Larger tires requires larger brakes?

[Captain Skip]

If you believe what you read on the internet, if you put larger tires on your Jeep your brakes will fail, you’ll burst into flames, the climate will warm, it will rain for 40 days and 40 nights and Armageddon will occur. So, what’s the truth? How much larger over stock do you need to go before you start negatively affecting your braking abilities? I’m looking at upgrading from stock 245/75/17s to Toyo OC 285/70/17s and I don’t think this is an issue. But, that brings up the question regarding Willys and Rubicon packages. They come with larger tires, do they come with larger brakes or are they the same ones as the Sport and Sport S have? Inquiring minds would like to know.

Sponsored

 

[Headbarcode]

The Mopar big brake kit comes standard on the Rubicons and Saharas. Maybe also on some special editions, but I don't pay much attention to them.

I upgraded my pads and rotors to the PowerStop Z36 package. It made a noticeable difference stopping my Rubi on over 4" of lift and 38x13.5's.

 

[blnewt]

If you believe what you read on the internet, if you put larger tires on your Jeep your brakes will fail, you’ll burst into flames, the climate will warm, it will rain for 40 days and 40 nights and Armageddon will occur. So, what’s the truth? How much larger over stock do you need to go before you start negatively affecting your braking abilities? I’m looking at upgrading from stock 245/75/17s to Toyo OC 285/70/17s and I don’t think this is an issue. But, that brings up the question regarding Willys and Rubicon packages. They come with larger tires, do they come with larger brakes or are they the same ones as the Sport and Sport S have? Inquiring minds would like to know.

Sahara & Rubicon have the larger brakes, not really larger but the rotors are thicker (for less fade, better heat-dissipation). Not sure for the Willys pkg, when going larger in tires & wheels I would go w/ the Powerstop Z36 pads that @Headbarcode mentioned. You won't need the rotors unless yours are deeply scored, warped, or worn down to the minimum thickness spec. rockauto has nice pricing on those pads (be sure to confirm part#s as it's been awhile since I bookmarked these) Also be sure to bed-in the pads, will make them perform much better from the start.
https://www.rockauto.com/en/parts/power+stop,Z366005,brake+pad,1684
https://www.rockauto.com/en/parts/power+stop,Z361589,brake+pad,1684

Skip, FWIW my braking noticeably declined going from the stock 245/75 to a 295/70, the new tire wheel set probably weighs about 35 lbs more per

 

[Shots]

Well the size change you're talking about is 4% larger diameter and 8% wider so they will add some mass. Rotational resistance isn't a 1:1 thing so the actual dimensions and weight of what you get will determine how the Jeep is effected.
The change you're looking at (+1" dia, +1.5" width) isn't that extreme and shouldn't give you any issues. Where something like that really rears its head is with wear and fade. You may end up replacing the pads/rotors more frequently (not by a lot either), and if you drive through the mountains you may notice brake fade more quickly.

So how does tire size effect braking in an emergency? Minimally. Here's where my post gets long, to explain.
WARNING SCIENCE AHEAD:
For daily driving, you may notice a little more pressure is required to stop in the same distance. Don't worry, I do mean a little. Likewise using the same amount of pressure you're used to may require an extra foot or two to come to a stop depending on the initial speed. But that's daily driving, what about an emergency?
When you're skidding, it doesn't matter if you're diving a Corvette or a Neon. The roadway only offers a certain amount of grip to a skidding tire. This is the coefficient of friction (or Mu). Mu can be affected by grade and braking efficiency. By braking efficiency I mean something like, your rear brakes don't work, so only 2 of the 4 tires can apply braking. If all 4 tires are locked, you have 100% braking efficiency. If you're skidding, that's the maximum brake pressure that vehicle can apply. Pushing the brake pedal harder and/or tire size changes nothing after that point. The tires are already stopped and sliding, they can do no more.
Therefore, if all 4 of your 31" tires are locked, and all 4 of your 32" tires are locked, you have the same braking efficiency (100%). Since Mu is determined by the surface on which you're sliding (asphalt, concrete, grass, etc) the size of the tire sliding on it doesn't matter. Therefore a 31" tire will skid the same distance as a 32" tire on the same surface. Ergo, when skidding the tire will have no impact.

"But wait" you say, "we've all seen high end cars showing off their impressive stopping distances." And you'd be correct. That's because ABS will allow a tire to rotate rather than skid. There is a point at which a tire can slow the car more effectively than skidding. This is called threshold braking. When doing this the tire is at the threshold of skidding, and can achieve a higher Mu value.
That's why I kept emphasizing "skidding." In an emergency skid the size will have no effect. Prior to skidding there will be a slight difference, depending on the tire size/weight and your brakes. Likewise, if ABS is allowing the tires to rotate while "skidding" you'll need a little more distance to stop and say will have an effect.

TLDR:
For a 1" tire difference, you won't notice a difference. You may wear out brake pads/rotors a bit quicker than before, but not excessively. You may need to push the brake a bit harder to stop, depending on your speed. Skidding to s a stop will be so minutely affected that you won't notice.

 

[Captain Skip]

Well the size change you're talking about is 4% larger diameter and 8% wider so they will add some mass. Rotational resistance isn't a 1:1 thing so the actual dimensions and weight of what you get will determine how the Jeep is effected.
The change you're looking at (+1" dia, +1.5" width) isn't that extreme and shouldn't give you any issues. Where something like that really rears its head is with wear and fade. You may end up replacing the pads/rotors more frequently (not by a lot either), and if you drive through the mountains you may notice brake fade more quickly.

So how does tire size effect braking in an emergency? Minimally. Here's where my post gets long, to explain.
WARNING SCIENCE AHEAD:
For daily driving, you may notice a little more pressure is required to stop in the same distance. Don't worry, I do mean a little. Likewise using the same amount of pressure you're used to may require an extra foot or two to come to a stop depending on the initial speed. But that's daily driving, what about an emergency?
When you're skidding, it doesn't matter if you're diving a Corvette or a Neon. The roadway only offers a certain amount of grip to a skidding tire. This is the coefficient of friction (or Mu). Mu can be affected by grade and braking efficiency. By braking efficiency I mean something like, your rear brakes don't work, so only 2 of the 4 tires can apply braking. If all 4 tires are locked, you have 100% braking efficiency. If you're skidding, that's the maximum brake pressure that vehicle can apply. Pushing the brake pedal harder and/or tire size changes nothing after that point. The tires are already stopped and sliding, they can do no more.
Therefore, if all 4 of your 31" tires are locked, and all 4 of your 32" tires are locked, you have the same braking efficiency (100%). Since Mu is determined by the surface on which you're sliding (asphalt, concrete, grass, etc) the size of the tire sliding on it doesn't matter. Therefore a 31" tire will skid the same distance as a 32" tire on the same surface. Ergo, when skidding the tire will have no impact.

"But wait" you say, "we've all seen high end cars showing off their impressive stopping distances." And you'd be correct. That's because ABS will allow a tire to rotate rather than skid. There is a point at which a tire can slow the car more effectively than skidding. This is called threshold braking. When doing this the tire is at the threshold of skidding, and can achieve a higher Mu value.
That's why I kept emphasizing "skidding." In an emergency skid the size will have no effect. Prior to skidding there will be a slight difference, depending on the tire size/weight and your brakes. Likewise, if ABS is allowing the tires to rotate while "skidding" you'll need a little more distance to stop and say will have an effect.

TLDR:
For a 1" tire difference, you won't notice a difference. You may wear out brake pads/rotors a bit quicker than before, but not excessively. You may need to push the brake a bit harder to stop, depending on your speed. Skidding to s a stop will be so minutely affected that you won't notice.

Believe it or not I completely understand what you’re talking about with skidding tires. I was an accident reconstruction investigator in a previous life and tested the coefficient of friction on a lot of road surfaces with a drag sled. I will add to your examples that the tire compound/hardness has an effect too. Harder tires will slide further than a tire with a softer compound. I had a dial indicator that I used to measure tire hardness at an accident scene so I would know which tread pad to use on the drag sled. One of the reasons I would not put a lift kit on my Jeep is because of what I learned in training. One of the seminars I went to had a program run by the VA State Police. This was back in the early 1980's when Bigfoot was very popular. Seems a lot of the good ol boys in Virginia were jacking up their F250's as high as they could...and rolling them over at 35mph on the I95 exit ramps. We did a bunch of hands on experimentation calculating the shift in center mass when you modify the suspension. It can be scary how easily a vehicle has the potential to roll when the suspension is modified - a lot -. Anyway, I appreciate your input and understand what you're saying. I only put 4 to 6 thousand miles on a year so my brakes should last me. I don't plan on going any bigger for tires as the Commander in Chief is vertically challenged - don't you dare call her short - and she wouldn't be able to get in the JL without a stepladder!

 

[Captain Skip]

Sahara & Rubicon have the larger brakes, not really larger but the rotors are thicker (for less fade, better heat-dissipation). Not sure for the Willys pkg, when going larger in tires & wheels I would go w/ the Powerstop Z36 pads that @Headbarcode mentioned. You won't need the rotors unless yours are deeply scored, warped, or worn down to the minimum thickness spec. rockauto has nice pricing on those pads (be sure to confirm part#s as it's been awhile since I bookmarked these) Also be sure to bed-in the pads, will make them perform much better from the start.
https://www.rockauto.com/en/parts/power+stop,Z366005,brake+pad,1684
https://www.rockauto.com/en/parts/power+stop,Z361589,brake+pad,1684

Skip, FWIW my braking noticeably declined going from the stock 245/75 to a 295/70, the new tire wheel set probably weighs about 35 lbs more per

Thanks Brad. Yup the 295's are a bigger jump from the 245's, like almost 6% in height/circumference and 20% in width so I would imagine that those would have an effect. I don't know how much the stock JL rims weigh but the wheels I'm looking at, either the Method 316 or Fuel D665 Shok are around 25lbs each. The stock GY Adventure w/Kevlar is listed at 36lbs while the Toyo OC AT/3 LT 285/70/17 LRC are listed at 55lbs each. So I'm looking at 19lbs more on each corner with the tire weight, I think the wheels are pretty close. The P285/70/17 OC AT/3 are listed at 46lb each, but I really want a tire that's got some more sidewall and tread depth. Funny thing is the mileage warranty on the P tire is 65k miles while the LT is 50k miles, but the LT has thicker tread, go figure.

If I go through with the 285/70's I'll keep a close watch on my brakes. If they start wearing quicker I'll definitely look into the pads you guys recommend. You've already been there, done that and got the t-shirt.

 

[Yellow Cake Kid]

For any given friction coefficient accorded to the pavement and the tire material, when the contact patch increases the friction between the two surfaces increases. This will tend to increase the potential for stopping, but also increase the requirements of the mechanism that is trying to overcome the inertia.

When the moment arm, or radius of the tire increases, the torque force which your brake calipers must overcome increases.

These factors are a great influence, probably more so than an 100lb +/- increase in wheels and tires weight.

 

[Shots]

....... It can be scary how easily a vehicle has the potential to roll when the suspension is modified - a lot -. Anyway, I appreciate your input and understand what you're saying. I only put 4 to 6 thousand miles on a year so my brakes should last me. I don't plan on going any bigger for tires as the Commander in Chief is vertically challenged - don't you dare call her short - and she wouldn't be able to get in the JL without a stepladder!

Like you, I put very few miles on my Jeep every year. As for tires, I have 34's on mine (IIRC actual measure is 33.8"), and I don't feel any difference in brake performance. It's also worth noting that I've got a Sahara with the tow package so I've got the HD brakes. Although I'm sure any Jeep can handle a 1" - 2" increase in tire size. These aren't designed to be at the brink of brake failure. Although to be clear, I don't recommend modding to that point. Anyone adding a lot of weight, and/or much larger tires should upgrade the brakes too. It would be foolish to drive around on the limit of what your brakes can handle.

PS, smart move keeping the missus happy. If she doesn't like getting in/out of the Jeep you'll end up driving whatever she has and unless that's a Power Wagon it won't be as enjoyable as your Jeep.

... Funny thing is the mileage warranty on the P tire is 65k miles while the LT is 50k miles, but the LT has thicker tread, go figure....

It's probably a softer compound that will wear quicker, resulting in better grip, but less tread life.

....When the moment arm, or radius of the tire increases, the torque force which your brake calipers must overcome increases.

These factors are a great influence, probably more so than an 100lb +/- increase in wheels and tires weight.

Good point. Yes the increased radius, and subsequent torque is an important factor. Although I wouldn't negate that extra 100 lbs. Not only do you increase the moment arm, but you've also increased rotational resistance by adding weight.
Both the diameter and weight effect acceleration and braking. We have greatly simplified things, but it's worth sharing the basics of what is happening when you make changes to the design. Much like the increased COM with a lift.

Sponsored