Tyres and sidewall heights
Discussion
ClassicChimaera said:
hillclimbmanic said:
I've fitted 215/45x17 front, on 7.5"
245/40x17 rears, on 8.5"
The closest Rolling circumference available, to standard
And you have no rubbing issues with that front size. 245/40x17 rears, on 8.5"
The closest Rolling circumference available, to standard
Cool
I've just checked...215/40x17 =1821.02 mm THAT'S what I've fitted...
Verses: 205/55x15 =1829.16 mm
Plus I've fitted 10mm spacers, and Ford Motorsport studs, to clear the big discs and Brembos...
Rear: 225/50x16 = 1904.26 mm - 245/40x17 = 1893.4
Edited by hillclimbmanic on Sunday 1st January 16:18
Edited by hillclimbmanic on Sunday 1st January 16:30
Edited by hillclimbmanic on Sunday 1st January 16:32
Edited by hillclimbmanic on Sunday 1st January 16:41
QBee said:
Not sure I understand your maths - what exactly is the 1.8 to 1.9 metres to which you refer? The width of the car?
It is the circumference of the tyre...Which determines how accurate your Speedo will read...On the rear, at least.!!
Please excuse the shaking New Year's Day hand...
Edited by hillclimbmanic on Sunday 1st January 17:27
hillclimbmanic said:
Oops...My bad.!!
I've just checked...215/40x17 =1821.02 mm THAT'S what I've fitted...
Verses: 205/55x15 =1829.16 mm
Plus I've fitted 10mm spacers, and Ford Motorsport studs, to clear the big discs and Brembos...
Rear: 225/50x16 = 1904.26 mm - 245/40x17 = 1893.4
ETA by Al !
Fronts
The calculator I'm using suggests
205/55/15 = 1905.4 circumference 606.5 diameter
215/40/17 = 1896.9 mm 603.8
Rears
225/50/16 1983.6 631.4
245/40/17. 1972.3. 627.8
Essentially both front and rears are smaller than standard. This is the set up I have at the moment.
I've just checked...215/40x17 =1821.02 mm THAT'S what I've fitted...
Verses: 205/55x15 =1829.16 mm
Plus I've fitted 10mm spacers, and Ford Motorsport studs, to clear the big discs and Brembos...
Rear: 225/50x16 = 1904.26 mm - 245/40x17 = 1893.4
ETA by Al !
Fronts
The calculator I'm using suggests
205/55/15 = 1905.4 circumference 606.5 diameter
215/40/17 = 1896.9 mm 603.8
Rears
225/50/16 1983.6 631.4
245/40/17. 1972.3. 627.8
Essentially both front and rears are smaller than standard. This is the set up I have at the moment.
Edited by hillclimbmanic on Sunday 1st January 16:18
Edited by hillclimbmanic on Sunday 1st January 16:30
Edited by hillclimbmanic on Sunday 1st January 16:32
Edited by hillclimbmanic on Sunday 1st January 16:41
hillclimbmanic said:
I quoted from: Wheel and Tyre Bible...Sorry if they're wrong.!!
Paul
I wouldn't know which is correct, just pointing out the slight difference, looking at your post again and using the calc Paul
You appear to be suggesting the rear 245/40/17 is 1893 which looks like a slight mix up on the 225/50/16 1983
But then I could have easily mixed up a few numbers when writing it all down
I'm known to do it when half cut 
It's a few mm here or there anyway, if they fit in there as Qbee says those with adjustable suspension can dial out some difference.
The Rainsports that I've seen appear to have a soft sidewall as well as soft tread blocks, I'm hoping I can play with tyre pressures a bit more as the higher tyre wall allows me greater scope.
The 215/40/17 on my car as the size suggests, has a very low side wall and causes me to bottom the tyre out and the rim edge takes the strain, I'm trying to soften the car more for localised speed limit restricted driving, I have other wheels I might put some summer rubber on but for now I'm looking at the ride and comfort level for the easy drives I tend to actually do. It's not that often I'm ripping it up, and if I am I'm puffed out in seconds so we go back to sleepy mode.
How many of us actually hoon about. If I get better grip this way for slow road based use and can protect my electrics by taking up more of the bumps,, I'll be into the car more.
It's not hard to change one of these cars with sets of shocks and wheels from race b
d tough guy to old fart softie. I'm liking old fart softie at the moment
Nearly forgot to mention, nice chassis and that wheel is just simply showing off, lovely
Edited by ClassicChimaera on Sunday 1st January 21:20
QBee said:
Not sure I understand your maths - what exactly is the 1.8 to 1.9 metres to which you refer? The width of the car?
Its pi x dia to get the circumference measurement.Once you work out the dia of various wheel and tyre combinations from standard the pi X d will give the measurement to compare from standard the % difference it will make on your standard speedo. Obviously the rear wheel and tyre combo is that that changes the speedo readout.
RobXjcoupe said:
QBee said:
Not sure I understand your maths - what exactly is the 1.8 to 1.9 metres to which you refer? The width of the car?
Its pi x dia to get the circumference measurement.Once you work out the dia of various wheel and tyre combinations from standard the pi X d will give the measurement to compare from standard the % difference it will make on your standard speedo. Obviously the rear wheel and tyre combo is that that changes the speedo readout.
QBee said:
RobXjcoupe said:
QBee said:
Not sure I understand your maths - what exactly is the 1.8 to 1.9 metres to which you refer? The width of the car?
Its pi x dia to get the circumference measurement.Once you work out the dia of various wheel and tyre combinations from standard the pi X d will give the measurement to compare from standard the % difference it will make on your standard speedo. Obviously the rear wheel and tyre combo is that that changes the speedo readout.
. I've noticed a lot of emphasis on this subject without wheel width and offset being taken into consideration. Like a 245 40 17 tyre needs an 8j wheel at least but a 245 45 16 is quite happy mounted on a 7.5j wheel. But a rear wheel on the front needs an offset change via wheel spacers and a reduction in tyre width to enable lock to lock steering with no rubbing. Ride height shouldn't make any difference unless you go below the standard bump stops and the suspension geometry should allow clearance with increased camber as the wheel moves into the wheel arch. Getting the wheels to sit perfectly in line with the wheel arch and low isn't always possible as the wheels will have increased camber as you go lower. Sitting higher will possibly give the impression of fitting much wider wheels but once the suspension arm moves upwards the wheel will rub on standard arches more so with higher profile tyre walls that look like they fill the clearance gap in the wheel arch.Does that make sense?
ClassicChimaera said:
It all makes sense, I'm using 7.5 J alround so if anything I need spacers on the rear as the fronts are now 1/2 in wider so to keep the track proportionally correct.
I know my wheels fit with a similar tyre size as the ones I'll be fitting.
This is what gets a bit complicated because original fit front wheels have a different offset to compensate for front and rear track widths. Fitting a 71/2j front wheel with et35 rear wheel offsets virtually keeps the track the same as the rear of the wheel sits further inboard on the front axle so the outer rim edge sits in nearly the same place as the original 7j rim with an offset of I think is et25.I know my wheels fit with a similar tyre size as the ones I'll be fitting.
If you use the et25 7j front wheel the width of wheel in front of the hub mounting face is 63.9mm and 113.9mm behind the mounting face.
If you fit a et35 71/2j wheel on the front, the width of wheel in front of the hub mounting face is 60.25mm and 130.25mm behind the mounting face.
Using the above dimensions the wider front wheel actually gives a narrower track by 7.3mm overall or 3.65mm per wheel as the extra width is behind the mounting face due to the et35mm.
Does that make sense to you?
RobXjcoupe said:
This is what gets a bit complicated because original fit front wheels have a different offset to compensate for front and rear track widths. Fitting a 71/2j front wheel with et35 rear wheel offsets virtually keeps the track the same as the rear of the wheel sits further inboard on the front axle so the outer rim edge sits in nearly the same place as the original 7j rim with an offset of I think is et25.
If you use the et25 7j front wheel the width of wheel in front of the hub mounting face is 63.9mm and 113.9mm behind the mounting face.
If you fit a et35 71/2j wheel on the front, the width of wheel in front of the hub mounting face is 60.25mm and 130.25mm behind the mounting face.
Using the above dimensions the wider front wheel actually gives a narrower track by 7.3mm overall or 3.65mm per wheel as the extra width is behind the mounting face due to the et35mm.
Does that make sense to you?
Yes entirely If you use the et25 7j front wheel the width of wheel in front of the hub mounting face is 63.9mm and 113.9mm behind the mounting face.
If you fit a et35 71/2j wheel on the front, the width of wheel in front of the hub mounting face is 60.25mm and 130.25mm behind the mounting face.
Using the above dimensions the wider front wheel actually gives a narrower track by 7.3mm overall or 3.65mm per wheel as the extra width is behind the mounting face due to the et35mm.
Does that make sense to you?
ET 25 is a wider track then ET35 because of the offset regarldless of the wider wheel, more of the wheel is inbound relevant to hub face.
I'm struggling to accurately identify my wheels online,
AZEV M type( looking at pics?)
17 x 7,5J H2
We know what the H2 is ( inherent strength loading) my wording!
We know the size
On AZEV site they show offsets for most wheels but these there's nothing blank!
25/35 I have no idea
If there's no offsets listed does this suggest zero offset ?
I've never seen any other markings on the wheels other than what's stamped on the outside inner rim.
My tyres should arrive by Friday so I'll have a wheel off soon enough, I'll take a close look at the inner rim etc, but I don't think there's anything stamped into,them. Hopefully I'm wrong.
If my ET is 35 happy days, rears are correct if I've understood it correctly, fronts need 4 mm spacers either side.
See I thought it turned in well, sometimes a bit to well
ClassicChimaera said:
RobXjcoupe said:
This is what gets a bit complicated because original fit front wheels have a different offset to compensate for front and rear track widths. Fitting a 71/2j front wheel with et35 rear wheel offsets virtually keeps the track the same as the rear of the wheel sits further inboard on the front axle so the outer rim edge sits in nearly the same place as the original 7j rim with an offset of I think is et25.
If you use the et25 7j front wheel the width of wheel in front of the hub mounting face is 63.9mm and 113.9mm behind the mounting face.
If you fit a et35 71/2j wheel on the front, the width of wheel in front of the hub mounting face is 60.25mm and 130.25mm behind the mounting face.
Using the above dimensions the wider front wheel actually gives a narrower track by 7.3mm overall or 3.65mm per wheel as the extra width is behind the mounting face due to the et35mm.
Does that make sense to you?
Yes entirely If you use the et25 7j front wheel the width of wheel in front of the hub mounting face is 63.9mm and 113.9mm behind the mounting face.
If you fit a et35 71/2j wheel on the front, the width of wheel in front of the hub mounting face is 60.25mm and 130.25mm behind the mounting face.
Using the above dimensions the wider front wheel actually gives a narrower track by 7.3mm overall or 3.65mm per wheel as the extra width is behind the mounting face due to the et35mm.
Does that make sense to you?
ET 25 is a wider track then ET35 because of the offset regarldless of the wider wheel, more of the wheel is inbound relevant to hub face.
I'm struggling to accurately identify my wheels online,
AZEV M type( looking at pics?)
17 x 7,5J H2
We know what the H2 is ( inherent strength loading) my wording!
We know the size
On AZEV site they show offsets for most wheels but these there's nothing blank!
25/35 I have no idea
If there's no offsets listed does this suggest zero offset ?
I've never seen any other markings on the wheels other than what's stamped on the outside inner rim.
My tyres should arrive by Friday so I'll have a wheel off soon enough, I'll take a close look at the inner rim etc, but I don't think there's anything stamped into,them. Hopefully I'm wrong.
If my ET is 35 happy days, rears are correct if I've understood it correctly, fronts need 4 mm spacers either side.
See I thought it turned in well, sometimes a bit to well
To measure the offset you need the tyre removed from the wheel. If you put a straight edge across the front and back of the wheel you should get approx 81/2" roughly an inch wider than the 71/2j inside width measurement. If the above is correct you should have a measurement of 142.95mm from the mounting face to the rear of the wheel to the straight edge. That would suggest an et35 wheel.
after looking at offset sectional drawings I was thinking a similar thing so as soon as I can get in the garage and the tyres off I'll take some measurements. I've been assuming therir ET 35 simply based on the position of the rear wheels ( rough guess) in the wheel arch.
I've been banking on finding some info on the wheel itself.
Thanks for taking the time to confirm some calculations I was making were actually correct,,,,,, for once.
It's only taken me 4 years of not having a clue about offsets to then find a nice calculator and a few other sites offering tech knowledge to start to understand it, I'm dyslexic, that's bad enough, no it's a right royal PITA
I'm flipping number scared too, divi
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