Smaller tires = better braking?

I've competed the brake rebuild on my 69 CJ5 (10 in drums), and as expected, they work, but are only so-so for stopping. I'm trying to keep it as original as possible, so looking into downsizing the tires from 30x9.5x15 to either 235 or 225/75R15. My neighbor, a Chevy Blazer guy, says the smaller tires will improve the braking. He also said a less aggressive tread will help.

Has anybody heard this before? I'm not sure I understand the physics if this is true.

Thanks

 

The physics don't lie smaller tires are going to have less angular momentum and thus will brake better. Lighter tires also help.

 

Smaller tires give the brakes more mechanical advantage but smaller tires turn at a faster speed so more friction so might over heat easier.

So from a user experience there will be less pedal pressure for the same rate of deceleration.

 

I agree that a smaller lighter tire will stop easier, but I don't see how tread design would factor in.
If anything, an aggressive tread has more rolling resistance.

 

The only way tread design would matter is wrt coefficient of friction. Usually an aggressive tread means less rubber in contact with the road (more and wider grooving), and the tire would be more likely to skid rather than stop the car. Rubber compound matters too. I suppose you could design a tire with a soft compound that would be less likely to skid than some conventional tires with a harder compound... so it's not an absolute rule.

 

10" brakes should stop a stock CJ5 reasonably well. I drove mine with a smallblock Chevy in it, 33's, and the stock 9 inchers on all 4 corners for quite awhile and that worked..It gave my right leg a workout, but I could lock the 33s. I have 11" all the way around now and it makes a huge difference, but maybe you have some other underlying brake problem you haven't discovered yet? Manual drum brakes will never feel like modern power-assisted discs, but they should work just fine...

 

Just use 11 inch drums. 10's are pointless.

 

More aggressive treaded tires are typically much heavier. Compare the weight of a mud and snow tire vs. an aggressive mud terrain tire.
A larger diameter tire has more leverage to stop and typically more rotating mass. Therefore a larger tire is harder to stop rotationally than a smaller tire.


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I was able to get my jeep lock up tires with 31 on 10 inch drums when I had everything adjusted right, but have sense switched to self adjusting 11 inch brakes its so much easier to get parts and they are self adjusting.

this next one is anecdotal evidence, I am not sure if it applies to jeeps sense they are a lot lighter then a 1965 C30 chevy truck with flatbed.
the previous owner switched to smaller tires P225/75r16, I would lock up those tires so fast at every stop light that it normally ended up me skidding about 14 feet. (in straight line.) It was a nice and easy pedal to press but with those stoplights and stopping at 65 mph to dead stop not for me.

the tires were not able to take the weight of the truck so I found a set of stockers which was equivalent to a set of 31-33 inches, it was old school sizing. (8.5x16.5?)
it took a lot more force to lock up the tires however I was able to stop better. No more skidding and watching the peoples face look scared when they heard my tires.
I think it was probably a balance of tire size and size of your brakes with weight factored in.

 

"Original as possible" means going down to 205/75/15. That's the closest metric equivalent to the 7.65 x 15s originally (likely) on it.

 

So is there a write up to switch to 11" or is it a bolt on etc. from later years?

 

Try the search function. This has been covered ad nauseum


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Or just remove the drum and bearing as one. That's how I did it when I replaced the brake cylinders.

 

So to check the 10" wheel cylinders on the '65 I just can't pull the drum ?

 

I wish we could, but no. Need a big socket for the front and a puller for the rear. Sorry.

 

http://web.archive.org/web/20100728...5.com/technical/brakes/early-cj-11-drum-swap/