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Old 10-02-2004, 03:51 PM   #1
Rupewrecht
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ABOUT: Getting new rims? All the info you need here

The series is only part of the rolling diameter. it combined with the width determines the rolling diameter. series is how tall the sidewall is in percentage compaired to the width.

Tyres are labeled www/hh/rr lls

Where www is the overall width of the tyre in mm

hh is the height of the side wall from the rim to the outermost point and is expressed as a ratio of the width (see twilightproteges post)

rr the total diamater of the rim in inches (some BMW use mm but they are rare and obvious), multipy by 25.4 to convert to mm

ll is the load rating, the higher the number the higher the load rating

s is the speed rating, a letter, the closer to 'Z' the higher the speed rating

So the diamater of the standard BA 1.8 185/65/14 is 65% of 185 = 120.25mm top and bottom of the rim = 240.5mm plus the 14" rim (355.6mm).
Total 596.1mm
BA alloy option 195/55/15 is 55% of 195 = 107.25mm top and bottom of the rim = 214.5mm plus the 15" rim (381mm).
Total 595.5mm

Use this to calculate your original diamater then you can make variations to plan

http://www.miata.net/garage/tirecalc.html

and calculate the offset you need here

http://www.1010tires.com/WheelOffsetCalculator.asp



Or this one which does both:

http://www.rims-n-tires.com/rt_specs.jsp
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Last edited by Rupewrecht; 29-10-2007 at 01:37 PM.
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Old 10-02-2004, 05:14 PM   #2
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Stage two, I've taken this from an American book about 20 years old but the information is sound (all be it in inches and MPH)

Tires Are “Gears” Too!
Having the latest hot-tip racing tire is nice — but not if its diameter fouls up your gear ratio. Can’t happen, huh? OK try this. You’ve been running the 5.50-9.20—14 tires on your 510 with a 4.11 rear gear. These have about 24.12- inch diameter at 24 PSI inflation. Get a gear/tire size computer wheel. This little circular slide rule calculator puts the whole thing in your pocket so you can impress your team mates and fellow racers with your knowledge of what’s happening. Set the tire diameter next to the gear ratio, then find 7,000 driveline RPM (same as engine RPM because this is measured in 4th gear, which is 1:1). Read the top speed (4th gear) of about 121 MPH.
Your ex-wife loses grasp of things one afternoon and write you an alimony check. You squander this new found fortune on a set of 10.45—14 tires with a shorter diameter (only 22.4 inches) than you had been running. Play the computer wheel game again and you’ll see that 7,000 RPM now gives 111 MPH. This might help you on a short track where you didn’t use much 4th gear — but if the next race features two long straightaways, you’ve got to figure out what to do. Change rear gears, twist the engine tighter, go back to a taller tire or just be content to finish somewhere in the pack with what you already have in the tire and gear departments? You might be better of leaving your new tires in their wrappers!

A 510 is the old Datsun 1600 '68-'73
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Old 17-02-2004, 01:42 PM   #3
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okay i didn't understand that, but thats why your supose to keep the same rolling diameter!!!!! thanks for the sites tho, i need to see pricing for tyres too they are expensive
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Old 17-02-2004, 02:02 PM   #4
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Tyres are what you pay for them, you can get cheap tyres that last a long time but they won't offer much in the way of grip, or you can buy Yokohama semi slicks (just road legal) which stick like you wouldn't believe but last 5000km if your lucky. Best people to quote you on tyre prices would be your local tyre shop, give 'em a call
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Old 17-02-2004, 08:16 PM   #5
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hahahaha! LMAO luv'd that explination! Nice work! U get to understand the same thing in grand turismo3 when setting all the gear ratio's! One race u luv it, the next u just keep watching the pack pull away!
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Old 04-04-2004, 12:02 AM   #6
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okay i didn't understand that, but thats why your supose to keep the same rolling diameter!!!!! thanks for the sites tho, i need to see pricing for tyres too they are expensive

Its mainly to keep your speedo reasonably accurate by fittng tyres of the same rolling diameter.
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Old 06-04-2004, 04:35 PM   #7
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Wheel offset calculator
Wheel offset explained
Hope this helps
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Old 07-07-2005, 09:00 PM   #8
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And a guide to Markings, Speed Ratings and Load Ratings

http://www.blackcircles.com/general/sidewall

Important when you've got a bloody BA with a ridiculously high load rating!
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Old 08-07-2005, 08:25 PM   #9
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hey its not that bad. integras and civics are 87
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Old 22-11-2005, 01:07 PM   #10
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When the sidewall height is indicated, ie 35, does it include the tyre bead in it's height? I ask this because my tyres are 215/35/18 and by the calculation I should have a sidewall height of 75mm, but my sidewalls are no where near that big. I think they would be lucky to be 50mm from the edge of the rim to the tread.
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Old 26-09-2006, 01:59 PM   #11
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Offsets and Stud Patterns:

BA Astina Hatch + Hardtop

I4 - 4x100 stud pattern, +45 offset
V6 - 5x114.3 stud pattern, +45 offset

Feel free to add BG and BJ, peoples
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Old 26-09-2006, 02:30 PM   #12
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BJ Astina (non SP20)

4x100 +45 offset
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Old 09-02-2007, 03:18 PM   #13
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Updated with an online offset calculator in the first post.
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Old 18-09-2007, 04:14 PM   #14
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will 18 x 8 wheels fit a hardtop if they're only 42 offset?
I know this is probably explained somewhere above but I'm not really mechanicly minded at all.
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Old 18-09-2007, 05:04 PM   #15
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thats what the offset calculater above is for, use it.
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Old 18-09-2007, 05:18 PM   #16
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Quote:
Originally Posted by Rupewrecht View Post
Offsets and Stud Patterns:

BA Astina Hatch + Hardtop

I4 - 4x100 stud pattern, +45 offset
V6 - 5x114.3 stud pattern, +45 offset

Feel free to add BG and BJ, peoples
+45 offset? Is offset tire wall. Ive been trying to work out the terminology, ive assumed things in the past but never been sure. If the BHA run a 45mm tirewall then ive F***ed up when getting new tires. But the ive always kind of known that.
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Old 18-09-2007, 08:00 PM   #17
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Offset is disatnce from the centre of the rim to the point where the hub bolts to the rim. Check out the link at the top for a diagram.

As for 8" wide rims and +42, you'd have to test fit them...but i reckon they'll rub on the struts!
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Old 18-09-2007, 08:32 PM   #18
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7" wide is as far you can go with the 45+ offset mine nearly hit the struts with this configuration
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Old 18-09-2007, 08:45 PM   #19
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would definitely rub i think after looking at the clearance i have on the 7" 17's i have now.
I did check the calculator above,,, and these wheels would be 11 mms closer to the strut and there isn't that much room now.
Would aftermarket struts or coilovers give more clearance?
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Old 18-09-2007, 09:04 PM   #20
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Depends how low you're running the coilovers - as the adjusting collars are level with the rim lip on mine, and i have about 8mm clearance with +37 rims.
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