Not a question unique to Subaru's but something I hope someone who understand this stuff a little better than I do could give me a bit of input on. I think I have it down but a confirmation would be helpful!

Quick background info.
Car is running BC coilovers set to 330mm front and 335mm rear heights (wheel centre to arch) but hasn't been aligned since fitting these + full new front hubs. Currently on 205/50/16 winter tyres on which are being swapped for 205/55/16 summer tyres before the alignment at the weekend. I'll be keeping the winter tyres to refit later in the year.

The height difference between the 205/50 and 205/55 is just over 10mm.
Roughly: New diameter - old diameter = difference / 2 = height change
631.9mm - 611.4mm = 20.5mm / 2 = 10.25mm

Assuming the car is aligned on the 205/55 tyre, when I swap back to the 205/50, the car (in theory) will sit 10mm lower, mucking up the geo to some degree.

My assumption is, and correct me if i'm wrong, but when i refit the 50 profile tyres, if I also raise the suspension 10mm, the geo should remain the same as it was when running the 55 profile?

Cheers.

 

The ride height (wheel arch to wheel centre) will not change whatever profile tyres you fit as it doesn't take that into account.
Similarly all "geo" measurements are taken using the hub/wheel face (not the tyre) as the datum for measurements.

In other words changing the tyre profile has no effect on the geo settings. The only difference is the car will be approx 10mm higher/lower off of the ground.

 

Thanks Don, that makes sense. I couldn't get it into my head why the ride height measurement was taken from wheel centre to arch but that makes perfect sense now I know it eliminates the tyre as a factor. In theory then, if one corner was flat, the ride height measurement should remaining the same, arch centre (if we discount any other effects a flat tyre would have on the chassis), but one side of the would simply sit lower to the ground.

As you say, changing the tyre profile, which adds or removes 10mm in my case, happens to the car's ground clearance as a whole rather than the wheels in relation to the chassis which is what changes when you adjust ride height using the suspension.

Thanks for the help

 

You could use 205/55 16 Michelin Pilot Sport 4 for the summer tyre? I like the extra compliance in the sidewall.

 

Thats what I had in mind when I started looking. Deliberately went for 205/55 because of your recommendation on Type RA forums and because they only do PS3's in the stock 205/50 size.

PS4's are being fitted Friday

 

what do you guys run as a winter tyre?

 

Dunlop sp winter sport 4d.

I think it's been replaced with the winter sport 5?

 

My Impreza came with some Goodyear Eagle Ultra Grip's which worked well last winter. I've used Nokian WRA3's on another car in the past which were better in my opinion.

 

 

I had replied to this but it went into moderation queue for some reason and hasn't appeared yet!

I've been using a set of Goodyear Eagle Ultra Grips as they came with the car when I bought it. Do a good job but I'd used some Nokian WRA3's on one of my previous cars which I thought were better.