I have a '97 4wd shortbed, V6 F150 with stock 235/75/16's on it, and I'd like to go to the largest tire I can without modifying the truck. I wanted to put 265/70/16's on it, but I was told that would "screw up" the antilock brakes on the rear wheels. Is this true? Will this tire size fit? Thanks...

 

kinger26

Yes, it's true. But I am still trying to learn that part. I supposed that, since tire diameter is affected some parameters will change and that creates variation for the computer to analize, being different than the stock settings may send the wrong information.

I am in the process of changing tires. We'll see how it goes.

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1998 F150 4x4 Lariat Supercab Flareside 5.4L in Oxford White/gold, ORP, Tow Package, 3.55 Gears.

Modifications: K&N FIPK, Stepshield door sill protectors, Performance Accesories 3" body lift, Lift Lips and Daystar front anti-sway bar polyurethane bushings.

See photos at http://www.communicomgroup.com/jmtruck

 

From a practical standpoint the ABS should not know any difference.

The larger rolling radius (taller tire) means a different leverage on the brakes. So does adding weight (load) to the truck. The computer doesn't care that your truck is heavy any more than it cares that you're going 5% faster.

During a very transient (mostly theoretical) point in the braking 'adjustment' by the computer, it [computer] looks at how quickly the wheel rolls up following that [theoretical] wheel lock -- for the purpose of determining the friction coefficient of the street surface. The tire's actual response against the computer's pre-programmed 'map' of what to expect could be shifted by the 5 or 10% that you've changed the speed of the tire by the increased rolling radius.

This is largely a theoretical issue, however, since in practice tires rarely make a good first 'lockup' -- because another part of the computer's brain had already 'forseen' that impending lockup -- and had made the proper brake release.

From a practical standpoint, the wheels are less likely to lockup under any scenario with an increased rolling radius -- simply due to the lessened braking effect which is the natural result of an increased rolling radius (longer arc) -- so you're less likely to experience a wheel lockup -- and even if you did, the computer adapts to relative changes in wheel rotational speed -- not absolute changes in wheel rotational speed.

Fog-factor high enough for you now?

Don't sweat it -- there are so many calculations and double-checks and reality checks that go on during a brake application that you've really got to try to fool it (like going from ice to hot rough cement to wet ice to rough cement -- and split the wheels so that they're doing things different on the left than on the right, etc.) in order for the computer to approach 'tilt'.

Lastly: You will experience a greater tendency for high-speed brake fade with taller tires -- due to the leverage created by the increased rolling radius (fixable with higher-cooeficient lining materials and/or greater clamping pressures).

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Y2K™ Jim

Toreador Red, Keyless XLT SC SB 5.4L E4x4 4wDisc/ABS, 3.73LS, Skid, HD 7700# Towing, LT-245's on Chrome, Tube-Steps, Captain's, 6CD, Tonneau, named: "Nick"

 

kinger26

Just want to be short and concise about this. If you go to the third paragraph of Y2K 7700 4X4 the conclusion is it may affect the ABS operation by 5-10% like he said. For detail just go back to his reply.

And since the system is looking for wheel speed it will be looking for wheel deceleration. If any of the wheels are slowing more rapidly than the others, the computer can tell it's about to lock up and then and only then it will send a signal to the hydraulic valve block for that wheel.

But it's not looking for the coefficient of friction. Like Y2K 7700 4X4 said the computer does not care that your truck is heavier or not, and it does not care on what tires are you using. So, knowing that every brand of tires uses different compounds to manufacture their tires, that would be an added variable for the computer to analize friction of that one tire against the road.

Our systems will still work with our modifications but not as when stock with all the OEM configuration.

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1998 F150 4x4 Lariat Supercab Flareside 5.4L in Oxford White/gold, ORP, Tow Package, 3.55 Gears.

Modifications: K&N FIPK, Stepshield door sill protectors, Performance Accesories 3" body lift, Lift Lips and Daystar front anti-sway bar polyurethane bushings.

See photos at http://www.communicomgroup.com/jmtruck

 

The tire size DOES affect the ABS. I have 35s on my truck, and the antilock hardly works at all anymore. When I slam on the brakes they always lock now. I think one or 2 sizes bigger than stock shouldn't make much difference, but the bigger you go, the more they will be affected.

 

How many millimeters difference in HEIGHT?

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2000 F-150 XL,RC,LB,5.4,4R70W,3.55LS,
Class III tow/Payload #3/Convenience pkgs.,
4-wheel disc/ABS,Chestnut/Parchment 40/60,
Ford bedliner & gas/wheel/spare locks,
3" cold air box modification, Superchip,
Dynomax ultra-flow welded 3" cat-back,
3" exit Y-pipe, Hellwig rear anti-sway bar,
Rancho RS9000s.

 

the difference in hight on mine was going from a 29" tire to a 35" so its 6" difference, so you have to divide it by 2 and the total difference in height of the tire was 3"

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99 black f150 4x2 ext. cab sport w/4.6 V8. external mods. include:
3" body lift, 3.5" fabtech lift, airaid FIPK, JBA headers, Superchip headlight/taillight covers, wiper schroud, shadeblade, ventshade, bug deflector, bed liner, tonno cover, and custom duel exhaust out the sides with 3.5" chrome tips and NO muffler. Tires: Mickey Thompson Baja HP 35 12.50 17 internal mods. include:
Pioneer indash cdplayer, 2 12" subs, 400 watt amp, remote start, and aftermarket alarm
http://www.4tssinc.com/truck
http://www.mifordtrucks.com

 

Chris- yes, that is a large difference. Thanks for the insight on the changes. Sorry for the confusion, but I was asking Kinger26 because his difference seems small. The 235 to 265 is width, and since he wanted to go from 70s from 75s, the overall height difference there seems small to me.

 

Kinger26,
I bought my 97 4x4 new and paid for the ABS option front/rear. Two years ago, I changed the original 255/16s for 265/75/16s (Goodrich TAs). I haven't locked my brakes yet. Maybe if I stump them a little harder...