VIDEO : SCRUB RADIUS - What it's All About & The Dynatrac PRO 1550 Knuckle

So does castor effect the scrub radius? Curious cause when we lift our jeeps we usually lose a couple degrees of negative castor for the sake of the front drive line.


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cameleer said:
A Jeep JK stock wheel backspace is 6.25". With 32" tires this has the stock factory scrub radius.
Changing to aftermarket wheels with a backspace of 4.5", the scrub radius is now off from stock.
Adding 37" tires changes the scrub radius again, moving it closer to stock.
I would like to know, with aftermarket wheels backspaced at 4.5". What size tires will put you back to the stock scrub radius, or the best scrub radius.
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Holy crap...

I swear that this was on my 5th grader's math homework last week!?!?!
(I didn't know the answer then either) Lol.
 
So the scrub radius is better the taller the tire. 40 is better than a 37. How does the 1550 compare to the pro 60 wheel end? Does it have a higher inclination angle? Mel said spicy chicken steers better even though he's running 3.5 bs.
 
Add to my weekend-to-do list: Remove wheel spacers

Great video. Isn't it amazing that just when you think you got this shit figured out, along comes new info that shows how off base you were? :crazyeyes:
 
scrhino said:
So the scrub radius is better the taller the tire. 40 is better than a 37. How does the 1550 compare to the pro 60 wheel end? Does it have a higher inclination angle? Mel said spicy chicken steers better even though he's running 3.5 bs.
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The PRO 1550 knuckles were designed with 40" tires in mind and they would be the optimal tire size for them. Spicy Chicken does steer better even though Mel is still running 3.5" of back spacing and it is because of the PRO 1550. Measuring from the far outside edge tire to tire, Spicy Chicken's stance is 1" wider than Rubicat in spite of her 72.5" axle. We physically measured this.
 
cameleer said:
Yes, that was my point. The steering axis inclination line will be at a different point with larger tires. If I understand it right, this will change where the scrub radius is according to tire size.
I'm running a Dynatrac ProRock 44 with Reid racing knuckles, but the scrub radius does not change with this setup. Also have wheels backspaced at 4.5" and 37" tires. I'm thinking this may bring me close to the proper scrub radius.
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Do you have any rubbing issues at full lock with the 4.5" bs wheels?
 
This is why Dynatrac is at the top of the pack! They are constantly evolving and improving their products unlike their competitors.
 
VIDEO : SCRUB RADIUS - A Look into the Design of the Dynatrac PRO 1550 Knuckle

wayoflife said:
The PRO 1550 knuckles were designed with 40" tires in mind and they would be the optimal tire size for them. Spicy Chicken does steer better even though Mel is still running 3.5" of back spacing and it is because of the PRO 1550. Measuring from the far outside edge tire to tire, Spicy Chicken's stance is 1" wider than Rubicat in spite of her 72.5" axle. We physically measured this.
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I'd be curious to what the scrub radius measurement is on a pro 60 wheel end, 40 in tire, and 3.5 bs. I'm guessing it's more than 2.94.

And while I'm asking, I'm sure the PR44 is similar to the pro 60 as far as inclination angles go?
 
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scrhino said:
I'd be curious to what the scrub radius measurement is on a pro 60 wheel end, 40 in tire, and 3.5 bs. I'm guessing it's more than 2.94.
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Moby is running a Pro 60, 40" tire and 4" of back spacing and while I don't know what the actual scrub radius is, you can clearly see it in the print left behind after turning the steering wheel from lock to lock while parked.
attachment.php


While the PRO 1550 is optimized for a 40" tires, you can see the HUGE difference it made when we did the same thing with Rubicat. Note, she's only running 37's but on a PRO 1550 and 5.5" of back spacing. As you can see, the tread rotates more around a center point close to the center instead of arcing around a point outside of the tread.
attachment.php


You can see more photos regarding this here:
http://wayalife.com/showthread.php?...550-Front-Axle&p=867609&viewfull=1#post867609

And while I'm asking, I'm sure the PR44 is similar to the pro 60 as far as inclination angles go?
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Not sure what it is on a ProRock 44 but it is a different animal in that it still uses unit bearings. You will get better steering with.
 
Found some interesting info.
The YJ/TJ knuckle has a 10 degree inclination vs 12 degrees on the JK.
Can someone tell me what the XD60 inclination is?

Also what I found, it sounds like you want some positive scrub or negative scrub radius to keep from getting Squirm.

Squirm occurs when the scrub radius is at zero. When the pivot point is in the exact center of the tire footprint, this causes scrubbing action in opposite directions when the wheels are turned. Tire wear and some instability in corners is the result.

A positive scrub radius will allow the tire to rotate some when turned, you just don't want an excessively scrub radius.

I got to thinking about this because almost all of the scrub radius diagrams show a positive scrub radius where the 2 lines are at the bottom of the tire. Scrub Radius.gif
 
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chitown35 said:
Could you explain any relationship between scrub radius and turning radius?

Thanks!

Edit: I guess even just having a wider axle, instead of less backspacing, helps the arc that the wheel turns in to be smaller. Maybe I misunderstood about it being related to scrub radius?
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Turning radius and scrub radius are completely different, and for the sake of discussion, unrelated. Turning radius is the tightest circle the car can turn in without backing up and doing a multi-point turn. The number is typically published as part of the vehicle technical specs by the manufacturer. Rarely is scrub radius published.

Front wheel end geometry does have an effect on the turning radius of the vehicle, but other things (non-axle related) have a larger impact. We did not test the turning radius on Rubicat before or after the axle was installed, but maybe Eddie can chime in with his impressions so far.

We will publish more info on this at a later date.
 
Dynatrac said:
Turning radius and scrub radius are completely different, and for the sake of discussion, unrelated. Turning radius is the tightest circle the car can turn in without backing up and doing a multi-point turn. The number is typically published as part of the vehicle technical specs by the manufacturer. Rarely is scrub radius published.

Front wheel end geometry does have an effect on the turning radius of the vehicle, but other things (non-axle related) have a larger impact. We did not test the turning radius on Rubicat before or after the axle was installed, but maybe Eddie can chime in with his impressions so far.

We will publish more info on this at a later date.
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Awesome, thanks for the confirmation on scrub radius not affecting turning radius. Yes I'm sure other things affect turning radius more, however I do have to say wheel end components affect turning radius enough to matter though! Try taking a 4 door jeep off road on tight trails on the same axle with 2.75 backspacing vs 4.25 or even 6.25.

Anyway it seems like even just the wider axle with less wheel backspacing should help out turning radius, would love to hear how much from you, Eddie.
 
chitown35 said:
Awesome, thanks for the confirmation on scrub radius not affecting turning radius. Yes I'm sure other things affect turning radius more, however I do have to say wheel end components affect turning radius enough to matter though! Try taking a 4 door jeep off road on tight trails on the same axle with 2.75 backspacing vs 4.25 or even 6.25.

Anyway it seems like even just the wider axle with less wheel backspacing should help out turning radius, would love to hear how much from you, Eddie.
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Actually, I think if you take a wide axle and compare it to a narrow axle with spacers or more wheel offset, which sets the tires to the same width as the wide axle, the narrow axle many have slight advantage in turning radius, but it would be very slight.
The only reason this would happen, the narrow axle with the wells moved out would hit the control arms a little further out, that is without stops.
 
Dynatrac said:
We did not test the turning radius on Rubicat before or after the axle was installed, but maybe Eddie can chime in with his impressions so far.
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Funny you should say as much, Cindy and I just did a test earlier to see what we would get and this is what I can tell you:

For reference, a bone stock JK Unlimited running 225/75R16 tires measures 73.9" wide and has a curb to curb turning radius of 41.20'.

Rubicat with her new 72.5" XD60 with PRO 1550 steering knuckles running 37x13.50R17 wheels with 5.5" of back spacing measures 85" wide. Curb to curb, we measured a turning radius of 45.27'. Basically, 4' more than stock.

Here's the real kicker, Moby with his 68.5" ProRock 60 and Pro 60 steering knuckles running 40x13.50R/17 wheels with 4" of back spacing measures 85" wide. Curb to curb, we measured a turning radius of 50.72'. That's almost 10' more than stock! :shock:
 
wayoflife said:
Funny you should say as much, Cindy and I just did a test earlier to see what we would get and this is what I can tell you:

For reference, a bone stock JK Unlimited running 225/75R16 tires measures 73.9" wide and has a curb to curb turning radius of 41.20'.

Rubicat with her new 72.5" XD60 with PRO 1550 steering knuckles running 37x13.50R17 wheels with 5.5" of back spacing measures 85" wide. Curb to curb, we measured a turning radius of 45.27'. Basically, 4' more than stock.

Here's the real kicker, Moby with his 68.5" ProRock 60 and Pro 60 steering knuckles running 40x13.50R/17 wheels with 4" of back spacing measures 85" wide. Curb to curb, we measured a turning radius of 50.72'. That's almost 10' more than stock! :shock:
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Great info, thanks.
 
wayoflife said:
Funny you should say as much, Cindy and I just did a test earlier to see what we would get and this is what I can tell you:

For reference, a bone stock JK Unlimited running 225/75R16 tires measures 73.9" wide and has a curb to curb turning radius of 41.20'.

Rubicat with her new 72.5" XD60 with PRO 1550 steering knuckles running 37x13.50R17 wheels with 5.5" of back spacing measures 85" wide. Curb to curb, we measured a turning radius of 45.27'. Basically, a 4' more than stock.

Here's the real kicker, Moby with his 68.5" ProRock 60 and Pro 60 steering knuckles running 40x13.50R/17 wheels with 4" of back spacing measures 85" wide. Curb to curb, we measured a turning radius of 50.72'. That's almost 10' more than stock! :shock:
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Well that should come in handy [emoji15] awesome info. Makes me curious how a jku with stock axle width and wheels with 3.5" back spaced wheels would fare.


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