2WD mismatched tire sizes

[IdahoDoug]

Jdirge,

The actual calc would be a loss of braking power in direct proportion to the % increase in tire diameter, so I just tossed those out without knowing actual tire sizes. You'd also lose a bit of brake performance due to the inertia of a larger/heavier tire but that's certainly splitting hairs. But the answer to your thought is that no, the slower movement of the larger tire won't give you back any braking power or reduce stopping distances.

General Comment

In an overall sense, the unchanged braking at the other axle vs the % changed at the axle with the larger tires is a pretty big deal even at small %'s. Yes, losing a percent of total braking power could result in a collision vs a safe stop in a straight line stop. But most collision avoidance braking involves some element of turn/swerve and that's where the imbalance from front axle to rear axle comes in. Things end poorly when a van or other high center of gravity gets sideways or leaves the roadway.

Having two different tire sizes on a vehicle not designed for that is simply deteriorating margins of safety in ways and under conditions that even the most experienced Vanagon driver cannot account for in an emergency. Unless someone here has done instrumented testing on a Vanagon's stopping distance wet/dry/mixed traction and a variety of cornering attitudes, weights and road surfaces with mismatched tires, then I don't think it's right to say "I've had no problems" as though that is a valid input. I could drive around with under rated passenger tires for years and make this same comment, but I'd be absolutely wrong to recommend it to someone posting here on The Samba who is wondering if it's OK.

Specific comment to the tire contact patch

When a vehicle has wider tires mounted, its contact patch remains the same surface area. However, it becomes shorter in the direction of travel and longer laterally. A tire generates braking force from tension across the contact patch in the direction of travel and the longer the tire patch the greater the potential braking force. Shorten this, and you reduce max braking force, all else equal.

DougM
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1987 2WD Wolfsburg Vanagon Weekender "Mango", two fully locked 80 Series LandCruisers. 2017 Subaru Outback boxer. 1990 Audi 90 Quattro 20V with rear locking differential, 1990 burgundy parts Vanagon. 1984 Porsche 944, 1988 Toyota Supra 5 speed targa, 2002 BMW 325iX, 1982 Toyota Sunrader

[Wildthings]

I have adjusted the front/rear bias on many vehicles by changing the size of the rear cylinders. The available increments in wheel cylinder size are typically in 1/16 of an inch. For a one inch cylinder this means that the smallest step you can take is about 13%. A change in diameter from a 185R14 to a 27/8.50R14 is about 1.2" or about 5%, so less than half the minimum affect I can get by changing a cylinder on the vehicles I have worked on. People who go to larger front brakes probably change their front/rear bias much more than this.

When you go to a wider tire you can also probably run a lower tire pressure, which is going to cause the length of the tire batch to be about the same as a narrower tire running more pressure.

[IdahoDoug]

I wonder if it might be more efficient to alter the front/rear balance with one of those simple units designed to do this based on load? They are simple - just an arm to the rear suspension such that a heavier load increases rear braking force at a splitter above the rear axle in the brake line that comes from the front.

Lots of vehicles have them from the factory. Known as a brake proportioning valve. LandCruiser guys who install lifts will bend the arm to restore the original settings.

DougM
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1987 2WD Wolfsburg Vanagon Weekender "Mango", two fully locked 80 Series LandCruisers. 2017 Subaru Outback boxer. 1990 Audi 90 Quattro 20V with rear locking differential, 1990 burgundy parts Vanagon. 1984 Porsche 944, 1988 Toyota Supra 5 speed targa, 2002 BMW 325iX, 1982 Toyota Sunrader

[j_dirge]

[IdahoDoug]

Good to know about the proportioning valve. Is it a manual type that's preset by the factory, or a dynamic type that responds to load. The latter is often referred to as a "load sensing proportioning valve". On older cars the brake pressure proportioning was done in the brake booster/master cylinder and was preset.

On the 16-er, yep if it simply had larger diameter tires fitted then its brakes would not be able to stop as fast under certain conditions. Would be unusual for a German company NOT to augment the brakes here, though...

DougM
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1987 2WD Wolfsburg Vanagon Weekender "Mango", two fully locked 80 Series LandCruisers. 2017 Subaru Outback boxer. 1990 Audi 90 Quattro 20V with rear locking differential, 1990 burgundy parts Vanagon. 1984 Porsche 944, 1988 Toyota Supra 5 speed targa, 2002 BMW 325iX, 1982 Toyota Sunrader

[Crankey]

235/50/17 front 245/45/17 rear ususally. my winter tires are all the same, 235/55/17.

[Beygon]

I have been running Michelin 205-70-15s on the front and 215-75-15s on the back for quite a while, my speedo is exact with the gps and it helped drop my rpms a little at 70. Can't say anything about the saftey issue, haven't had any problems with them and it seems to stop fine.

[VisPacem]

Beygone,what RPM are you at, at ~70mph ?


I can't stop as well in the Peterbilt as I can in the 911. Must be those wiiiide duellies in the back.

So.. I tell my foot and my brain

You foot, and you brain, PAY ATTENTION you are driving the Peterbilt. NOT the 911 !!

Capisce ??

Or is it that for some the vehicle is driving them and not vice versa

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LG aka VisPacemPB, *The* party Pooper

No Regrets (Nothing to do with Vanagons)