Hi Nir,
changing to wider tyres can also affect the steering and make it less direct, you have more rubber which means more filters.
The chnage your'e about to make is pretty drastic make sure you know what you are getting into, ride, handling, accaleration are about to change.
One more thing where did you read the Michelin won?
Have the best of luck
Udi.

Nir,

The important thing to try to maintain when you change wheel/tyre sizes is the rolling radius of your tyres. In the case of your standard 205/55 R16, the rolling radius is 315.95mm on a new tyre (half the wheel diameter = 8" or 203.2mm - plus the section height of the tyre - 55% of 205mm = 112.75mm; add these figures together to get the rolling radius = 315.95mm).

If you go for a wider tyre, without changing the aspect ratio (e.g. going up to 215/55), the effect will be to increase the rolling radius, which will have a major effect on the gearing. In this example, it would add nearly 31mm to the rolling radius, equivalent to around 194mm more distance travelled for each wheel revolution. So your speedo would be under-reading, the car wouldn't accelerate as quickly, but your top speed would be greater. For road driving, and with the plice and cameras as active as they are, it's important to try to keep as near as you can to the original rolling radius so your speedo is reliable - or, as you say, you can have it calibrated. Beware though that the rolling radius changes by as much as 6mm as your tyres wear (which is why most speedos are fairly inaccurate and read higher than you're going).

Still with me? To maintain a rolling radius and rolling circumference as close to your original wheel/tyre combination (315.95mm and 1984.17mm respectively), you could look at these alternatives (I've no idea how easy these tyre sizes might be to find, though!):
225/45 R17 (rr 317.15mm; rc 1991.70mm)
245/35 R18 (rr 314.35mm; rc 1974.12mm)

Take note of Suresh's comment about fouling the arches - 245 rubber is A LOT wider than 205.

Best of luck,

Brian

Has anyone tried installing 215/45 R17 wheels
on an Impreza?
Does it feel good?
No problems?

Hello all
I've been involved in a car accident (no
casualties) and my tyres were hit
- it's an opportunity to change them.
I'd appreciate quick replies as the car in
the garage now.
It's a non-scooby FWD car with 205/55 R16 P6000 tyres weighing 1355 Kg.
What could improve handling?
There are 3 issues: tyre profile, brand, and width.
How about Bridgestone Potenza RE71, 205/50 R17? or Michelin Pilot Sport?
Or should I leave the diameter R16 with a lower profile (this will
make the car lower)?
What do you think about nuts who fit tyres like
205/35 R18 on a Golf GTI? What does it give them?
Do they suffer loss of comfort?
In short, what is the best practical mod to
harden the steering, and improve the grip?

Thanks
Nir


[This message has been edited by Nir (edited 12-06-2000).]

[This message has been edited by Nir (edited 12-06-2000).]

Nir,

Sorry to hear about you accident. Glad nobody was hurt.

A few ramblings:

1. Changing the profile on the same diameter wheel will also alter gearing of your car as well as lowering it slightly. Speedo / Odometer will both over-read. Not advised.
2. Wider tyres may be prone to tramlining and could put more strain on the steering gear than car was designed for.
3. Bigger wheels may foul the arches. Check before you buy!
4. Bridgestone SO2's are extremely grippy compared to the Pirellis.

Not an expert's view, I'm afraid - but you said you wanted speed of response, not quality!!

Suresh

Thanks, I've been lucky.

Regarding your points:

1. you mean more frequent gear changes will be required?
lowering the car is ok, it's pretty tall anyway.
Speedo - well I'll calibrate it

2. On what scale? I mean, going from 205 to
215 is that dramatic for the steering gear?
the steering is pretty light, I wish it was
harder

3. Ok. I hope they allow me to change the wheel diameter, so I can get extermely low profile (35)

4. It lost to Michelin Pilot in recent test,
(http://www.tirerack.com/tires/tests/max_rd.htm)
Is choosing brand really important? changing dimentions
is for sure, but brands? (as long as it's a
well known company)

5-No problem... yes I am I'm in rush

Nir

Brian,

You refer to changing a 205/55 R16 to a 215/55 R16, and that it will affect the gearing and speedo. The actual circumference of these tyres is the same, so neither gearing or the speedo will be affected. Acceloration will be, but for different reasons.

Triggaaar,

The 205/55R16 and 215/55R16 tyres do NOT have the same circumference and WILL affect the speedo, gearing etc. I do not know how you think that 55% of 205 and 215 is the same http://bbs.scoobynet.co.uk/confused.gif!

Nir, take Brian's advice and keep the rolling radius the same and you should be O.K.

Triggaaar,

No, the circumferences are different because the aspect ration of the tyre relates to its width. So, a 205/55 R16 has a section height of 112.75mm (55% of 205mm) whereas a 215/55 R16 has a section height of 118.25mm. That makes the 205 tyre's circumference 1984.17mm and the 215 tyre's circumference 2018.71mm. Ok, so 34.54mm isn't much for each revolution of the wheel (not far off 2%) but it is a difference.

All the best,
Brian

Hmmm..
I didn't think about the fact that profile is stated in PERCENTAGES...
Nevertheless, the proportions I think determine
the tyre's behaviour.
In fact, due to to licensing problems I am experiencing now,
I think I will go for putting 205/50/16 in the FRONT,
and leaving the 205/55/16 in the REAR.
(reason is partially financial)
If I think right, this will help my understeer
problem; the front of my car will be closer to
the road.
I wish I could use 215/45/17; I have 235 bhp
and most of it is wasted on wheelspin.

Nir

Wider tyres will also aquaplane more easily in the wet.

Nir,

I doubt whether what you're thinking about would make any difference to your understeer problem. One thing to consider though - do you have a full size spare wheel, or a space saver? If full size, you really need to make sure all your tyres are the same size, otherwise you could put a strain on your diff if you end up with different-sized tyres on each side at the front (and the rolling circumferences are different) following a puncture. Same issue with a space saver, you could put a bit of strain on your diff if you run that on the front.

Brian

OOooopps
Is there any way around this? (spare)

Oops - I thought the 55 meant a height of 55mm, not 55% of the tyres width.

Cheers Brian

Nir,

As long as you used the spare as a "get you home" replacement and have the same tyre aspects re-fitted on the same "axle" as soon as possible, there shouldn't be a problem. Diffs, by their vary nature, are designed to allow the wheels at either side to turn at different speeds. My point is that you shouldn't run odd sizes on the same "axle" for longer than necessary. If you have a limited-slip diff, however, this would result in some "wind-up" which you would feel through the steering as the diff unwound from time to time.

Brian

Well, I actually thing it would be worse -
if one wheel has a smaller overall diameter,
even a bit, the car will NOT move in straight line....

~N

Hmmm. No, I think the car would still drive straight. The diff would sort the circumference differential (which is why it's called a diff) on the front wheels and the difference would probably be imperceptible on the rear (a bit like having one well-worn tyre and one brand new). Still, you're having second thoughts, which is good. I'd recommend keeping the same size tyres all round unless you're prepared to a) carry two spares, or b) call out the AA for a lift home if you get a puncture. IMHO, running tryes of different sizes carries more downside than benefit.

Brian

You all talk about the size of the tyre but there is also the rim issue, 16 is not the same as 17.
regarding the width and wet roads, it is acttualy the other way around as you can see in the rally, a wider tyre takes longer to get rid of the water and tends to float over the water.

Wider is not always better , often worse look at a "fat boy" vitara on a wet road and see a frightened driver without a clue what dirrection the wheels are pointing
for a good guide to up-grading wheel size without altering rolling radius follow this simple patern 155/80 13 = 175 /70 13 = 195/60 13 = 185/60 14 = 205/50 14 = 195/50 15 then the patern tends to go a bit sqiffy the bigger and lower you go see the Toyo web site for a full chart

You also have to take into account the rim size. 7" wide is often the maximum you can get. A 7" is ideal with a 205 wide tyre if you want some accurate road handling. Of cause you can mount a 215 tyre on a 7" rim but you will get a less responsive steering.
I have tried both 215/40 17 and 205/45 17 on a Subaru with 7" OZ's. I prefer the 205 as it give a better turn in.
Ok it was not same tyre manufacture.

Lars, what you said is really interesting!
Is 215 really responds less than 205?

I was told it makes the wheel "harder" (mine
is soft "disconnected"! a 620ti!)
What about acceleration?
Currently, I am loosing much of the power on wheelspin.
Wider tyre means better straight line grip, at least.
Someone said that lower overall diameter,
gives better acceleration (imporant) and lesser
top speed (not important).
I think I'll stick with the same rim, and
will cut in sidewall size, this way I will gain both
grip and acceleration, and save buying new rims..

Stancer has provided this great chart:

my original overall diameter- 205/55R16 633mm
my desired now- 215/45R16 (couldn't find it in
Stancer's chart though)

Wonder if the effect on comfort will
be severe...


~N

Hi Ian,

<B>Don't go too wide with that Turbo rover , serious Tourque steer</B>

No Torque steer at all.
I have found out that if you do not *accelerate*
in corners, the problem doesn't exist *at all*
If is anyone is buying 4x4 just to avoid Torque Steer,
they are making a mistake.
What is too wide, anyway?

<B>dad had a 623i, which was ok at 60%effort But push it too hard and it lost interest
</B>

Huh?
Lost you completely.

Anyway, it looks like I'm going to demand new tyre
setup from my Rover dealer.
I should have recieved 205/50, the brochure said
205/50, but the car was supplied with 205/55.
This is a very basic thing. A 10% grip difference.
If they don't change, I'll sue them.

~N


[This message has been edited by Nir (edited 14-06-2000).]

Nir,

Didn't understand the last part of your post - a 55 profile tyre will not have either 10% more or 10% less grip than an 50 profile tyre - it's merely a measure of the height of the sidewall.

Higher profile= better ride comfort but less responsiveness.

You'll get nowhere suing. Read the brochures - they ALWAYS say that specifications can be changed without notice 'as part of our continual efforts to improve quality'.

More grip needed? Fit stickier tyres, rather than go for much wider than normal. You'll only end up with aquaplaning in the wet and tramlining in the dry.


Nick

I would guess Toyo don't do a tyre that size ,ring someone like elite to see who does , 215 is a bit of an odd ball size for some reason
Don't go too wide with that Turbo rover , serious Tourque steer could ruin ,an already "light" set-up (dad had a 623i, which was ok at 60%effort But push it too hard and it lost interest )
good luck
Ian

I don't know about responsiveness, it was said
that low profile is less prone to bend when cornering,
which means better grip (wonder if similar effect
is obtained by raising the tyre air pressure,
as small raise, 2-3 psi makes ride much stiffer)
It looks like responsiveness has more to do with width.

Regarding sueing, yes you are right about the
small letters in the brochure.
I'll find a way around

Stickier tyres? I already have P6000 which is
good.
Doubt that top-of-line Michelin or whatever
would be noticeable.
Should I put 800 Pounds on this?

I went lower and wider on my old 205gti and it produced so much grip there were diamond - shaped stress dents in both the front wings after a short while, the car simply couldn`t take it.

It also ruined the subtlety of the handling and steering so spoiling the driving experience.

Zoog,
You haven't been specific enough...
How extreme did you go with that tyre mod,
that is, what was the "before" and "after" dimensions?
Could you please explain on "ruining the subtlety of the handling and steering"
Has this occured only some time later, too?
Has the wheel resistance strengthened?
(did you have power steering?)
Has oversteer/understeer behaviour changed?
How severe was lost in comfort?
If your car could handle the load, would you
be doing this again?

~N