Tire circumference with the illusion of speed and handling

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timberstone
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Tire circumference with the illusion of speed and handling

Post by timberstone » Sat Nov 30, 2019 4:55 pm

The actual diameter of original unworn Dunlop 19" tires for the MGTC is estimated around 27.5 inches.
In the past, for economic reasons, we have used 19" tires that were meant originally for the Ford Model A. These tires fit well but had an actual diameter estimated around 32 inches, when not worn.
The difference in circumference between the 2 tires (from 602 inches to 804 inches) is a total of 202".
The total circumference difference is 33%. This means that the Dunlop must spin around 4 times for each 3 times that the Model A tire spins.
This explains to me why the engine must revolve at higher RPM in order to keep up with highway speeds and why shifting more frequently is necessary. It also explains the good highway speeds we attained in a younger, more frugal, time when all we could afford were the Ford versions.
With the original Dunlops back on the wheels now, we have the illusion of going faster because of the sound of the engine revolving at higher RPM, but when looking at the speedometer, the MPH just reads lower than expected.
Such a disappointment.
Question, would there be any real saving by not having to pay for a lower geared transmission (such as the T9) or differential? Either one of those modifications would cost several thousand dollars, but the difference in price for each of the 4 tires would only be about $80 for a total of $320 additional shelled out.
How would the handling be affected?
How much sacrifice in performance is there between top end speed and acceleration?
Octagonally yours,
Boxley (Robert and MGTC0820)

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Steve Simmons
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Re: Tire circumference with the illusion of speed and handling

Post by Steve Simmons » Sat Nov 30, 2019 5:40 pm

I regularly tour with a friend who has fitted 5" tires. I run the original 4.5" Dunlops. The difference in our RPM at 60 MPH about 3-400 as I recall. When I'm at 4K he's at about 3.6K if memory serves.

You could change your rear end gearing for a lot less than a new gearbox. Both 4.875 and 4.625 are available and fit with no modification. I think they range from about $600 to $750 for the gears. A 4.3 Spridget pumpkin can also be fit with minor modification to the new pumpkin. I run 4.875 and am about to swap it for a 4.625. The only downside is losing climbing and passing power in 4th gear. But on flat ground it makes about a 250-300 RPM difference for each jump in gearing.

I think the only real downside of bigger tires other than appearance is that the rear wheel can become difficult to get on and off because of a lack of clearance. I can't imagine handling would change all that much. You get more rubber on the road but also more sidewall flex. The ride does seem to get a bit softer on bigger tires, so that's a plus I suppose.
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1949 TC9849 EXU / XPAG 10507
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Duncan M
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Re: Tire circumference with the illusion of speed and handling

Post by Duncan M » Sat Nov 30, 2019 8:18 pm

There are online calculators where you plug in tire diameter, rpm, mph and rear end ratio. Leave one field blank and it will calculate it for you. http://www.advanced-ev.com/Calculators/TireSize/

4.75/5.00 x 19 tires these days commonly sold for Model A's run about 30 inches diameter. There is some minor variance between makers. Your 27.5" number for the original 4.50 Dunlops sounds correct. Using 4,000 RPM as a "redline" if you have 30" dia. you can hit 69.7 mph. With 27.5"dia. you can hit 64 mph. Both examples are using the stock 5.125:1 gearing. Another way of looking at is if you are running 27.5" tires, you will need to run a 4.7:1 rear end to duplicate the effects of running 30" tires with stock 5.125:1 gearing.

Steve is correct about possible problems installing the larger dia tires, which are also wider and have a wider road contact patch. The problem is you may need to deflate the tire some to get it on and off the axle. Once installed and inflated, there is not any sort of fitment problem. The other problem is fitting the spare tire in place. A 30" dia tire will simply not fit back there, unless you "shim up" the spare tire mount.

SteveW
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Re: Tire circumference with the illusion of speed and handling

Post by SteveW » Sun Dec 01, 2019 9:54 am

"The difference in circumference between the 2 tires (from 602 inches to 804 inches) is a total of 202".
The total circumference difference is 33%. This means that the Dunlop must spin around 4 times for each 3 times that the Model A tire spins."

I think that there is something wrong with your calculation. The circumference of a 27.5" tyre is 86.4" and the diameter of a 32" tyre 100.5", which is a 16.3% difference.

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Re: Tire circumference with the illusion of speed and handling

Post by Steve Simmons » Sun Dec 01, 2019 11:27 am

You also have to account for "squish". The most accurate way to measure is to mark the tire and roll the car exactly one revolution, then measure the distance it went. This is how we used to calibrate cycling computers to be accurate to 1/10 MPH or better. Simply entering the tire circumference without any weight on it would result in a significant error.
1949 TC8975 / XPAG 9609
1949 TC9849 EXU / XPAG 10507
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timberstone
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Re: Tire circumference with the illusion of speed and handling

Post by timberstone » Mon Dec 02, 2019 7:15 pm

Yes, agreed that the wrong formula was used to calculate tire circumference. Therefore the differences are not as great, but are really as observed in Steve W's comment.
The order has been placed for tires with diameter of 31.40 inches, and using the proper formula, will equal circumference of 98.65 inches.
The improvement over the original Dunlops, with diameter of 27 inches (with a circumference of 84.8 inches), will be the 16.5% as observed by Steve W.
Thinking that the improvement is worth a try and since we needed 2 tires for the 2 wheels (which we spoked with stainless steel) anyhow, and the cost is only about $200 difference -- then this exercise costs far less than a differential or gearbox change.
Perhaps we will see an improvement --
Will report any performance observations when these are mounted.

Octagonally yours,
Boxley (Robert and MGTC0820

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Duncan M
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Re: Tire circumference with the illusion of speed and handling

Post by Duncan M » Mon Dec 02, 2019 10:32 pm

What size and brand tire is 31.4 inches? Such a tire would give you the equivalent of a 4.5:1 rear end. Interesting to note that circumference is not a stat tire makers ever mention.

timberstone
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Re: Tire circumference with the illusion of speed and handling

Post by timberstone » Tue Dec 03, 2019 5:36 pm

Yes, it is hard to get information about tire circumferences. The measurement usually must be derived mathematically from looking at the "overall" tire diameter -- but even that information is not always available, or perhaps totally dependable.
And then, like dummy me, the wrong formula can be applied to the calculation for arriving at circumference based on diameter. The correct formula is Pi (3.1417) x Diameter = Circumference. (I was using the formula to calculate area -- please forgive me, Euclid)
The tires are "BF Goodrich -- 600/650 by 19" with listed diameter of 31.4 inches and "recommended rim width" of 3.00-3.50.
Could have gotten 19" tires that were 32 inches diameter but the "recommended rim width" went up to 4.00-4.50. (Maybe these would have worked, but would not like to trust it.)
Other, cheaper, tires with the same "recommended rim width" were not as large diameter. Some were close, though.

The equaling of the tire circumference specifications to differential ratio was what just recently dawned upon me, and is the basis for this whole exercise. If we wanted to carry this to the logical end, suppose that the high end speed could really be boosted by adding the lower rear end ratio gears, 5 speed transmission, etc. to the larger tire circumference.
Maybe we could then cruise comfortably at 75 mph on the Interstates. ...
(Though probably would have to take off the front fenders in order not to become airborne.)

I learned about the importance of tire circumference a few years back while talking shop with a farm tire supplier who was very knowledgeable about racing cars. He highlighted the fact that almost everything about street performance boiled down to tire circumferences and that both wheels on the same axle should match circumference as nearly as possible -- not just rim diameter.

Also, as mentioned above, the "squish" factor may affect the actual circumference of the rubber that meets the road. "Squish" can be variable depending on temperature and pressure. So, maybe the vendors shy away from listing circumferences in the tire specifications because of the possibility of too many variables.

Octagonally yours,
Boxley (Robert and MGTC0820)

Also, the concern about getting the wheels off and then on the car again, with such large diameter tires, is very real. MGTC0820 is one of those that were assembled with one rear fender slightly inboard in comparison to the opposite rear fender. (It was real trouble to get the Model A Ford tires on and off due to that problem.) During the latest restoration, that dimension was not altered. But now with larger tires, the assembly dimensions of the rear fender openings will have to be revisited.

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Duncan M
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Re: Tire circumference with the illusion of speed and handling

Post by Duncan M » Tue Dec 03, 2019 8:48 pm

Those Goodriches are meant to be installed on a 3.0-3.5 inch rim. Standard TC rim is 2.5 inch. Though the actual outside width of an original TC wheel is 3.7 inch, that is not how it is measured. Doubtful anybody would suggest it OK to put such fat tires on 2.5" rims. Or have you already got 3" rims from Bob G? Should be a real challenge getting those Goodriches on the rear axle. https://www.cokertire.com/600-650-19-bf ... gKKUvD_BwE

The largest diameter tire you will find that is designed to fit properly on a 2.5" rim is something like this 30" 475/500 x 19 - https://www.cokertire.com/475-500-19-un ... -tire.html

timberstone
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Re: Tire circumference with the illusion of speed and handling

Post by timberstone » Thu Dec 05, 2019 7:15 pm

Thank you Duncan M. for warning me about the tires not being capable of fitting into the wheel wells when mounted on the wheels. No, we do not have the wide rim wheels from Bob Gruneau.

I do remember having to wrestle with getting the wheels mounted when the Model A Ford tires were in use. I did not then consider rim width as any factor to be considered -- just rim diameter (and cheap price) -- so I do not know what rim width those tires were.

But I am not sure that even those were as large as the tires which I just received from Coker!

Why would not these tires work (which do have recommended rim widths of 2.50 - 3.00 and overall diameter of 32 inches): https://www.cokertire.com/550-600-19-fi ... kwall.html ??
After all, this is an experiment in process -- with the advantage of group enlightenment from this forum.
Octagonally yours,
Boxley (Robert and MGTC0820)

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Steve Simmons
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Re: Tire circumference with the illusion of speed and handling

Post by Steve Simmons » Thu Dec 05, 2019 7:39 pm

It's a pretty big tire for a 2.5" rim. Model A's don't usually go around corners very fast, but TCs do, and you may find that such a large tire will introduce a lot of sidewall flex. This can make the car less stable in the corners as well as being more likely to wander at highway speeds. Higher inflation would likely help reduce the effect.
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Duncan M
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Re: Tire circumference with the illusion of speed and handling

Post by Duncan M » Fri Dec 06, 2019 1:23 am

Robert, if you recall the problem getting the standard Model A tires (475/500 x 19)on the rear axle, you will remember the difficulty was the getting thickest part of the tire past the gap between hub and lowest part of rear fender. Deflating the tire will sometimes do the trick. The firestones you found do indeed say they were designed to fit on a 2.5" rim. Problem is the tire is 3 inches wider than the 475/500 x 19. Steve mentioned the flex problems of such a tall tire. A 32" dia tire on the rear axle will give the equalivelant of a 4.4:1 rear end. Acceleration will be sluggish, but you will be able to hit 74.3 mph in top gear at 4000 RPM eventually. Thing about sluggish acceleration is most drivers compensate by revving the engine higher through the gears. Back in the day when lots of folks were using Model A tires on the TC's they seem to have found the best comprimise size was the equalvalent of 475/500-19. The other alternative Model A size is the new radials. Coker makes one radial that will fit on a 2.5" rim, and it is also said to be 30" diameter. https://www.cokertire.com/american-clas ... 00r19.html At $206 it still equal to about a 4.7:1 rear end, but less than many of the more pricey Model A tires. Handling and driveability of this radial is said to be quite noticably better in a Model A, but so far I have seen nothing from TC owners trying the radial.

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Re: Tire circumference with the illusion of speed and handling

Post by Steve Simmons » Fri Dec 06, 2019 9:11 am

It's also worth noting that a large tire will introduce a lot of rotating mass as well as unsprung weight. From a performance standpoint, it's better to stay with a lighter tire and change the rear end gearing.
1949 TC8975 / XPAG 9609
1949 TC9849 EXU / XPAG 10507
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