20" Tire Problems

GM Dude · March 20, 2005

Just installed new 20" wheels and tires (P275/55R20 Hankook) on my new 2005 and I'm showing about 2 MPH off at about 65 (based on a roadside radar) . I'm not so worried about a couple of MPH at the legal limit of 70 but I am worried about the bad shuddering that is produced when applying the brakes. At first I thought it might be the ABS system doing its thing but I pulled the fuses and the wires on the ABS and it still shudders when applying the brakes.

Can anyone give me some advice on what might be causing this and/or how to fix it?

Thanks

Axxlrod · March 21, 2005

Big wheels (20" or larger) are extemely heavy, and therefore have much more rotational mass. The stock brakes are only designed to stop the stock size wheels. And its even worse if the wheels are chrome. Best thing to do is try to purchase wheels that are forged and not cast metal. I also recommend upsizing your brakes... like the Eradispeed +1 from Baer Brakes.

GM Dude · March 21, 2005

Big wheels (20" or larger) are extemely heavy, and therefore have much more rotational mass. The stock brakes are only designed to stop the stock size wheels. And its even worse if the wheels are chrome. Best thing to do is try to purchase wheels that are forged and not cast metal. I also recommend upsizing your brakes... like the Eradispeed +1 from Baer Brakes.

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I would agree to a certian extent if they were chrome plated steel. The wheels I have are aftermarket BOSS chrome plated aluminum and even with the heavier tires they are only about 12 lbs. more than the stock 17's. GM offers 20" aluminum wheels with Goodyear LS P275/55R20 as an option that they will install on your truck off the lot (would assume they are as heavy or heavier than aftermarket and much more expensive). Since you are able to get these as an option off the lot, I do not believe they change the brake system when they install them. I may be wrong and I'll give them a call to see if this is right.

Thank you for your opinion though and keem them coming.

asepgrad98 · March 21, 2005

maybe they warped the rotors and/or wheels with an impact gun due to improper torque on install.

Axxlrod · March 21, 2005

Big wheels (20" or larger) are extemely heavy, and therefore have much more rotational mass. The stock brakes are only designed to stop the stock size wheels. And its even worse if the wheels are chrome. Best thing to do is try to purchase wheels that are forged and not cast metal. I also recommend upsizing your brakes... like the Eradispeed +1 from Baer Brakes.

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I would agree to a certian extent if they were chrome plated steel. The wheels I have are aftermarket BOSS chrome plated aluminum and even with the heavier tires they are only about 12 lbs. more than the stock 17's. GM offers 20" aluminum wheels with Goodyear LS P275/55R20 as an option that they will install on your truck off the lot (would assume they are as heavy or heavier than aftermarket and much more expensive). Since you are able to get these as an option off the lot, I do not believe they change the brake system when they install them. I may be wrong and I'll give them a call to see if this is right.

Thank you for your opinion though and keem them coming.

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Since GM now offers 20" wheel options from the dealer, I'm sure they have tested their braking systems with the optional 20" wheels. And the GM optional wheels are cast, not forged, so they are also heavy. Keep in mind that a 12 lb. increase in wheel/tire weight is a large increase per wheel. Think of it as a percentage... maybe 30% - 40% increase!

All I know is that I put my 20" wheel/tires on, I definitely noticed an increase in braking distance, and a drop in mpg. But my brakes never shuddered when applied as you are stating. Have the brakes looked at.

builderpaul · March 21, 2005

maybe they warped the rotors and/or wheels with an impact gun due to improper torque on install.

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I agree with this. If you don't use a torque wrench and torque these wheels in the correct sequence the brake rotors can warp. Keep the air gun away from the installation.

jimi · March 21, 2005

maybe they warped the rotors and/or wheels with an impact gun due to improper torque on install.

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I agree with this. If you don't use a torque wrench and torque these wheels in the correct sequence the brake rotors can warp. Keep the air gun away from the installation.

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Anyone know what a stock 17" Sierra Z71 alloy wheels weighs? Seems to me if you can find 20s that weigh about the same, or less, braking shouldn't be a big issue. Agree?

Scyry · March 21, 2005

Even if you find 20" rims that are the same wieght as stock, you are still putting more force into your drivetrain and brakes. You are pushing the weight farther away from the center and that adds a lot of force to everything. Say the weight per rim is 30lbs. Stock rim is 17" that is a 8.5" radius and a torque of 21.25 ft-lbs. A 20" rim is 25 ft-lbs. Now say that rim is 12 lbs. heavier, you get a torque of 35 ft-lbs, which is a 65% increase.

For rotational energy, the energy is equal to half mass, times the radius from the point of rotation to the center of mass squared, times the angular velocity squared. So say you are moving along att 50mph with a 31" tire, the angular velocity is 4.518 rotation per second. So the kinetic energy of a 17", 30 lbs rim is approximately 22,000 ft-lbs of work. A 20" rim assuming 12 lbs heavier, is approximately 43,000 ft-lbs of work. Or about a 90 to 95% increase in kinetic energy.

These are just numbers I made up to show the changes, and I was guessing at approximate weights and made a few assumptions. But you can get a general idea of why GM doesn't want to warranty stuff when you change tires and/or rims. the amount of work I figured would be the work done by the brakes to slow your truck from 50 to 0 mph or the work done by your drive train to go from a stop to 50 mph.

*Almost forgot, not responsible for math calc errors, d**n basic algebra was my bain in college, calculus is much easier than algebra And I am just going off equations from memory, so it is possible I screwed up, but you get the general jist of the changes effects, all I do is basic structural calcs anymore, none of this moving object BS *

GM Dude · March 22, 2005

Big wheels (20" or larger) are extemely heavy, and therefore have much more rotational mass. The stock brakes are only designed to stop the stock size wheels. And its even worse if the wheels are chrome. Best thing to do is try to purchase wheels that are forged and not cast metal. I also recommend upsizing your brakes... like the Eradispeed +1 from Baer Brakes.

<{POST_SNAPBACK}>

I would agree to a certian extent if they were chrome plated steel. The wheels I have are aftermarket BOSS chrome plated aluminum and even with the heavier tires they are only about 12 lbs. more than the stock 17's. GM offers 20" aluminum wheels with Goodyear LS P275/55R20 as an option that they will install on your truck off the lot (would assume they are as heavy or heavier than aftermarket and much more expensive). Since you are able to get these as an option off the lot, I do not believe they change the brake system when they install them. I may be wrong and I'll give them a call to see if this is right.

Thank you for your opinion though and keem them coming.

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Since GM now offers 20" wheel options from the dealer, I'm sure they have tested their braking systems with the optional 20" wheels. And the GM optional wheels are cast, not forged, so they are also heavy. Keep in mind that a 12 lb. increase in wheel/tire weight is a large increase per wheel. Think of it as a percentage... maybe 30% - 40% increase!

All I know is that I put my 20" wheel/tires on, I definitely noticed an increase in braking distance, and a drop in mpg. But my brakes never shuddered when applied as you are stating. Have the brakes looked at.

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I kinda expected loss of MPG as well as stopping distance so that was no problem. I thought about warpage and brakes but to eliminate that I removed the 20's and reinstalled the stock 17's and the shuddering disappeared. Put the 20's back on and shuddering / shaking reappears. Got to be a problem with the wheels so I may take them back and have them look at it on thier dime.

Scyry · March 22, 2005

I kinda expected loss of MPG as well as stopping distance so that was no problem. I thought about warpage and brakes but to eliminate that I removed the 20's and reinstalled the stock 17's and the shuddering disappeared. Put the 20's back on and shuddering / shaking reappears. Got to be a problem with the wheels so I may take them back and have them look at it on thier dime.

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Aftermarket rims are notorious for being out of round or having a slight wobble to them. I forget, did you increase the tire diameter? If so you could easily be fooling the ABS and it can kick in and out and make you feel a shudder. Otherwise also make sure you got round tires.

MyBigToy · March 22, 2005

Are the wheels hub-centric or do they have the hub-centric spacers? You will get bad wobble if they are not.

If so, it could just be the wheel balance. Get them road-force balanced on a Hunter machine and odds are your issues will go away.

GM Dude · March 22, 2005

Are the wheels hub-centric or do they have the hub-centric spacers? You will get bad wobble if they are not.

If so, it could just be the wheel balance. Get them road-force balanced on a Hunter machine and odds are your issues will go away.

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I'm going to play stupid because I'm not sure if they are hub-centric or not because I'm not sure I know what hub-centric is. I will say they are BOSS wheels made by American Eagle and 6 on 5.5 and are secured with acorn style nuts. I can also say that the center hole of the factory wheels was a perfect fit where the raised part of the drums/disc fit and the BOSS wheels have a much larger center hole. Should there be some sort of spacers that would fit onto the drum/disc that would take up that difference??

When purchased I was told it was road force balanced but not sure what brand equipment it was.

Thanks

GM-Tech · March 23, 2005

I'm going to play stupid because I'm not sure if they are hub-centric or not because I'm not sure I know what hub-centric is.

Hub-centric means the wheel is centered by the hub. In other words, the hole in the center of the wheel is a snug fit on the hub. Lug-centric means the wheel is centered by the lugs, which is not nearly as accurate. They do make adapters that allow a larger wheel hole to fit the hub snugly.