Hey, thanks for the message. Sorry it's taken me a while to get back to you..
Ok so to answer your questions:
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You wrote that you bought 4 OZ rims which were mounted on a MX5 (NA, I'm right?) and you've fitted them on the 107.
The bolt hub to studs conversion is necessary to mount that kind of wheel or is possible to maintain the OEM bolt hubs?
Yeah I bought them from a guy who raced mx5's and mr2's - I think they were fitted to an early mx5. They're 6Jx14 et45. I'm using 10mm slip on spacers to take them to et35, it's a nice fit and I'd be cautious of the wider wheel rubbing on the damper without that spacer there. The spacer covers the spigot ring on the hub so the wheel doesn't locate properly; using studs keeps it a bit more secure and easy to fit/centre. You could use bolts as long as they're the right taper and narrow enough to fit in the wheel but I recommend studs when fitting spacers.
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How do you feel with the Nankang NS2-R on the 107? You tryied different tyres on 185/60 r14?
I first fitted NS2-R's in that size and honestly it was a bad choice on this car. They're a bit too stiff and they cause a lot of drag for a small engine to overcome. It was too grippy for my driving style, like cornering on rails but there was no feel and it was actually difficult to get the car rotating.
I was talking with the guy who runs the new BRSCC CityCar Cup a few weeks ago (they also use ns2r); I noticed they spec softer 80a front wishbone rear bushes with 95a front bushes and he told me this was for the same reason: there's no feel in the tyre so they fit softer bushes at the back of the wishbone to try and compensate for that by adding a bit of flex into the suspension.
They're great tyres on slightly bigger/heavier cars like the type r but they just dont suit the 107.
The other problem is gearing. That tyre size is huge and it makes the already long gearing even worse!
I've since switched to a 185/50r14 yokohama a539 and it's my favourite tyre by far. It feels really good, predictable, works in any condition and you can use the weight shift to rotate the car properly; there's less grip overall but I find I can drive faster on these just because the car is more fluid and lively. They're also slightly smaller radius than the standard 155/65r14 which helps the gearing.
I'd recommend looking at 175 and 165 options too.
I had 175/60 a539's on this and they were good, I then went to the 185/50's just for the smaller radius to help gearing.
If you're budget limited you could try a grippy 155/55 tyre on the steel rims - The C1 Racing Club use nankang as1's, they do a 24h race at Spa without issues. This size would allow better speed with less drag than wider tyres and I think they'd be fun in the corners - It depends on your driving style and confidence really.
That's my experience anyway.
Also have a look at the *Weichers front lower strut brace which fits between the front wishbone mounting points - they're great for track, cost £36 here and make a huge improvement to steering feel/response especially with grippier tyres.
Hope this helps.