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 Post subject: Tire stretching ?
PostPosted: December 9, 2018, 12:15 pm 
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Doing the ground work for a replica of a 908-3 Porsche. The front tires were 10.0 / 20.0- 13. Its about 22.4" tall and was mounted on a 10" x 13" rim.
To get that look I'm think I can get it by doing a tire stretch on a 10"x 15" rim using a 225/50 r 15" . Ideas and experience's ?
I see some info from the Miata guy's are doing this.

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 Post subject: Re: Tire stretching ?
PostPosted: December 9, 2018, 1:36 pm 
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I've used a small diameter bicycle inner tube with very low pressure in it to reduce the air loss enough to make the tire sidewall push out enough to seal.


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 Post subject: Re: Tire stretching ?
PostPosted: December 9, 2018, 1:40 pm 
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seven13bt wrote:
I've used a small diameter bicycle inner tube with very low pressure in it to reduce the air loss enough to make the tire sidewall push out enough to seal.

Disregarding the wheel will be much wider than any tire mfgr will recommend, can you say "Propane"? I knew you could. What can be more fun a big BOOM ?

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 Post subject: Re: Tire stretching ?
PostPosted: December 9, 2018, 2:40 pm 
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Thanks. I don't think the mounting will be a big problem ? We will see.

It is more a question of what can I expect from this stretch ? Handling ,general performance ?

Here is a photo of the race wheel used on the car. Plan is to re-scale the body for the taller front tire.


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 Post subject: Re: Tire stretching ?
PostPosted: December 9, 2018, 8:08 pm 
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My $0.02,
If you were going to change just the wheel, then the car would react faster to inputs (steering, cornering). But, since you're also apparently going to change to a taller tire (ie more sidewall compliance) the net effect may be slower reactions. Performance - will depend more on the compounds used before and after. Tire wear - most tires are "dead" long before the tread is gone.


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 Post subject: Re: Tire stretching ?
PostPosted: December 9, 2018, 8:32 pm 
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13,its more about using a 15" wheel because their are no 13" street tires that could fit on a 10" wide rim.
Then its designing the car to work with that. The stretch is more for the look.

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 Post subject: Re: Tire stretching ?
PostPosted: December 10, 2018, 7:27 pm 
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Using a tire on a rim with a width that is not within the range recommended by the mfg is a bad idea. I change all my own tires and I would expect it to be difficult. I'd be surprised if you find a shop willing to try to mount it. As a teen, I ran tires that were not wide enough for the rim. Had to keep the pressure higher than optimum to keep the bead seated.

I suggest a 15x8 with 5 inch back space. If it doesn't clear you ubj, use a standard length stem ubj and design around that.

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 Post subject: Re: Tire stretching ?
PostPosted: December 11, 2018, 2:49 pm 
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Location: BC, Canada. eh?
Personally, I'm with Miatav8 on this one. The look you're looking for would be pretty darned good with a 15X8" wheel and, say, 225-50X15 tires or something similar. That's what I have on my Locost, and I think they look great. Interestingly, by the way, they're 22.5" tall...very close to the height you mentioned. Additionally, there are some seriously good tires still available in this size. As you say, 13" tires are becoming almost unobtainable on this continent. Some still available in the UK, of course, but shipping costs make importing fairly unrealistic.

There are pics of mine on my build log....about halfway down the page in this link -

viewtopic.php?f=35&t=17662&start=180

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 Post subject: Re: Tire stretching ?
PostPosted: December 11, 2018, 4:44 pm 
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Think I could use a 245/40r15 on a 10" x 15" rim. Only a 1/4" stretch on each side and the tire can be mounted on a 8 to 9 1/2" rim so not far off.

Been playing with the chassis pickup points and its funny how a chassis needs to be built from the tire and rim up .

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 Post subject: Re: Tire stretching ?
PostPosted: December 11, 2018, 9:16 pm 
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Location: BC, Canada. eh?
Well, that might work. Tires in that size sure ain’t cheap, though! :shock:

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 Post subject: Re: Tire stretching ?
PostPosted: December 12, 2018, 12:53 am 
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With the time I have left I will only need one set ? LOL

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 Post subject: Re: Tire stretching ?
PostPosted: December 12, 2018, 1:57 pm 
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I wouldn't do this. Above you're deciding to build your whole project outwards from here - and you're basing it on a hope that is clearly violating a simple spec. There are all kinds of reasons not to do this.

Here is the thing. Take a tiny step back. Consider... You must make the decision to *instinctively* make choices that you can rely on. Taking a path where people's experience with a Miata are used to validate your choice on a replica of the quickest endurance racer for a decade, you can imagine a problem - so take it off the table.

A quick look at Summit for 15" wheels shows 7441 choices for 10" and 2700 for 9" but only 1 9.5" wheel. The folks who make those tires would like to tell you to put them on 10" rims. I think saying 9.5" is a clue bat from them. I wouldn't even run the max width rim, I've never heard that to be the fastest setup. Maybe it would be if there was some other problem, it's always hard to tell when you look at someone proving something with an individual case.

Look for things like the lowest load rating tire you can find. There is no reason to think that a tire built for hard race use at a high load will behave well or predictably on a light car. People learned in the 70's - 80's that NASCAR oval slicks could be a bad idea on a light rod racer. I just remember the gist of it was "be 'fraid!".

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 Post subject: Re: Tire stretching ?
PostPosted: December 12, 2018, 2:28 pm 
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Thanks for you input.I can pull back to 8" x 15" . But may get one 10" x 15" to test ?

I will just need to make the upright fit the deep offset.

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 Post subject: Re: Tire stretching ?
PostPosted: December 12, 2018, 9:10 pm 
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245/40/15 on a 10” will work perfect. It's pretty much been proven in the turbo Miata world that it's fastest to run a wheel the same width as the tire section, ie. 225 on a 9" or 245 on a 10". I'm running 205 on an 8" and 225 on a 9".


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 Post subject: Re: Tire stretching ?
PostPosted: December 12, 2018, 11:15 pm 
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This is a drawing I made of a 8" and 10" rim with a 245/40r15's.
I think the 8" is too small . And to me the 19" looks like it can work ?


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