Tire size setting driveline angles

[pete63]

I am working on setting up driveline angles on my 63 with air bags and was wondering how much tire size effects it. I have my ride height set at 5" with a 27" tires(what I had laying around). What my question is will the angles change if go to 29-30" tire later, or should I get the taller tires now to set driveline angles? Thx in advance

[hellbent72]

Imo You will want the tire you plan to run in order to set the angles.
With a 27 inch tire your suspension will be more extended than with the 30 inch at your 5 inch ride height. Maybe not to much to make a huge difference but when it comes to drive line angles its good to be spot on any small change can throw it all off. You maybe able to find someone with the tire size you plan to use and measure its height and shim your 27 inchers with some wood under them and then measure for your 5 inch ride height.
I'm no susp guru just a thought.

[hyatt]

I dissagree Tire size has nothing to do with driveline angle unless you have to change it for tire clearance you can set your angle without any wheels at all just set your suspension where you will drive it and set your angles

[LONGHAIR]

I would agree that it matters, since you are trying to keep the same ride height. A taller tire would put the axle centerline higher in relation to the frame.
I have no problem with the suggestion of shimming the current tires up to the new height, but remember to raise it only half of the difference.

[tinydb84]

You set your drive line angles relative to the trans output and rearend. It doesnt matter if your truck is upside down.

If your rearend angle changes that much through the suspension cycle (like a 2 link) then you have other issues to deal with

[pete63]

Thanks for the replies. I can see each point of view, I think will try shimming the tires 1" (for 29" tire) and go from there. Thanks again.

[SCOTI]

Originally Posted by tinydb84

You set your drive line angles relative to the trans output and rearend. It doesnt matter if your truck is upside down.

If your rearend angle changes that much through the suspension cycle (like a 2 link) then you have other issues to deal with

Wouldn't those angles change if the tires were @ different heights though? Ex: front tire 27" now 28" later; rear 28" now 30" later. There should be an impact on the measured angles (unless the truck is upside down).

It might be minimal, but it would impact the measurements. The amount of rake on an adjustable set-up seems like it could impact things as well if one were to adjust heights based on the 'new' tire OD dimensions. That could be used to put things back where they were originally before the tire size changes or possibly amplify any change.

[tinydb84]

Originally Posted by SCOTI

Wouldn't those angles change if the tires were @ different heights though? Ex: front tire 27" now 28" later; rear 28" now 30" later. There should be an impact on the measured angles (unless the truck is upside down).

It might be minimal, but it would impact the measurements. The amount of rake on an adjustable set-up seems like it could impact things as well if one were to adjust heights based on the 'new' tire OD dimensions. That could be used to put things back where they were originally before the tire size changes or possibly amplify any change.

If you put huge tires on the rear end then the whole truck (driveline included) would change its "rake" but the transmission and rearend wouldn't change relative to each other.

As the suspension cycles there will be change (depending on the type of suspension would dictate the amount of change) but changing the tire size would not change the position of the rearend relative to the transmission.

[_Ogre]

your ride height is determined with your front a-arms at the correct ride height, level on most vehicles.
set your front a-arms level, set your rear axle at the same height (unless you want some rake or gasser look)
and set your pinion angle at that height.
yes you can still cruise down or up from there, but you will have the best ride at that height

[PBFAB.COM]

This is one of those questions where there are multiple right answers depending on which rear suspension system is being utilized.

In this case, assuming it is the factory 63 suspension with trailing arms... tire height does matter. With the trailing arm suspension, the pinion angle changes as the differential bumps and droops. The tire height change will cause the rear pinion to change as well.

If you go from a 27" tall tire to a 30" tall tire, the rear axle center line will be 1.5" higher with the taller tire. The pinion change might not be much, but it will change. I would simulate the taller tire height and see what the actual change is.

If you know for sure that you will be running a specific size tire in the future, set the rear end up for the known tire size and purchase a shim if necessary for the time being with the smaller tire.

Nate

[tinydb84]

Originally Posted by Porterbuilt Street Rods

In this case, assuming it is the factory 63 suspension with trailing arms... tire height does matter. With the trailing arm suspension, the pinion angle changes as the differential bumps and droops. The tire height change will cause the rear pinion to change as well.

But the tire size would not effect the rearend's position relative to the truck, correct? Only suspension changes would do that (i.e. shorter or taller springs).
If you go from a 27" tall tire to a 30" tall tire, the rear axle center line will be 1.5" higher with the taller tire. The pinion change might not be much, but it will change. I would simulate the taller tire height and see what the actual change is.

Wouldn't the rear end of the truck also be about 1.5 inches taller as well?

Nate

My input/questions in red. Thoughts?

[hyatt]

angle to ground will change but that makes no difference Angle to trans stays the same if you had it standing on front bumper

[PBFAB.COM]

Originally Posted by hyatt

angle to ground will change but that makes no difference Angle to trans stays the same if you had it standing on front bumper

Does the pinion angle change on a differential using trailing arms as the suspension compresses? YES.

What is being overlooked, is that he wants to maintain a 5" ride height.

If he wanted to go with a taller tire lets say from 27" to 30", and keep the 5" ride height, the axle will have to move up the 1.5" difference in height (just like you compressed the suspension).

If he wanted to add the extra 1.5" tire diameter AND raise the ride height to 6.5" then the driveline angles would remain the same.

Hope that makes sense to everybody.

Nate

[SCOTI]

For clarification during discussions like this, when referring to "maintaining a 5" ride height", we're referring to scrubline or what?

[pete63]

Originally Posted by Porterbuilt Street Rods

Does the pinion angle change on a differential using trailing arms as the suspension compresses? YES.

What is being overlooked, is that he wants to maintain a 5" ride height.

If he wanted to go with a taller tire lets say from 27" to 30", and keep the 5" ride height, the axle will have to move up the 1.5" difference in height (just like you compressed the suspension).

If he wanted to add the extra 1.5" tire diameter AND raise the ride height to 6.5" then the driveline angles would remain the same.

Hope that makes sense to everybody.

Nate

This makes sense to me, so in this case I will shim the tire to my desired height of tire that will be running later and set all my angles. This is pb suspension sorry didn't say that before just thought it was a generic question, sorry to cause confussion. And yes 5" to frame.

[hyatt]

I have had a repair shop for 23 years and again I dissagree no matter what size tires you have the pinion angle does not change unless the springs or trailing arm moves It will be the same even with no tires sitting on jack stands Rake of truck will change but drivshaft angle stayes the same Angle is measured in relation to trans yoke not the ground