Down here in New Zealand, transport laws state that if a vehicle is lifted more than 50mm, it has to obtain a 'Low Volume Certificate'.

As part of the cert, the modified vehicle is placed on a 'Rolling Road' and braking effect is scrutinized.

I have liased with my local Low Volume Inspector, who has informed me that some 4X4's struggle to pass this test after having been fitted with 33" tyres and bigger.

Have any of you guys with modded trucks, had your braking ability impaired as a result of the bigger tyres??

 

Bigger tires are just that, bigger. They have more mass and more weight. So if you are driving with bigger tires, they are going to have more momentum, so the braking is going to be impaired.

 

Yep, I'm aware that bigger tyres have some impact, but was just wondering how much of an impact.

My last 4x4 was an Isuzu (you guy's probably know them as an Isuzu Amigo??) which i had modded and fitted 33" Super Swampers. Braking was affected, but was still able to scrape through to above mentioned test.

Standard, the Isuzu was fitted with disc brakes. The XJ has disc/drum setup, which I hope is up to the job.

 

Disc brakes are definitely more effective than drum brakes because they dissipate heat more efficiently. That being said, the law in NZ sounds very stringent, and if you've got the money, you may want to look into a rear disc brake conversion. If the Cherokee can pass with stock brakes, go for it; however, if you're uncertain if it will pass, then go for the conversion. I hope this helps.

 

Thanks for the response.. I have just been looking at an Australian XJ site http://go.jeep-xj.info/. This site has some very good write-ups on just what you have suggested. If my XJ fails the test, I'll look into the conversion.

Cheers - Bill

 

The braking decrease will be reduced by the amount of proportional increase in the diameter of the tire. For instance, on an XJ they have 27" tires stock and if you put a 33" on it will be 27/33 or 82% of the original braking force.