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 Post subject: More Contact Patch!
PostPosted: Sun Apr 02, 2017 8:21 am 
356 Fan
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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!


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 Post subject: Re: More Contact Patch!
PostPosted: Mon Apr 03, 2017 10:49 pm 
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Joined: Sat Mar 21, 2015 3:27 am
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Location: Orange County, Ca
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.


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 Post subject: Re: More Contact Patch!
PostPosted: Mon Apr 17, 2017 6:04 pm 
356 Fan

Joined: Sat Jun 21, 2008 11:16 am
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Location: Santa Monica, CA
Peter, isn't a 6" rim too wide for 165mm tires?

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 Post subject: Re: More Contact Patch!
PostPosted: Tue Apr 18, 2017 3:09 am 
356 Fan

Joined: Sat Mar 21, 2015 3:27 am
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Location: Orange County, Ca
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.


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 Post subject: Re: More Contact Patch!
PostPosted: Sat Jul 08, 2017 3:58 pm 
356 Fan

Joined: Sat Jun 21, 2008 11:16 am
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Location: Santa Monica, CA
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.

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 Post subject: Re: More Contact Patch!
PostPosted: Sat Jul 08, 2017 5:13 pm 
356 Fan

Joined: Wed Nov 23, 2011 10:52 am
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Location: Cleveland, Ohio
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.

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 Post subject: Re: More Contact Patch!
PostPosted: Sat Jul 08, 2017 6:07 pm 
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Location: Santa Monica, CA
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.

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 Post subject: Re: More Contact Patch!
PostPosted: Sat Jul 08, 2017 11:32 pm 
356 Fan

Joined: Wed Nov 23, 2011 10:52 am
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Location: Cleveland, Ohio
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

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 Post subject: Re: More Contact Patch!
PostPosted: Sun Jul 09, 2017 11:17 am 
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...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.

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 Post subject: Re: More Contact Patch!
PostPosted: Sun Jul 09, 2017 12:17 pm 
356 Fan

Joined: Wed Nov 23, 2011 10:52 am
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Location: Cleveland, Ohio
Norm, lets not confuse things with facts!

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 Post subject: Re: More Contact Patch!
PostPosted: Mon Jul 10, 2017 5:17 pm 
356 Fan
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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?

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 Post subject: Re: More Contact Patch!
PostPosted: Mon Jul 10, 2017 6:29 pm 
356 Fan

Joined: Sat Jun 21, 2008 11:16 am
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Location: Santa Monica, CA
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.

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 Post subject: Re: More Contact Patch!
PostPosted: Mon Jul 10, 2017 10:19 pm 
356 Fan

Joined: Wed Nov 23, 2011 10:52 am
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Location: Cleveland, Ohio
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.

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 Post subject: Re: More Contact Patch!
PostPosted: Mon Jul 24, 2017 11:00 pm 
356 Fan

Joined: Sat Jun 21, 2008 11:16 am
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Location: Santa Monica, CA
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.

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 Post subject: Re: More Contact Patch!
PostPosted: Mon Jul 24, 2017 11:17 pm 
356 Fan

Joined: Wed Nov 23, 2011 10:52 am
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Location: Cleveland, Ohio
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.

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