Tyres/Wheels For Fiesta ST

Tyres/Wheels For Fiesta ST

Author
Discussion

dotdog

Original Poster:

457 posts

219 months

Friday 3rd March 2017
quotequote all
Hello,
Not a suspension and brakes topic, so apologies for that, but there is no wheel/tyre section..
My question is - I have a 2016 ST fiesta on 205/40/17 and am thinking of changing to 195/55/15 as a back up set for the awful roads where I live - is this safe? (Rolling diameter is the same with this swap) and what tyre pressures suggested? (Current ST on the 205/40/17 is 36F and 26R.
Thanks in advance.

E-bmw

9,106 posts

151 months

Friday 3rd March 2017
quotequote all
It isn't per-se unsafe but you will have to remember that with less surface area of the tyre in contact with the road, your grip level will be lower.

I would consult your handbook for pressures, you will probably find a similarly tyred similar weighted Fiesta to give you an idea of the pressures.

Edited by E-bmw on Sunday 5th March 12:48

HustleRussell

24,602 posts

159 months

Friday 3rd March 2017
quotequote all
First of all, do 15" wheels definitely fit the ST (which probably has bigger brakes than lower spec Fiestas with 15" wheels from the factory)

If so I think it's a good idea, you might find a table of Ford approved tyre sizes and pressures in the handbook? I wouldn't expect the pressures to be dramatically different from those given for 17" tyres.

HustleRussell

24,602 posts

159 months

Friday 3rd March 2017
quotequote all
E-bmw said:
It isn't per-se safe but you will have to remember that with less surface area of the tyre in contact with the road, your grip level will be lower.
I disagree with this part.

dotdog

Original Poster:

457 posts

219 months

Sunday 5th March 2017
quotequote all
Thanks for feedback - I have been looking at various forums and pics from USA where people have been asking about this with relation to winter tyres etc - seems you can go down to 15" (with 195/55/15)- but it depends on the spoke style as to whether they will fit over the brakes. 16" seems to be fine - so looking at 205/45/16 still on 7" which should improve the ride a bit or perhaps 195/50/16, although this seems to be a rarer size and thus more expensive and would also mean coming down from 7" rim to 6"..
There is also an issue with having to have the TPMS (tyre pressure monitoring system)valves installed in the new tyres - pain in the arse and additional cost. Have been speaking to Wheelbasealloys.com who have been helpful (and am considering their very nice Wolfrace alloys.

Edited by dotdog on Sunday 5th March 08:38


Edited by dotdog on Sunday 5th March 08:39

E-bmw

9,106 posts

151 months

Sunday 5th March 2017
quotequote all
HustleRussell said:
E-bmw said:
It isn't per-se safe but you will have to remember that with less surface area of the tyre in contact with the road, your grip level will be lower.
I disagree with this part.
205mm is bigger than 195mm and therefore has a greater contact area, the end. What can you possibly dissagree with?

I am OBVIOUSLY applying the caveat that the tyre would be the same when comparing one size to the other.

E-bmw

9,106 posts

151 months

Sunday 5th March 2017
quotequote all
dotdog said:
Thanks for feedback - I have been looking at various forums and pics from USA where people have been asking about this with relation to winter tyres etc - seems you can go down to 15" (with 195/55/15)- but it depends on the spoke style as to whether they will fit over the brakes. 16" seems to be fine - so looking at 205/45/16 still on 7" which should improve the ride a bit or perhaps 195/50/16, although this seems to be a rarer size and thus more expensive and would also mean coming down from 7" rim to 6"..
There is also an issue with having to have the TPMS (tyre pressure monitoring system)valves installed in the new tyres - pain in the arse and additional cost. Have been speaking to Wheelbasealloys.com who have been helpful (and am considering their very nice Wolfrace alloys.

Edited by dotdog on Sunday 5th March 08:38


Edited by dotdog on Sunday 5th March 08:39
Looking here

https://tiresize.com/tyre-size-calculator/

It seems that 205 45 16 are a pretty close size match & will maintain the tyre width.

These a re a fairly common size so shouldn't be too difficult to track down.

HustleRussell

24,602 posts

159 months

Sunday 5th March 2017
quotequote all
E-bmw said:
205mm is bigger than 195mm and therefore has a greater contact area, the end. What can you possibly dissagree with?
Yeah it's definitely that simple.

GreenV8S

30,150 posts

283 months

Sunday 5th March 2017
quotequote all
If you run a narrower tyre then the contact patch will be longer and narrower for a given tyre pressure. This will alter the way the tyre generates lateral grip. In general, the peak grip will be lower and will occur at higher slip angles, and the tyre will break away more progressively so there is less of a peak at the peak grip if you see what I mean. You might also find the grip doesn't fall off as sharply in wet conditions.

The sort of change you're describing will be enough to be noticeable but not excessive - if you pay attention to the handling it might feel as if the tyres are a few PSI soft. If you don't drive it hard or don't pay attention to the handling, you might not notice the difference. I suspect the same will be true of the ride quality - should be better with the higher profile tyres but probably not dramatically better.

Mr2Mike

20,143 posts

254 months

Sunday 5th March 2017
quotequote all
HustleRussell said:
I disagree with this part.
People that disagree with this tend to remember O-level/GCSE physics lessons demonstrating friction, where smaller contact area was exactly compensated by the increased pressure within that contact area. The tyre/road interface doesn't behave like this at all, it's far from an ideal friction model.

That said simply going up on tyre width does not guarantee an increase in contact patch area, which is primarily determined by air pressure, but with contributions from carcass/sidewall stiffness.

HustleRussell

24,602 posts

159 months

Sunday 5th March 2017
quotequote all
This one always generates discussion.

In response to the two posts above I'd explain my thinking as follows;

My opinion doesn't come from GCSE physics so much as it comes from experience. The change suggested above from a slightly wider, lower profile tyre to a 10mm narrower higher profile tyre has plusses and minuses, In addition to what GreenV8S said the narrower, taller tyre might manage choppy road surfaces better, depending on power and weight it might also actually perform better in longitudinal grip / traction due to the longer, narrower contact patch.

IMO the narrower tyre size suggested is not an inappropriate choice for a Fiesta ST and the slight reduction in outright dry lateral grip will definitely be within reasonable limits and in my opinion does not constitute a safety issue.

By reasonable limits I would suggest that the difference in dry outright grip between the 205/40 and the equivalent 195/55 might be comparable to the range of performance of the same 205/40 tyre in a nearly new vs. worn to 3mm condition for instance.

E-bmw

9,106 posts

151 months

Sunday 5th March 2017
quotequote all
HustleRussell said:
This one always generates discussion.

In response to the two posts above I'd explain my thinking as follows;

My opinion doesn't come from GCSE physics so much as it comes from experience. The change suggested above from a slightly wider, lower profile tyre to a 10mm narrower higher profile tyre has plusses and minuses, In addition to what GreenV8S said the narrower, taller tyre might manage choppy road surfaces better, depending on power and weight it might also actually perform better in longitudinal grip / traction due to the longer, narrower contact patch.

IMO the narrower tyre size suggested is not an inappropriate choice for a Fiesta ST and the slight reduction in outright dry lateral grip will definitely be within reasonable limits and in my opinion does not constitute a safety issue.

By reasonable limits I would suggest that the difference in dry outright grip between the 205/40 and the equivalent 195/55 might be comparable to the range of performance of the same 205/40 tyre in a nearly new vs. worn to 3mm condition for instance.
Having said what I said, I will also be the first to admit that there are other things at play & will agree whole heartedly with both your final conclusion and your statement that it will not be an inappropriate choice.

Obviously I was considering ONLY the tyre width as a variable in my (overly simplistic) statement above.

E-bmw

9,106 posts

151 months

Sunday 5th March 2017
quotequote all
GreenV8S said:
The sort of change you're describing will be enough to be noticeable but not excessive - if you pay attention to the handling it might feel as if the tyres are a few PSI soft. If you don't drive it hard or don't pay attention to the handling, you might not notice the difference.
Which is exactly why I mentioned it in my original post.

I used to have an e39 which I used winter tyres 10mm narower on & while the maximum grip was reduced it was never un manageable unless you decided to forget what boots the car was wearing.

Mr2Mike

20,143 posts

254 months

Monday 6th March 2017
quotequote all
HustleRussell said:
This one always generates discussion.

In response to the two posts above I'd explain my thinking as follows;

My opinion doesn't come from GCSE physics so much as it comes from experience. The change suggested above from a slightly wider, lower profile tyre to a 10mm narrower higher profile tyre has plusses and minuses, In addition to what GreenV8S said the narrower, taller tyre might manage choppy road surfaces better, depending on power and weight it might also actually perform better in longitudinal grip / traction due to the longer, narrower contact patch.
The specific part of your response that I was commenting on was that you disagreed that a smaller contact area would reduce grip - nothing to do with the shape of the contact patch. Try pumping up your tyres to twice the recommended value (which will significantly reduce contact patch area) and see how much grip that gets you.

dotdog

Original Poster:

457 posts

219 months

Monday 6th March 2017
quotequote all
Okay - got all that.. Thanks.. as said 205/45/16 pretty easy swap - but probably wont make that much difference - also it's turning out to be hard to find 7j width of rim with 16" wheel - well, nice ones anyway where the offset is not too compromised (another whole issue!)and the 205s need a 7J rim.
SOooo.. looks like I may well be going down to 195/50/16s on a 16x6j rim which will mean higher profile and more lateral movement of the tyre on the rim. There is a Wolfrace available for this in matt black:

http://www.wheelbasealloys.com/alloy-wheels/wolfra...


I only do road driving and therefore will be happy to sacrifice ultimate grip in favour of a more forgiving ride and it may even make the handling a bit more progressive - which I like. Tyre choice will either be the Dunlop Sport Blue Response (which I ran on a classic impresa and are good tyres), or the new Yokohama Advan Fleva V701.
If I do this I will post some pics of the new wheel/tyre comb.

E-bmw

9,106 posts

151 months

Monday 6th March 2017
quotequote all
What makes you think you can't fit 205s on a 6J rim?

http://www.tyresizecalculator.com/tyre-wheel-calcu...

Says 205 is OK on a 6J rim.

GreenV8S

30,150 posts

283 months

Monday 6th March 2017
quotequote all
Mr2Mike said:
The specific part of your response that I was commenting on was that you disagreed that a smaller contact area would reduce grip
I did not read his post as saying that. He was disagreeing with a post that asserted that a narrower tyre had a smaller contact area and therefor less grip. I also disagree with that conclusion and I hope you do, too. My disagreement is not based on any naive theories about coefficients of friction - it is based on the fact that the contact area is unchanged and all conclusions based on that assumption are unfounded.

dotdog

Original Poster:

457 posts

219 months

Monday 6th March 2017
quotequote all
http://www.carbibles.com/tyre_bible_pg4.html

Have been using this calculator - looked at the one you suggested and yes does say it's okay - hmm - even more lateral movement of the tyre with 205 on 6j... not sure if maybe too much..

HustleRussell

24,602 posts

159 months

Monday 6th March 2017
quotequote all
Mr2Mike said:
you disagreed that a smaller contact area would reduce grip
Just to clear this up, I was disagreeing with e-bmw's suggestion that it wasn't 'per se safe' to run the narrower tyre and went on to say in a later post that the reduction in grip would be within reasonable limits.

(It turned out to have been a typo on e-bmw's part anyway)

Edited by HustleRussell on Monday 6th March 12:52

Mr2Mike

20,143 posts

254 months

Monday 6th March 2017
quotequote all
HustleRussell said:
Just to clear this up, I was disagreeing with e-bmw's suggestion that it wasn't 'per se safe' to run the narrower tyre and went on to say in a later post that the reduction in grip would be within reasonable limits.

(It turned out to have been a typo on e-bmw's part anyway)

Edited by HustleRussell on Monday 6th March 12:52
Ok, just the way I read it then, sorry about that.