More Contact Patch!

For those who couldn't care less how their 356 left the factory!
Message
Author
User avatar
Craig Geiger
356 Fan
Posts: 11
Joined: Mon Nov 10, 2014 5:48 pm

More Contact Patch!

#1 Post by Craig Geiger » Sun Apr 02, 2017 8:21 am

The tires on my stock 63 356b T6 need to be replaced (due to age). I'd also like to add more contact patch - currently it's got 165's and I'm looking to move to probably 185 all around. I believe I can get "stock" wheels in 5.5", but am a Fuchs fan. I understand my car (drum brakes) has a 5 x 205 bolt pattern and Fuchs run 5 x 130. I have seen some options (adapter or replicas):

1. Does anyone know the backspacing and offset numbers that will work with the stock setup (I can add spacers / change studs if need be, I'm more concerned with rubbing on the body)?

2. I saw an adapter that converts from 5x205 ->5x130. Has anyone actually used one of these? http://www2.cip1.com/ProductDetails.asp ... C10%2D6710

3. I also saw Fuch replicas but with a 5x205 bolt pattern. Same question - anyone with experience of these? http://www2.cip1.com/ProductDetails.asp ... 05%2D5515B

I am not interested in converting to the car to discs at this point.

Thanks!

Peter Chifo Jr
356 Fan
Posts: 30
Joined: Sat Mar 21, 2015 3:27 am
Location: Orange County, Ca

Re: More Contact Patch!

#2 Post by Peter Chifo Jr » Mon Apr 03, 2017 10:49 pm

I just installed Coddington 6" with 3.625" back space and Vredestein 165-15 tires on the rear of my 65 356 SC. Perfect fit with about .375" clearance on drivers rear but a bit less on passengers rear. Keep in mind that for some reason many 356's have less clearance on passenger rear. Also note that adjustment of rear bumper will effect fender width a bit.

Edwin Ek
356 Fan
Posts: 1921
Joined: Sat Jun 21, 2008 11:16 am
Location: Santa Monica, CA

Re: More Contact Patch!

#3 Post by Edwin Ek » Mon Apr 17, 2017 6:04 pm

Peter, isn't a 6" rim too wide for 165mm tires?
#6386

Peter Chifo Jr
356 Fan
Posts: 30
Joined: Sat Mar 21, 2015 3:27 am
Location: Orange County, Ca

Re: More Contact Patch!

#4 Post by Peter Chifo Jr » Tue Apr 18, 2017 3:09 am

Edwin Ek wrote:Peter, isn't a 6" rim too wide for 165mm tires?
I actually researched the specs and they show that the tire is good for up to a 7" rim width. That seems like a bit of a stretch to me but the 165 on the 6" looks fine. In years past I was always advised that rim width should be about 75 to 80% of tire cross section.

Edwin Ek
356 Fan
Posts: 1921
Joined: Sat Jun 21, 2008 11:16 am
Location: Santa Monica, CA

Re: More Contact Patch!

#5 Post by Edwin Ek » Sat Jul 08, 2017 3:58 pm

If you want to be careful about things, a wider tire does not increase the contact patch.

Pressure = Force / Area

In this case, pressure and force don't change when you widen the tire, so area can not either. All is not lost though: a wider tire makes the contact patch longer and narrower. That extra length does improve handling.
#6386

Martin Benade
356 Fan
Posts: 3754
Joined: Wed Nov 23, 2011 10:52 am
Location: Cleveland, Ohio

Re: More Contact Patch!

#6 Post by Martin Benade » Sat Jul 08, 2017 5:13 pm

I can't say what actually happens to the amount of traction, but a wider tire can't help but have a wider (larger) contact patch.

Edwin Ek
356 Fan
Posts: 1921
Joined: Sat Jun 21, 2008 11:16 am
Location: Santa Monica, CA

Re: More Contact Patch!

#7 Post by Edwin Ek » Sat Jul 08, 2017 6:07 pm

Martin, that is the point. Wider but narrower, so the area doesn't change.

Edit: "Wider but narrower" may seem confusing. The contact patch is a rectangle, whose long sides are the width of the tire. The height of the rectangle shrinks as the width increases. The area of the rectangle (the contact patch) remains constant.
#6386

Martin Benade
356 Fan
Posts: 3754
Joined: Wed Nov 23, 2011 10:52 am
Location: Cleveland, Ohio

Re: More Contact Patch!

#8 Post by Martin Benade » Sat Jul 08, 2017 11:32 pm

Isn't the narrower dimension of the patch mostly dependent on the tire diameter? After a Google session, it seems to be much more complicated and unpredictable than either Edwin or I thought. If you search on Google for "tire contact patch lowering pressure gain traction- engine basics" you will find an interesting article with a lot of data from Avon. Things seem to be very non-linear. I now see that Edwin's answers are sort of correct, not totally wrong as I first thought.


Edited after looking up stuff

User avatar
Norm Miller
356 Fan
Posts: 2140
Joined: Thu Mar 27, 2008 9:14 am
Tag: Official curmudgeon
Location: near Ft Collins

Re: More Contact Patch!

#9 Post by Norm Miller » Sun Jul 09, 2017 11:17 am

...and the tire pressure is the same with both tires?

/: tire pressure a common term being the internal pressure caused by the amount of air contained within the tire.

Me thinks a little track time and experimentation might blow some of these theories to hell.
 

Martin Benade
356 Fan
Posts: 3754
Joined: Wed Nov 23, 2011 10:52 am
Location: Cleveland, Ohio

Re: More Contact Patch!

#10 Post by Martin Benade » Sun Jul 09, 2017 12:17 pm

Norm, lets not confuse things with facts!

User avatar
Craig Geiger
356 Fan
Posts: 11
Joined: Mon Nov 10, 2014 5:48 pm

Re: More Contact Patch!

#11 Post by Craig Geiger » Mon Jul 10, 2017 5:17 pm

Edwin,

Very interesting. For my own edification, under heavy braking, will the wider tire will provide a larger contact patch? More importantly, I'm curious if given two identical compounds of tires (same stickiness), and diameters, on the same car, will the wider tire provide shorter stopping distances? Coefficient of friction factors into the conversation somehow, right?

Thanks!

Edwin Ek
356 Fan
Posts: 1921
Joined: Sat Jun 21, 2008 11:16 am
Location: Santa Monica, CA

Re: More Contact Patch!

#12 Post by Edwin Ek » Mon Jul 10, 2017 6:29 pm

Speculating here (in other words applying first principles): frictional force is a function of the area of the contact patch (and the coefficient of friction). Since the area of the contact patch doesn't change (much?) under braking, I bet the braking force is the same (more or less). I bet stability under braking would be improved.

Tire engineers and people who have raced can give practical insight.
#6386

Martin Benade
356 Fan
Posts: 3754
Joined: Wed Nov 23, 2011 10:52 am
Location: Cleveland, Ohio

Re: More Contact Patch!

#13 Post by Martin Benade » Mon Jul 10, 2017 10:19 pm

The more powerful a modern car is (Porsche, Corvette, etc) the wider the tires are, so I imagine it accomplishes something somehow. I am certainly not one to explain it.

Edwin Ek
356 Fan
Posts: 1921
Joined: Sat Jun 21, 2008 11:16 am
Location: Santa Monica, CA

Re: More Contact Patch!

#14 Post by Edwin Ek » Mon Jul 24, 2017 11:00 pm

Martin, as I have posted a couple of times already, a wider tire is indeed advantageous because the shape of the contact patch changes: it widens. That greater width provides greater stability under accelerative and de-accelerative (braking) forces.

Of course the reality of the details is more complicated. That is the nature of engineering. But understanding underlying principles is critical. You are lost without them.
#6386

Martin Benade
356 Fan
Posts: 3754
Joined: Wed Nov 23, 2011 10:52 am
Location: Cleveland, Ohio

Re: More Contact Patch!

#15 Post by Martin Benade » Mon Jul 24, 2017 11:17 pm

In your mention of Pressure = force/area, I understand the area, but what are pressure and force relating to? May be a dumb question, but I don't know the answer and I am trying to improve my understanding.

Post Reply