Hobo overdrive - mismatched tire sizes question

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Jack Tarlinton
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Hobo overdrive - mismatched tire sizes question

Post by Jack Tarlinton » Fri Oct 22, 2021 12:29 am

Ok, stupid question time yet again from me.

The fellow I bought my pile of parts from suggested that I should use mismatched tyre sizes for a little overdrive:

450/21s on the front

525/21s on the back

Yes, I can see I'll have to carry one of each as spares for touring.

Has anyone trued such a combination?

I am predicting an increased load on the engine, which I am beefing up with stronger crank and higher comp head, .280 cam etc.

Is there anything else you guys can comment on? Please feel free to chime in - your advice is always appreciated.

Best regards and thank you in advance.

Jack

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ewdysar
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Re: Hobo overdrive - mismatched tire sizes question

Post by ewdysar » Fri Oct 22, 2021 1:44 am

First, 525/21 tires calculate out to 1.5 inches taller than 450/21 tires (31.5” versus 30”), but in practice, they tend to be just about an inch taller (31.5” versus 30.5”). That extra inch is only 3% taller, so instead of 30mph, the new tires will run at 31mph for the same RPM. If you’re already in the market for new tires (and you can find them in inventory), then consider switching to the new size all around.

Coincidentally, my ‘27 Runabout is wearing 525/21 tires all around, the extra tread width is more important to me than the tire height. The tires don’t look oversize, even parked next to a car with 450/21s, the height difference is not very noticeable.

For a somewhat noticeable “hobo overdrive” effect, you could go up to 600/21 tires, about 3” taller than 450/21. 30mph becomes 33mph, etc. But remember that making the overall drive taller will reduce acceleration, hill climbing, and braking ability. In this case not by much, but not the direction that most of us want to go. I doubt that I would go this route. It’s probably cheaper to get a 12 tooth pinion for basically the same overdrive, less money but more labor to make the change.

Keep crankin’
Eric


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Re: Hobo overdrive - mismatched tire sizes question

Post by Jack Tarlinton » Fri Oct 22, 2021 3:21 am

Hi Eric,

I already have 2 brand new 450/21s on the front, as I was able to find them cheaply.

The rears have ancient (probably original) 500/21s but I not going to deem them safe to drive, only useful for mock up purposes.

Do you think having using the taller, wider 525/21s on the back would improve braking etc with the wider tread?

I would think having narrower tires on the front would be better on the front as there is less load on steering, particularly when parking.


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Re: Hobo overdrive - mismatched tire sizes question

Post by TXGOAT2 » Fri Oct 22, 2021 10:33 am

I'd put 5.25s all around. To ease steering, always have the car moving when turning the steering wheel. 5.25s should give a little more traction for braking on most surfaces, but will make the brake work a little harder to stop the car due to the "gearing up" effect of the larger tire diameter. The "overdrive effect" of around 3% would be at its maximum at highway speeds, and less pronounced at lower speeds due to centrifugal force and higher tire running pressure at higher speeds. Most cars also generate some aerodynamic lift at higher speeds, and unloading the rear tires would enhance overdrive effect slightly. It might be practical to run around 25 PSI in the 5.25 tires, which would probably enhance ride comfort and offset the overdrive effect to some degree.


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Re: Hobo overdrive - mismatched tire sizes question

Post by John Codman » Fri Oct 22, 2021 10:48 am

I don't know if my '27 is typical, but as a practical matter the effectiveness (or lack of it) of the brakes is determined by one band in the transmission that is soaked with oil. My '27 will not lock the wheels using the transmission brake. Unless you have Rocky Mountain or other aftermarket additional brakes, I doubt that larger tires will do anything to increase stopping power. In fact, since the brake band has a 4:1 mechanical disadvantage (as opposed to wheel brakes), I would suspect that larger, wider, tires would hurt rather then help braking. I am assuming that you are not using the parking brake as a service brake; if you are - I would think that the larger tires might help stopping by a smell amount.
Last edited by John Codman on Sat Oct 23, 2021 1:57 pm, edited 1 time in total.


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Re: Hobo overdrive - mismatched tire sizes question

Post by Dennis Prince » Fri Oct 22, 2021 11:02 am

I have run 525/21s Firestones for years on my 26 roadster (have always called it Hobo Overdrive) it does give you about 20% more tread on the ground for increased braking, the 3% overdrive is noticeable and if you don't have a Ruckstell you will spend more time in low. The Firestones are a little taller than the other brands that I have put them against and they are 6ply rated, some of the places that I go that gives me peace of mind. People that know Model T's notice them right away, after a short time you get use to the difference in how they drive and how the look, they also last a lot longer than other tires, I get 35,000 to 40,000 miles out of a set before replacing and they still have some tread left. I love them.
tunnel exit (2).jpg


Dennis Prince
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Re: Hobo overdrive - mismatched tire sizes question

Post by Dennis Prince » Fri Oct 22, 2021 11:07 am

I forgot to mention that I run 36 psi but I carry a pretty good load, I have also noticed at lower pressure you get a fair amount of sidewall flex that you can feel as you drive.


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Re: Hobo overdrive - mismatched tire sizes question

Post by TXGOAT2 » Fri Oct 22, 2021 11:07 am

I think the axle gearing works to the advantage of the engine in driving the car, and to the advantage of the transmission brake in stopping it. The brake drum speed is 3.6 times the axle speed, but torque at the trans brake is proportionally reduced. For an emergency stop, you can apply the transmission brake and low pedal to lock the wheels. Best braking is developed just shy of locking the wheels. My '27 will slide the rear wheels using only the transmission brake on non-pavement surfaces. I haven't tried it on pavement.


Topic author
Jack Tarlinton
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Re: Hobo overdrive - mismatched tire sizes question

Post by Jack Tarlinton » Sat Oct 23, 2021 7:24 am

Thank you Pat, John, Dennis for your feedback and advice. I really appreciate it.

I think I will go with the mismatch of front and back sizes as I already have new front tires, and eventually upgrade the fronts to match the rears.

best regards

Jack


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Re: Hobo overdrive - mismatched tire sizes question

Post by John Codman » Sat Oct 23, 2021 1:59 pm

I have corrected my previous post in this thread to read that the transmission has a 4:1 mechanical disadvantage as opposed to the #:! that I originally posted.


John Codman
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Re: Hobo overdrive - mismatched tire sizes question

Post by John Codman » Sat Oct 23, 2021 2:03 pm

To Txgoat: You are correct in your post about the advantage - I was looking at the gear ratio from the tire end, not the transmission end. In my case, I have never attempted to lock the brakes on an unpaved road, and considering that he brakes are totally dependent on a 97-year-old driveline, I never will do so intentionally.
Last edited by John Codman on Sat Oct 23, 2021 5:42 pm, edited 1 time in total.


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Re: Hobo overdrive - mismatched tire sizes question

Post by TXGOAT2 » Sat Oct 23, 2021 2:31 pm

Larger diameter tires will require more brake pedal pressure. Braking force is minimal at best due to there being only rear wheel brakes. Backing down a steep grade would allow the car to develop more braking force, at least until the brake burned up or something broke. Steering with reversed caster can get interesting, too.

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