Differences in going from a 7'' wheel to 8''wheel?

[Jason1145]

I recently upgraded from 7x17'' alloys to 8x18''.... went down in profile from 45 to 40 series to keep the rolling radius the same on my '07 W245. but I think the turning either on or off power has deteriorated and the steering feels heavier now.
I am still using the same width tyre of 215 (garage said it was good for 7'' and 8'' alloys) so the tyre width hasn't increased, is it normal to experience this?
Note - went from et49 to et42 and the front tyres now sit quite a bit out from the arch than previously... this has made the track wider? Is this the cause?

 

[kid-jensen]

You've changed the profile to a lower one, so the width of the rubber on the road will be wider. Just Google it for the exact figures. It's a natural consequence of lower profile, but there are otherfactors that affect steering feel.

What about the offset? If you had 7" before with say, 45mm offset (ET Figure) rims, when going to 8" the offset would bneed to be 37 (or as close as you can get to it) to give you the same steering scrub setting.

If the offset is too far from this figure, it's all downsides...

 

[whitenemesis]

Did you have staggered set-up previously? Have they mounted the rims on the correct axle?

A wider track could lead to a heavier feel.

 

[Jason1145]

Standard 7x17 et49 215/40/17 (not staggered, same all round - only a little B Class)
Now 8x18 et 42 215/40/18

Yeah I can see the same 215 tyre is wider on the road now... and the offset figures not matching up would track it wider.... both have combined to affect my lovely steering!
Well I won't tell the Missus that's the reason, luckily she never drove it with the stock 17'' wheels on ;p

 

[renault12ts]

The tyre is the same width at 215...I'm not sure the slightly lower profile will do much for the contact patch.

But the wider track will make it feel heavier.

 

[Jason1145]

Actually... the contact path might even be smaller.... because the 215 width is now stretching across 8'' not 7'' anymore.... the sidewalls are more angled in I think making the contact path narrower!

 

[renault12ts]

Actually... the contact path might even be smaller.... because the 215 width is now stretching across 8'' not 7'' anymore.... the sidewalls are more angled in I think making the contact path narrower!

The other way round I think, but so little as to be inconsequential.

 

[Jason1145]

The other way round I think, but so little as to be inconsequential.

Lol you're right sir ;p

 

[hotrodder]

https://en.wikipedia.org/wiki/Scrub_radius is something that often gets messed with when changing wheels but in this case changing the ET has preserved the origional setup

Tyre contact patch size/area is mostly a function of the weight being supported and pressure in the tyre. Size of the tyre makes relatively little difference to the size of the contact patch but does affects its shape... replace a narrow tyre with a wider example and the contact patch remains about the same area but changes from relatively long and narrow to short and wide. Same width tyre on a larger (dia and width) wheel should have a similar effect albeit a lot more subtly.

The new combination is virtually guaranteed to be a chunk heavier though which is never a good thing from a feedback/handling/ride comfort POV.... rotational inertia, gyroscopic effects, sprung to unsprung mass ratio etc. This is almost certainly why the new wheels 'feel' worse from a driving POV

 

[RichM63]

Ah I remember the days when 155/13 and no power steering were the 'norm'.

but apart from all the good explanations above, am I not correct in thinking that having tyres that 'sit outside' the wheel arches is a no-no over in the UK? #1. Hence rolling the arches thing.

Even if it is OK to have this, the sides of the vehicle will get covered in road spray and all other crud very quickly.

 

[Jason1145]

No I've seen lots of cars with wide wheels that sit outside the arches with stretched tyres on..... think it's a VW status statement..... I didn't copy it intentionally lol.... but I'm sure it's not illegal.
Here is a quick pic to try and show my wide wheel sticking out past the front arches... and yes I'm peeved at the debris and grime it throws up the side of my car now... am thinking to fit some mug guards to try and reduce this

I think my B170 has Sport suspension as it is very stiff and I'm pretty sure there is no way the car wil ever dip down enough for the arch to catch the tyre... there's just no give in it

 

[whitenemesis]

I reckon you could get pulled for those tyres sticking out that far.

 

[Jason1145]

You're a bundle of fun pal, cheers for that ;p
Maybe the angle made it look worse than it is.. I could photo my bicep real close to camera to make it look bigger then Mount Everest ;p

Here's a few more shots to give a rounder view maybe.... what's the deal with that aspect of the law then?

 

[whitenemesis]

It doesn't look soo bad in those other shots...

IIRC the law requires the tyre to be covered so that it cannot come into contact with a pedestrian, of something like that.

Usually if the arch covers the top of the tyre it's ok, looks better in the pics posted above..

ETA --

"Tread is important not only for grip, but is one key part to the rules. The laws states that NO tread must be visible outside of the wheel arch. So, in effect, the parts of your tyre with tread MUST be under the wheel arch. The reason? When it’s wet water gets sprayed up and impacts other drivers. Rims/parts of alloys or the sidewall (stretch) do NOT form part of this. If you have tread showing outside of your wheel arch be prepared to be ticketed. If you can’t loose your wheels maybe consider rolling your arches, remove the spacers if you have any or increase the camber.

RULE 1: TREAD INSIDE THE ARCHES!"

http://trafficlawadvice.co.uk/tyre-stretch-poke.html

 

[Jason1145]

Tread inside the arches seems sensible too..... I'd hazard a guess mine are ok.. compared to this one here anyway lol
Stretched Tyres - Are they legal or not? - Roads Policing - UKPOLICEONLINE Discussion Forum

Saying that... most wheel arches don't run in a straight vertical line down the vehicle, they max out at the top of the arch and then taper in towards to bottom at the front and rear of the arch... so the top of my tyre has my tread under the top of the arch, but as you go down the tyre and the arch then tapers in, the tread then does stick out past the arch at the bottom.
I'll creep past the next traffic bobby I see and use my Harry Potter invisiblity cloak just in case.
Looking at fitting mud guards on the front anyway, this might help the line of the tread keep inside the arch (mudflap)
I certainly don't want to cause any nuisance to other drivers from my spray

 

[rtm4676]

Heres the link you need.

This is the best link you'll find which will tell you exactly what your wheels will do if you play around with sizes, better than the one I found on the site the other day.

Rim & Tire Size Calculator. Custom Offsets - Wheel-Size.com