Let's talk tire size and braking. - JeepForum.com
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post #1 of 18 Old 11-04-2019, 11:49 PM Thread Starter
Nucking1Futs
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Let's talk tire size and braking.

Everyone talks about upsizing their tires for clearance, looks, or both. Followed by lift kits and axle gearing. From the threads I have seen, they are all about getting the Jeep (or any auto) going down the road/trail with current or ideal gear ratio sets. But that is only part/half the issues.

NO one talks about stopping. Jeeps did not have the most stellar brakes. To compound the effect, installing larger tires exacerbates this issue. When it comes to stopping the Jeep, it can get a bit hairy in traffic with large tires. The smaller brakes and bigger and heavier tire requires more force to be applied to brakes slow the vehicle down, excessive brake force that is enough to lock the wheels from the force applied. A panic stop? Nightmare on spin ally!

My CJ7 has ended up being my DD now, and 33" Hercules Trail Diggers tires are not the ideal commuter tires. This leads me to a logical conclusion, smaller and appropriate tires. I would like to have 31's. With the tires I want to run, 31's are about 7% increase in tire circumference and 13% increase in weight over stock size.

Looking for input from real world experience. For those that jumped from 235/75r15 to 31/10.5r15, how significant was the decrease in braking distance from the new plus size vs the stock size? While it will be loads better than my 33", I am trying to see if 31 is still too much tire verses 30" or even 235's.

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post #2 of 18 Old 11-05-2019, 12:09 AM
Fourtrail
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First off, are you manual or power assist brakes? There are numerous ways to increase your braking before you give up on the 33's and go back to smaller tires.

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post #3 of 18 Old 11-05-2019, 12:13 AM
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Stock was 29" so a 31" is 7% greater circumference.

A well maintained Cj7 braking system will be able to bring 31" tyres to the point of locking quite easily, you can probably improve what you have by maintenance and changing the pad material to improve the feel, a servo is always a nice thing to have. If you are locking and spinning you may want to check you have an original setup and a working proportioning valve, though I suspect it is because you are trying to brake faster than mud terrains allow, something which can be quite unnerving.

I would not hesitate in a CJ7 to have 31" tyres. I doubt there is any difference in braking from a 29" tyre just from the size and weight. If you looked at 33" or 35" or even 37" tyres, I doubt you could control them as easily s they are starting to get heavy and have a significant straight on momentum of their own.

After that it is the tyre tread and compound that will determine the stopping distance. Here is a test in a Hilux on highway, all terrain and mud terrain tyres for various stopping tests from a low 50mph ish. They were all General Grabber 265/75/16 size (31.6 inches nominal diameter). For a DD the mud terrains take so much longer to stop on wet highway that you would not put them on a DD if you drove in rain. Many times I have nearly stuffed a CJ with 31" mud terrains in the rain, you need to hang well back as nearly any car in front will out brake you. I guess you have the same experience with your Trail Diggers, being a mud terrain.

Highway terrain (HT) All-terrain (AT) Mud Terrain (MT)

DRY BRAKING (from 80-0km/h)
28.7m 29.93m 31.21m

WET BRAKING (from 80-0km/h)
29.82m 36.26m 38.66m

My suggestion for a DD is to get some good highway tyres and have a set of mud terrains for off roading. A *** to swap round but if you did a lot of miles they would pay for themselves.

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post #4 of 18 Old 11-05-2019, 12:46 AM Thread Starter
Nucking1Futs
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Quote:
Originally Posted by Fourtrail View Post
First off, are you manual or power assist brakes? There are numerous ways to increase your braking before you give up on the 33's and go back to smaller tires.
Oh I am keeping the m/t tires. I just need a practical tire for commuting. Tires are cheaper than a new car.

I have manual brakes. Adding a booster just eases the input pressure from the operator. It does not up the braking power of the brakes. I have rebuilt the entire brake system, and everything works as it should. I can lock the tires up just fine. That is the problem.
The rotating mass and increase in diameter just makes things a bit sketchy when I get cut off in traffic and have to apply brakes suddenly. The m/t's have just got to come off for a dd. It is just not very practical.

Smaller tires will help in my mpg (currently getting 16 mpg), and with braking.
With the proper tire, an increase in dry/wet weather traction, reduced tire noise, and a more compliant ride. Really don't see it as losing anything. I am gaining.

I am kicking around buying either 30's or 31's. It is not about looks, it is about practicality and cost. As my new tires will see service off road, that is the only reason the 31's are considered.

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post #5 of 18 Old 11-05-2019, 03:03 AM
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You may get 17 mpg combined when you have the right tyres on it, not much more. However the 1984 has a particularly tall gearing and, if you not Nutterred, the maximum economy will only be there if you move to a smaller tyre, otherwise you will always have a small penalty from the larger tyres.

A skinnier tyre will also help with fuel economy. It could have come with 235/70HR15 high way tyres which were 28 inches in diameter, but you could go to narrower such as 215/75R15 and enjoy its original street oriented performance. If you have the T5 trannie, this would pep up the performance and allow a decent cruise speed. I would keep the larger tyres and put them on a separate set of rims for off-roading.

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post #6 of 18 Old 11-05-2019, 06:52 AM
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Quote:
Looking for input from real world experience. For those that jumped from 235/75r15 to 31/10.5r15, how significant was the decrease in braking distance from the new plus size vs the stock size?

Quote:
I have rebuilt the entire brake system, and everything works as it should. I can lock the tires up just fine. That is the problem.
Two things:
If you can lock up the tires "just fine", then perhaps the brakes are too aggressive. You want the fronts to lock up under panic braking only when you are "bending the steering wheel" with effort. The rears should never lock up except in gravel or other low resistance surfaces and under the same conditions. Since there is no mechanical or electronic anti lock devices on the Jeep, you need to mentally modulate your right foot at all times or purchase a manual brake modulator from such places as Wilwood.


Secondly, all that being said, the only difference in braking distance using your current braking system (which you state will lock up your oversized tires just fine) will depend on the tread design you run. Rough, aggressive mud tires will never stop as well as a conventional street pattern,regardless of size (within reason) as the street pattern will put more rubber on the ground.

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post #7 of 18 Old 11-05-2019, 07:32 AM
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I don't disagree with N1F. I noticed my 2000 XJ Limited, back when it was only a few years old, did not stop with near the surety after I installed 31 MTs. So much so, that I took them off again and replaced them with 29s ATs. The acceleration, braking, fuel mileage all improved with the 29s over the 31s. The wheeling was just as good also, I just had to pick a better line.
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post #8 of 18 Old 11-05-2019, 08:08 AM
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If your driving is going to be mainly on Northern California highways you will need AT--all terrain or better AP--all purpose tires. I have all 31" APs and when I bought them the owner of the tire store told me that the white letter side had to be out. The tread is more aggressive but not much on the inside and more like all season on the outside thus not so noisy. Wet road traction is good but without anti lock brakes they will still lock up. we've grown accustomed to anti lock brakes and have forgotten the art of stopping in wet or icy conditions.

I cringe when I read here that folks buy tires for looks. Tires should be for function and not looks. Having said that Jeeps have 235 tires just don't look right. A fellow jeep club member has a CJ7 with a a 2.5" lift and 235 mm tires. It looks like he is driving around on temporary spares.

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post #9 of 18 Old 11-05-2019, 07:08 PM Thread Starter
Nucking1Futs
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All, thanks for the replies.

We can talk all aspects about how different types of tires perform in controlled braking, but I generally know this. We all do. This is why I am changing tires on my Jeep. My brakes are not too aggressive either. But all these topics misses what I wanted to know. I was not clear enough.

I was trying not to delve into certain aspects of tire types as it would side track the post. Relevant, yes, but a subset of my general question.

I had a question about controlled braking with bigger tires over stock size, and I seldom see anyone else consider the effects of plus sizing your stock tires can create. So I thought I would start a post. The topic is more important than how fast you can get from 0 to 60mph, or how steep you can climb a hill. Speed, quickness, or crawl ratio is meaningless if you can't stop.

Does a 31" tire exceed the manufacturer's design on the brakes? I am not sure. I do know that an increase in mass and diameter will change the dynamics of the system. I just don't know by how much in proportion to the desired tire size I am interested in.

That is what I am looking for. Someone who can tell us all how much of an increase in braking distance going form stock tires too plus one or plus two tire size has created. I can calculate all sorts of fancy numbers, but real world knowledge is much nicer (and easier to digest).

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post #10 of 18 Old 11-05-2019, 08:19 PM
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Yes, they can handle 31" tyres. If you can lock them up, you can also bring them to a point near lock where you get maximum braking. A good brake system in a cj should be able to stop it in a straight line at near locking. I am going to guess (as I have no data) that somewhere around 0.70g is possible on modern highway tyres in the dry for an old Cj with a lift. I did find a 1978 test on a Cj5 with H78-15 Suburbanites (bit like A/T) on stock suspension that managed a max of 0.62g at 60mph, so maybe about right.

I read that in the 1980s the tyre industry could reach a value of cohesion of 0.8 and a vehicle could decelerate on roads at 0.6g to 0.8g. However modern tyres can improve this to 0.75g to 0.95g. With ABS 1g is achievable. clearly a CJ is going to be outclassed by a modern car.

The exact effect of changing tyre size is debatable as it depends on so many factors on your rig, including lifts, shocks, alignment, brake efficiency, repeated braking etc. The 1978 Cj5 maximum braking pedal force was about 70 lbs - 90 lbs before the wheels locked. No servo. Stopping distance at 60mph reported at around 180 feet (55m)

The only test I came across on tyre size was for a Ford F-250 which was tested with a range of suspension lifts and tyre sizes on a flat plate brake tester. There is no doubt from my seat of pants feel that larger tyres stop slower than smaller ones for same pedal force.

If too large my brakes may not be able to lock them up and I cannot maximise braking so I will just trundle on further than if my brakes were larger / better condition (which is why bagusjeep came off the road for a while, the brakes did not match the tyres, but I missed the motorcycle in front by a hair). However this was not part of the test.

So how much is it?



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post #11 of 18 Old 11-05-2019, 08:34 PM
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I would guess looking at those charts that a 3% drop in deceleration under hard braking (right hand column) on a lifted vehicle in the dry is possible by increasing the highway tyre size from 31 inch to 33 inch, say 0.69g to 0.67g.

If you start with a stock vehicle and lift it and put on 33" tyres, then you are looking at a higher change from 0.71g to 0.67g. But that is not what you are doing.

If you recall basic algebra (S.I. units of course) .......
v = u + at
v = u + 2as
s = ut + at

50mph = 22 m/sec
0.69g = 9.81 x 0.69= 6.77 m/s/s time to brake = 3.25 seconds distance = 35.7m (this is actual braking. you need to add reaction time to get total distance to come to rest in real life)
0.67g = 9.81 x 0.67 = 6.57m/s/s time to brake = 3.35 seconds diustance = 36.8m

At 60mph those figures are 50m and 51.6m respectively. Better than reported back in the day but that could be realistic on highway tyres.

So it will take longer to stop for same pedal force AT 50MPH, which we know from our seat, and is about 1m. Of course, if you overload the brakes TO THE POINT WHERE YOU CANNOT COMFORTABLY LOCK THEM UP it will not stop as well under emergency conditions as you cannot translate increased pedal force to more braking, and I suggest tyre type and tread has 3 times more effect than the size change on a dry road and a lot more on a wet road.

I would be surprised with 33" MT tyres that you are creating more than 0.55g in the dry, the car in front could be at 1g. Food for thought.

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post #12 of 18 Old 11-05-2019, 09:20 PM
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You should drive a older jeep. That is how a Jeep got the reputation of stopping being an experience. In the snow and ice you learn to drive, or not. Short wheel base. All drums. The parking brake is on the transfer box its a drum. Had 47cj top speed at best wind at your back foot to the floor 50 mph. At least it was slow enough most did not kill them self and it ran the Rubicon stock many times back in the 70's. The 66cj5 was in wrong hands a lot of jeep since only a bit heavier and the Buick v6 it was rather quick could do 80 and that was flying with drum brakes. Mine we had like 20 years. stock. did ok just hard on the driver, like the 47. could fix that jeep anywhere with very little. i gravity feed it once 30 miles out by a jerry can tied to the window feed the carb. fuel pump went out. No one called it DEATH wobble. At least no one i heard from and i was kind of in it then. wobbles, gots the shakes like Elvis, lol. People I did know the did die in a jeep to much power and i guess luck. The ones you hear about on trails is rather a lot too, every year. I still have not seen any list of dead people that was in direct cause. If there is i would want to know how many of them kept driving it and never had it repaired. its most cases not that hard to figure out. All straight axles can do it.

the real way to get more stopping force is larger disk brakes. more caliper. have to say the Jks stock do work rather well on 35 even 37 compared to say the 66 that could make stabbing brakes to hard at 75, an act of suicide.

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post #13 of 18 Old 11-05-2019, 09:49 PM
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Quote:
Originally Posted by 222Doc View Post
You should drive a older jeep. That is how a Jeep got the reputation of stopping being an experience. In the snow and ice you learn to drive, or not. Short wheel base. All drums. The parking brake is on the transfer box its a drum. Had 47cj top speed at best wind at your back foot to the floor 50 mph. At least it was slow enough most did not kill them self and it ran the Rubicon stock many times back in the 70's. The 66cj5 was in wrong hands a lot of jeep since only a bit heavier and the Buick v6 it was rather quick could do 80 and that was flying with drum brakes. Mine we had like 20 years. stock. did ok just hard on the driver, like the 47. could fix that jeep anywhere with very little. i gravity feed it once 30 miles out by a jerry can tied to the window feed the carb. fuel pump went out. No one called it DEATH wobble. At least no one i heard from and i was kind of in it then. wobbles, gots the shakes like Elvis, lol. People I did know the did die in a jeep to much power and i guess luck. The ones you hear about on trails is rather a lot too, every year. I still have not seen any list of dead people that was in direct cause. If there is i would want to know how many of them kept driving it and never had it repaired. its most cases not that hard to figure out. All straight axles can do it.

the real way to get more stopping force is larger disk brakes. more caliper. have to say the Jks stock do work rather well on 35 even 37 compared to say the 66 that could make stabbing brakes to hard at 75, an act of suicide.
Amen.

i had a death wobble experience on my stock 1951 Cj3A, i would call it an Elvis shake but in 1951 was probably the Delta Cats shudder.

More caliper only really works if your tyres can grip more than your brakes can stop (though the extra reserves are useful). As the OP is able to lock the wheels, it is not a lack of caliper, more like a lack of grip

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post #14 of 18 Old 11-14-2019, 01:17 AM Thread Starter
Nucking1Futs
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Originally Posted by BagusJeep View Post
If you are locking and spinning you may want to check you have an original setup and a working proportioning valve
So, funny thing happened when I went through the box of spare parts from the previous owner.

There was a proportioning valve in bottom of the box. That peaked my interest. So I crawled under and to look at the existing valve. Immediately, I can see it was not the stock valve as it had an adapter fitting to connect to the rear brake lines. Generally, these valves don't go bad so I don't know why it was changed. I figured what the heck and swap it in, what do I have to lose, right?. It might help the problem. So I cleaned up the valve the best I could and swapped it in (PITA too!).

WOW, a night and day difference! I can mash on the brakes and the back wheels don't lock up anymore! Gosh, what a relief. I don't know why it was swapped out. I don't care. My Jeep is stopping correctly now. I no longer have to pull the seat out of my rear after driving in traffic!

However, I still want to step down my tire size and tread design to a more appropriate tire while I use it as a commuter. Those Firestone Destination AT are looking to be a good compromise.

And maybe a vacuum booster in the future to give my thigh a little break in braking.

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post #15 of 18 Old 11-15-2019, 11:50 AM
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I've been noticing that the non-power brakes on my 80 CJ-5 are starting to remind me of my old 64 CJ-6 with 9" brakes. I run 32" tires on the CJ-5 and I don't think I can lock the tires up in an emergency stiop. Several years ago when I did the gear change I rebuilt the brakes since everything was apart. Braking was pretty decent following that but now I think I have to get back in and deglaze the rotors/ drums. I've been thinking of upgrading to power, in fact I already have the proper air cleaner.
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