Equivalent Scrub Radius Calculation

[EricLar80]

**Edit: updated the image with adjusted numbers**

Hi Everyone,

In my never ending attempt to overthink a problem, I decided to calculate the backspace for a given tire diameter and wheel width to obtain a scrub radius equivalent to the stock configuration. This assumes that the stock wheel is roughly a 28.9x9.3r15, with a stock wheel that has a backspace of 3.5" and width of 7". I read somewhere that the kingpin sits at 9 degrees, but I have not measured it to be certain. Anyone able to confirm it?

Scrub radius obviously plays into the handling of our vehicles, and the effect from tire size and wheel width is not always considered when choosing wheels. I thought it would help some, and perhaps be interesting to others. Note that this is not the actual scrub radius, I don't have enough information for that yet, but it does help show what you should get to be back to "stock", if that matters...

Calculating the actual scrub radius is the next, natural extension because I wonder how that plays into handling issues, such as death wobble.

Eric

 

[toddz69]

Very interesting stuff, sir. Thanks for putting that spreadsheet together. I've often wondered about the scrub radius equivalency as well. And less backspacing, i.e. greater scrub radius, definitely plays into the propensity for death wobble since you have a longer moment arm tugging on the balljoints.

Todd Z.

 

[EricLar80]

Very interesting stuff, sir. Thanks for putting that spreadsheet together. I've often wondered about the scrub radius equivalency as well. And less backspacing, i.e. greater scrub radius, definitely plays into the propensity for death wobble since you have a longer moment arm tugging on the balljoints.

Todd Z.

I wonder if less (near zero) is also likely to induce it because of squirm. A little scrub radius adds stability to the situation.

 

[nvrstuk]

SUBSCRIBED as I'm definitely an "over-think" the issue type guy also! lol

I checked into it with my narrowed D60 build and hope I'm close to what I need as all I can do now is put wheels on with less back spacing...

Keep us posted! Interesting read...

 

[DirtDonk]

Always touted the stock layout as well, so interested in hearing all the little tidbits you come up with.
For the king pin angle, there are several printouts posted up here in alignment discussions that mention the SAI (steering axis inclination) that should give you the exact number you need. Looking at and comparing several of them to get an approximate baseline should do the job.
Assuming we can find those charts in a search that is!

Good luck. Might as well make a second sheet showing the conversion of inches-to-degrees and degrees-to-inches of toe-in while you're at it!;D

Thanks

Paul

 

[Broncobowsher]

I don't get the image. Just get a circle with a line through it.

Scrub radius is one of those often overlooked things. I cringe when I see 3" wheel spacers and a set of deep dish wheels to get full width, without changing the axle.

I remember a long time ago with manual steering cranking the wheel to full lock while stopped. You could watch the tire roll a little bit as you went lock. Step on the brakes and turning while stopped just got a whole lot harder. Having to drag the tire instead of it rolling.

Few years ago I had a Ranger and got a set of mustang wheels. Same overall height, lot wider but all on the inside. The outside of the wheel was at the same location as stock. The steering got weird with those tires. Can't really explain it, but it wasn't right. Tracking and steering feel were not right. Went back to the stock wheels and it suddenly drove right. The tires drove just fine on the mustang. All I could figure was the scrub radius was way off.

I wish I saved the article from a very long time ago. It was in a stock car racing magazine. It had pages of info on scrub radius. Started off describing it since it is something most people don't know anything about. Then went into how changing it changed how things behaved.

Recently saw something (and just as quickly lost it) on how SIA/kingpin inclination altered how a car handled. I want to say the old school (60's car) was a rather steep SIA but modern cars have a much more upright SAI. Stuff that you really can't change on a straight axle. But an interesting read never the less. Allows steeper steering angles without the wheels leaning over at weird angles.

 

[DirtDonk]

Wait, there's an image? I don't even see a circle with a line.
Was kind of wondering what Todd was referring to with the "spreadsheet" comment. Figured it might have been something in another thread, or the OP's garage, or something.
What should we all be seeing?

Paul

 

[EricLar80]

Here is a link to the image; let me know if it doesn't work:
https://tinyurl.com/ybgczsn3

 

[EricLar80]

Updated the image and links.

 

[EricLar80]

SUBSCRIBED as I'm definitely an "over-think" the issue type guy also! lol

I checked into it with my narrowed D60 build and hope I'm close to what I need as all I can do now is put wheels on with less back spacing...

Keep us posted! Interesting read...

Take a look at the updated numbers compared to what you have. I am not sure of the kingpin angle is the same on the D60, but if it is different, I can easily pop that number in and tell you the result. Also if you have a particular tire diameter or whatever (aired down, etc.).

Eric

 

[Broncobowsher]

I can only see the chart on the phone. Both computers ignore it.

It may be answered in the chart, but what are you doing for inputting the distance between the SIA line and the wheel mounting surface? I can only guess that you measure from the axle U-joint center line to the end of the shaft, then subtract the distance from the end of the shaft back to the wheel mounting surface. This would also take into account duelly adaptors or wheel spacers. Gets you a fairly accurate offset off the pivot line.

 

[EricLar80]

I can only see the chart on the phone. Both computers ignore it.

It may be answered in the chart, but what are you doing for inputting the distance between the SIA line and the wheel mounting surface? I can only guess that you measure from the axle U-joint center line to the end of the shaft, then subtract the distance from the end of the shaft back to the wheel mounting surface. This would also take into account duelly adaptors or wheel spacers. Gets you a fairly accurate offset off the pivot line.

I am using a relative measurement only provided the 9 degree SAI line (steering axis inclination, or as I have referred to as the kingpin angle). Basically find the interception point of the SAI on the ground in stock configuration, and then the relative position if you raise the axle up with a taller tire. Then add in any effects due to the wheel width. The overall difference is added to the stock configuration to give you the final value.

So, I don't know the actual scrub radius, just whats needed to get it to stock. I would need to do the measurement exactly as you have stated to find the scrub radius value, but it doesn't provide much value to the layman - most want to get back to stock I would think. Maybe to avoid zero...

The most useful part of this spreadsheet, I think, is for someone to understand the impact of using a 10" wheel on a small tire (for example).

 

[EricLar80]

I can only see the chart on the phone. Both computers ignore it.

That may be a browser issue - I assume you are using internet explorer and not Google Chrome? Not sure why it makes a difference, but it doesn't show on IE for me.

 

[Broncobowsher]

That may be a browser issue - I assume you are using internet explorer and not Google Chrome? Not sure why it makes a difference, but it doesn't show on IE for me.

Probably so. IE is the only approved browser on the work computer, or I should say the option of one browser choice.

 

[Pops68]

I'm using google chrome and get no image......saw it early today (or maybe late last night) before you edited it. All I see now is the circle with the dash in it.

 

[nvrstuk]

I'm not a google account holder so I can't check it out...

 

[sykanr0ng]

That may be a browser issue - I assume you are using internet explorer and not Google Chrome? Not sure why it makes a difference, but it doesn't show on IE for me.

Does not show with Mozilla Firefox either.

 

[EricLar80]

Does this work?
https://i.imgur.com/PgGsj8r.jpg

 

[sykanr0ng]

Does this work?

That works.
Thanks.