalex
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Re: Changing tyre profiles

Post by mrscalex » Fri 08 Apr, 2016 17:14

Assuming it's programmed correctly this is a good calculator.

http://www.wheelcalc.com/

It advises a maximum of +/-5% variance and preferably no more than +/-2%. So working off 225/50 R16 it gives the following list based on +/-2% variance (Width, Profile, Rim, Diameter, Circumference, % Change):

205 40 18 60.29 (1' 11.74") 189.4 (6' 2.57") -1.8%
205 50 17 61.94 (2' 0.39") 194.6 (6' 4.61") 0.9%
205 55 16 61.37 (2' 0.16") 192.8 (6' 3.91") 0%
205 60 15 60.86 (1' 11.96") 191.2 (6' 3.28") -0.8%
205 70 14 62.52 (2' 0.61") 196.4 (6' 5.32") 1.9%
215 35 19 61.47 (2' 0.2") 193.1 (6' 4.02") 0.2%
215 40 18 61.05 (2' 0.04") 191.8 (6' 3.51") -0.5%
215 45 17 60.77 (1' 11.93") 190.9 (6' 3.16") -1%
215 55 16 62.33 (2' 0.54") 195.8 (6' 5.09") 1.6%
215 60 15 62.04 (2' 0.43") 194.9 (6' 4.73") 1.1%
225 35 19 62.23 (2' 0.5") 195.5 (6' 4.97") 1.4%
225 40 18 61.85 (2' 0.35") 194.3 (6' 4.5") 0.8%
225 45 17 61.56 (2' 0.24") 193.4 (6' 4.14") 0.3%
225 55 15 61.05 (2' 0.04") 191.8 (6' 3.51") -0.5%
225 60 14 60.77 (1' 11.93") 190.9 (6' 3.16") -1%
235 40 17 60.19 (1' 11.7") 189.1 (6' 2.45") -1.9%
235 45 17 62.52 (2' 0.61") 196.4 (6' 5.32") 1.9%
235 55 15 62.04 (2' 0.43") 194.9 (6' 4.73") 1.1%
235 60 14 61.94 (2' 0.39") 194.6 (6' 4.61") 0.9%
245 35 18 61.05 (2' 0.04") 191.8 (6' 3.51") -0.5%
245 40 17 60.96 (2') 191.5 (6' 3.39") -0.7%
245 45 16 60.77 (1' 11.93") 190.9 (6' 3.16") -1%
245 50 15 60.86 (1' 11.96") 191.2 (6' 3.28") -0.8%
255 30 19 61.85 (2' 0.35") 194.3 (6' 4.5") 0.8%
255 35 18 61.66 (2' 0.28") 193.7 (6' 4.26") 0.5%
255 40 17 61.75 (2' 0.31") 194 (6' 4.38") 0.6%
255 45 16 61.75 (2' 0.31") 194 (6' 4.38") 0.6%
255 55 14 61.75 (2' 0.31") 194 (6' 4.38") 0.6%
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1999 Z3 1.9 Boston Green (Dimitri) - My brother's car owned from new and restored by me
2001 Z3 2.2 Topaz Blue (Trudy) - My car rebuilt from a write-off
2002 Z3 2.2 Titan Silver (Okus) - Project car waiting for a new engine
2001 Z3 2.2 Topaz Blue (Debra) - Donor car/breaker
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Brian4
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Re: Changing tyre profiles

Post by Brian4 » Fri 08 Apr, 2016 19:31

Agreed rolling diameter should be the same or/similar front to rear otherwise the ABS/DSC could get confused.
I guess you have 205 55 16 tyres and biggest profile is 70, 205 70 16 and these are 60mm bigger diameter than the 55's the car travels approx 200mm further per rotation so as long as all 4 are the same the speedo should be within the 10% tolerance but don't be doing indicated 33 in 30 limit as you could be over the speed limit.
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mrscalex
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Re: Changing tyre profiles

Post by mrscalex » Fri 08 Apr, 2016 19:37

Does anyone know why the tyre width appears to be a consideration in these online calculators? Would it not just be the rim diameter and tyre profile that matters?
ImageImage
1999 Z3 1.9 Boston Green (Dimitri) - My brother's car owned from new and restored by me
2001 Z3 2.2 Topaz Blue (Trudy) - My car rebuilt from a write-off
2002 Z3 2.2 Titan Silver (Okus) - Project car waiting for a new engine
2001 Z3 2.2 Topaz Blue (Debra) - Donor car/breaker
1998 Z3 1.9 Boston Green (Allsworth) - Donor car/breaker now gone to Z3 heaven
Always happy to try and help with spares :)

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Robert T
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Re: Changing tyre profiles

Post by Robert T » Fri 08 Apr, 2016 19:47

The profile is given as a percentage of the tyre width. You need both figures to calculate the height of the sidewall.

Cheers R.

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mrscalex
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Re: Changing tyre profiles

Post by mrscalex » Fri 08 Apr, 2016 19:55

Robert T wrote:The profile is given as a percentage of the tyre width. You need both figures to calculate the height of the sidewall.

Cheers R.

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Got ya! Understand now :)
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1999 Z3 1.9 Boston Green (Dimitri) - My brother's car owned from new and restored by me
2001 Z3 2.2 Topaz Blue (Trudy) - My car rebuilt from a write-off
2002 Z3 2.2 Titan Silver (Okus) - Project car waiting for a new engine
2001 Z3 2.2 Topaz Blue (Debra) - Donor car/breaker
1998 Z3 1.9 Boston Green (Allsworth) - Donor car/breaker now gone to Z3 heaven
Always happy to try and help with spares :)

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Re: Changing tyre profiles

Post by Gazza » Fri 08 Apr, 2016 20:09

With the mention of speedo readings, my speedo with standard tyre fitment for an ///M reads 8mph over at 70mph
Gazza

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Alan W
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Re: Changing tyre profiles

Post by Alan W » Sun 10 Apr, 2016 10:49

The company that supply the wheels l'm interested in can supply them fitted with a variety of tyres if required and you just enter your car and their on line gadget works out the correct sizes which falls in with those suggested above.

These are the wheels which l think will look pretty striking on a black car...

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stevov
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Re: Changing tyre profiles

Post by stevov » Sun 10 Apr, 2016 12:57

Sort mate, a bit to chav bling fussy and a nightmare to clean.

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kjb1
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Re: Changing tyre profiles

Post by kjb1 » Sun 10 Apr, 2016 17:00

Alan W wrote:The company that supply the wheels l'm interested in can supply them fitted with a variety of tyres if required and you just enter your car and their on line gadget works out the correct sizes which falls in with those suggested above.

These are the wheels which l think will look pretty striking on a black car...

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Seen one with these rims and can confirm, look great!!


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Alan W
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Re: Changing tyre profiles

Post by Alan W » Sun 10 Apr, 2016 17:28

stevov wrote:Sort mate, a bit to chav bling fussy and a nightmare to clean.
Can see your point about keeping clean but my brothers shop now sell brake pads that don't create any dust.. goes without saying that will have to have those all round!!
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bertiejaffa
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Re: Changing tyre profiles

Post by bertiejaffa » Sun 10 Apr, 2016 17:30

I reckon they will look great
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c_w
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Re: Changing tyre profiles

Post by c_w » Mon 11 Apr, 2016 11:43

stevov wrote:Sort mate, a bit to chav bling fussy and a nightmare to clean.
I know what you mean about the looks when they're fitted to a golf or a civic but I think this design works on a BMW as similar designs have been used for years as an OE style (they look just like the BBS off a 328i Sport). Will be difficult to clean though!

Alan W
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Re: Changing tyre profiles

Post by Alan W » Tue 12 Apr, 2016 14:39

c_w wrote:Will be difficult to clean though!
What like really difficult?
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bertiejaffa
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Re: Changing tyre profiles

Post by bertiejaffa » Tue 12 Apr, 2016 22:48

Alan W wrote:
c_w wrote:Will be difficult to clean though!
What like really difficult?
Depends on how anal you are about detailing. You would need to clean each spoke edge
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Southernboy
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Re: Changing tyre profiles

Post by Southernboy » Wed 13 Apr, 2016 05:40

Cleaning is easy enough with the right cleaner and a bottle brush, but is considerably more tedious than a simpler 5 spoke wheel.
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Robin82
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Re: Changing tyre profiles

Post by Robin82 » Fri 22 Apr, 2016 13:00

I fitted 18" wheels with low profile tyres last year and I've just had it lowered by 30mm last week by fitting performance springs.

I can confirm - RIDE IS MUCH BUMPIER - I feel everything in the road. BUT!! handles like a go cart and looks cool so definitely worth it. Point and it follows :D
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