510 scrub radius and wheel fitment calc

Suspension, including wheel, tire and brake.
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kayakdude07
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510 scrub radius and wheel fitment calc

Postby kayakdude07 » 13 May 2012 21:29

So I got shipped off to the bush for work and thought I'd spend a bit of time trying to figure out my scrub radius situation for my planned brake/rim/tire/strut upgrade.

After a bunch of reading and research last night and some time today I think I have figured out the relationships between the tire sizes, struts, and offsets used on the 510 that determine the scrub radius. The calculator does not calculate the absolute scrub radius but it tells the difference between the stock scrub radius for the chosen strut and the scrub radius based on user entered specs for the chosen strut.

To calculate, it uses trig and the spindle angle of the strut and the change in tire height to calculate the change in scrub radius from this. (Shorter tires than stock increase the scrub radius [ceteris paribus])
This is added to the change in "effective" offset; "effective" because spacers change the perceived offset of the wheel. (Decreasing rim offset increases scrub radius [ceteris paribus])

Below is the list of specs I used to create the formulas used in the calculator.
Tire heights correspond to 175/60/13 for the 510, 205/60/15 for the S12, and 195/70/14 for the 280zx.
Tire sizes were found online.
Info for 510 and zx was found on Kurt's page at http://kmhafer.datsun510.com/wheelFAQ.htm
S12 info was found with a bit of web searching and using Kurt's page again....
Image

The calculator is hosted in google docs and requires drop box entries for the rim size and width, the tire width and profile as well as keyboard entries for the offset and spacer thickness (if used). Finally the strut used is selected from the final box (510, 280zx, or S12). Instructional photo below..
Image

A few notes:
This calculator assumes that the oem hubs were used with their respective struts. This is often not true but with more time and more data it could be fixed up....
Another flaw is that it cannot tell if the rims will actually fit. Again, this can be remedied with more time and data....
A positive scrub radius change means that the scrub radius has increased (i.e. the center of the new tire contact patch has moved more outboard than the stock location).
A negative scrub radius change means that the scrub radius has decreased (i.e. the center of the new tire contact patch has moved more inboard than the stock location).

Try it out and let me know what you think!! :D

http://tinyurl.com/510-scrub-calc

I'd love to hear from some of you 510 suspension gurus out there (ahem Dave and others :wink: ) to see if this makes any sense...

I would like help from you guys out there to get at least 2 different inboard clearance limiting rim sizes/widths/offsets for each strut because I can make a formula that will then throw out rim selections based on each strut choice.
If you have any issues with the data used in the formulas above feel free to post your issues....

Thanks in advance!
Last edited by kayakdude07 on 12 Jun 2012 19:06, edited 2 times in total.
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Byron510
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Re: 510 scrub radius and more calculator..... Needs YOUR hel

Postby Byron510 » 13 May 2012 23:05

Just for the record - the spindle angle on a ZX strut is 10 degrees, just like the PL510, (HL)A510, 200SX (S10 and S110) and 300ZX (first gen) among others.

I set out to gather this information many years ago when I too was searching to reduce scrub radius as much as possible when redesigning or choosing the components for my front suspension on the Bronze. I too had blindly believed that the 280ZX struts were also a different angle. But as you can see, it has been bad information that has been floating around for many years. I measured all of the above and published this list included below about 5 years ago. I hope it helps (file link is just above the photo)

BTW, I ended up using a 280ZX spindle/rotor on an A510 strut to get the wheel as close to the strut as possible, I then lengthened the LCA by ¾” to compensate on for the lost track width…
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kayakdude07
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Re: 510 scrub radius and more calculator..... Needs YOUR hel

Postby kayakdude07 » 15 May 2012 15:34

I have done some reading on the bluebirds list and it seems like the 280zx strut spindle angle is slightly disputed...
http://www.datsun510.com/bbmail/2006-2/5878.html

This image from datsun510.com also supports your findings....
Image

I have 2 sets of zx struts. 1 set with 510 perches that I ran on the car before with the 14" early 200sx rims and the set I'm am running now that are done up with coilovers and zxT snowflakes.

The first setup with the 200sx rims and 510 perches/random springs suffered visible from positive camber issues (no camber plates first year).

After doing up the coilovers on my other set of zx struts and installing them with camber plates and everything this year, I am maxing out my camber plates (positive) to give me front camber of between 0* and -0.5*.....

I am thinking that I may be a victim of crazy angle zx struts. :shock:
When I get home I will most definitely take them all apart to compare and try to get to the bottom of this
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Re: 510 scrub radius and more calculator..... Needs YOUR hel

Postby HudsonMC » 16 May 2012 06:41

kayakdude07 wrote:After doing up the coilovers on my other set of zx struts and installing them with camber plates and everything this year, I am maxing out my camber plates (positive) to give me front camber of between 0* and -0.5*.....

I am thinking that I may be a victim of crazy angle zx struts. :shock:
When I get home I will most definitely take them all apart to compare and try to get to the bottom of this


Wow, what does the rest of your front suspension look like? Any other adjustable bits that could be causing your issues?

Edit: sorry about the thread jack. This is a really cool spread sheet, and the prospect of wheel selection is very exciting to me. Let me know if you need any measurements. I've got a bare 2 door shell in my garage right now :)

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Re: 510 scrub radius and more calculator..... Needs YOUR hel

Postby kayakdude07 » 16 May 2012 18:48

HudsonMC wrote:Wow, what does the rest of your front suspension look like? Any other adjustable bits that could be causing your issues?

Setup #1
Well the setup I started with was 280zx struts/hubs/brakes with 510 perches and random springs with a 510 top hat. Stock TC rods and stock LCAs with no spacer. Rims were Datsun 200sx rims 14x5.5+25 with 195/60/14 without spacer.
Image
Image

Setup #2
After getting coilover bits, T3 RCAs, T3 TC rods, and GC camber plates, I set up coilovers on another set of 280zx struts. I did check the p/n's and realized that they were different but I didn't think much of it. The 14s had a positive offset and didn't clear the steering arms after installation of the T3 RCAs. I was in a pickle and managed to borrow those sweet snowflakes off of a friend. They are 15x6+10 with 195/45/15 without spacer. LCAs are still stock and the TC rods are not adjusted much at all because of the stock bushings on the LCAs. The camber plates are nearly max positive because I wanted to get rid of my "JDM yo" camber before I actually drove the car.

Image

The only camber adjusting part of setup 2 is the camber plates. I was under the impression that most 510 guys got camber plates to fix their positive camber from the zx struts (or get negative). My camber plates are doing the exact opposite. I'm confused and my car is far away but I will figure it out once I'm back home.

Another thing that changed camber slightly were the T3 RCAs because they alter the distance from the strut to the LCA mounting bolt. This happens because they change the point where the strut attaches in relation to the LCA arc. Assuming the LCAs are 11" long, we are talking about a lateral movement of the strut of <3/32" (assuming the LCA deviated from perfect horizontal; less otherwise).

Thanks for the comment on the spreadsheet; I'm glad you liked it. The main focus was on scrub radius but the inner rim clearance would be a nice addition for most. Outer clearance is impossible to account for because just about everything can change for people (camber, caster/wheel placement, LCA length, flared fenders, and the list goes on...)

On second thought I only really need to know one rim with very tight inside clearance for the 510 strut and 280zx strut. I've got the s12 under control. I am concerned with the clearance to the bare strut and not to coilover sleeves or anything like that. If you could help out with that it would be sweet. :D
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Update!

Postby kayakdude07 » 16 May 2012 20:46

The S12 wheel fitment option is up. A quick note is that it uses 4 or 5 lug Z31 hubs like these for the calculations and not the stock S12 struts for slip on rotors.
Image
Once I figure out the offset difference between stock s12 hubs and z31 hubs I will factor this into the calculator to fix the scrub radius calculation...I might add strut+hub options later on :D

I also added tire height in inches because I see that someone calculated that.
I still need maximum backspace data for the 510 and 280zx struts to be able to build the inner clearance calculator for those struts. Any helpers?? :D
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Re: 510 scrub radius and wheel fitment calc..... Help please

Postby kayakdude07 » 12 Jun 2012 18:55

I didn't realize you uploaded your data sheet Byron... :oops:
That's exactly what I was looking for.

Thanks!
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Re: 510 scrub radius and wheel fitment calc..... Help please

Postby Byron510 » 12 Jun 2012 21:41

kayakdude07 wrote:I didn't realize you uploaded your data sheet Byron... :oops:
That's exactly what I was looking for.

Thanks!


No worries, glad you found it. It does get hidden as an attachment on this site.

I'd sure like to find the mythical 12 degree 280ZX struts. I have measured many of them over the years, all were 10 degrees....

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Boy Blunder
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Re: 510 scrub radius and wheel fitment calc

Postby Boy Blunder » 14 Feb 2015 20:54

I'd like to revise this old thread after doing some research...

Does anyone know what the "original" scrub radius of the 510 is?

By my calculations.. running 13x7 libres with a 20x9.5x13 slick increases the scrub radius by nearly 34mm!

At what point does increasing the scrub radius become a hinderance? By playing with wheel offset and size it appears you can curve the increase scrub radius back towards the original figure, whatever that is.

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Re: 510 scrub radius and wheel fitment calc

Postby JordanTr » 14 Feb 2015 21:26

Wow. This is an old thread.

That sounds correct. To keep the scrub radius constant you need to maintain both the tire height and rim offset. Or increase tire height and decrease rim offset (more negative). Or decrease tire height and increase rim offset. It can be pretty easily visualized with the image below.

It sounds like you know what's up with that. Both your short tires and low offset means the scrub radius will increase quite a bit.

In general, larger scrub radius means easier parking (rolling instead of scrubbing) but the ruts in the road have that much more leverage on the steering. I run a 22.8" tire at an equivalent offset of +25 on S12V6 struts. Feels fine but I'm no expert.

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Re: 510 scrub radius and wheel fitment calc

Postby Boy Blunder » 14 Feb 2015 21:34

Having a race car that you need to learn about forces you do dive into the depths of the internet :shock:

Even more interesting.. having adjustable camber plates and lower control arms effectively alters the scrub radius as well... :shock:

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Re: 510 scrub radius and wheel fitment calc

Postby Sleepys-14 » 15 Feb 2015 10:15

Well, scrub radius doesn't change with the camber plate or lower control arms. It only changes between the strut and the wheel. On our spindle type front suspension, then you can only change it by changing either the hub offset or wheel offset. If you had the 2 bolts on the knuckle that bolt to the strut, then you can change the scrub radius.
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Re: 510 scrub radius and wheel fitment calc

Postby bertvorgon » 15 Feb 2015 10:36

While I have not the most optimum scrub (+), the increase in track width at the slaloms more than made up for that deficiency in my experience. I was stuck at the time( 1983 ) as our only option was the 280ZX strut. To that point, I made the car as wide as possible and ran as wide a slick as I could, 9.5".

I made sure my bump curve was good, and of course totally minimized any rear steer characteristics, so the car at least responded to my commands as best as possible.
"Racing makes heroin addiction look like a vague wish for something salty" - Peter Egan

1973 2Door Slalom/hill climb/road race / canyon carver /Giant Killer 510
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icehouse
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Re: 510 scrub radius and wheel fitment calc

Postby icehouse » 18 Feb 2015 21:27

Keith what is an "optimal scrub radius"? Seems like everything in suspension is a compromise.
I liked Hainz better...... Can we trade back?

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Re: 510 scrub radius and wheel fitment calc

Postby bertvorgon » 19 Feb 2015 07:45

Hey Jeff. You are right, EVERYTHING in the suspension is a compromise, no one thing can be perfect, as so many OTHER factors will change that one perfect setup.

Is there an optimal scrub...NO. That being said, we want about ½” POSITIVE on our 510’s.
Now, I am no expert on this, so this morning, I phoned a suspension guru, North American A- MOD champion, and builder of his own chassis”s. He also used to race a 510 and helped me before I ran into Andy at Specialty. He concurred with me that on our 510 sedans, some positive scrub is good. It gives a “feel” to the steering, but, done to an extreme, with too wide an offset wheel and/or spacers, it will cause weird “kicks” and other steering issues in corners with some bumpy issues. That’s what a lot of hot rod guys do, totally screw up their scrub, by putting on big offset rims, then even space it out.
On my car, I used my adjustable lower control arms to move the strut out (increase’s track ), then use a zero offset wheel, with a ½” spacer. This also helps big time to decrease lateral weight transfer, which is a big bonus for a car that likely has a more under steering tendency than not.

I would sometimes put another spacer on the front, depending on how tight the course was, to really help decrease front under steer. I then ran 6” degrees of castor ( I also lifted weights for a few year and we made a steering rig to work those arm muscles) I made sure my bump never toed OUT and put the 280Zx steering arms on my car to quicken the steering.

So, bottom line I guess, is getting any car setup as I always say, to be what YOU like and feel, and for the type of driving you do. I compromise my ease of low speed parking and steering by running 5.5 castor, for the wonderful high speed stability and straight line feel of the car.

cool visual graphic animation:

http://www.racecartuner.com/03/307.html

I have never been one to get caught up in the perfect world of mathematics, graphs, or other stuff. I read all the books and then tried to apply the fundamentals to achieve what worked for me at the time. The biggest thing on my 510 was getting rid of the rear positive toe change, THAT just transformed the car back in the early 80's and has now benefited most on this site, to keep the rear inline with an imperfect independent suspension.

These two books were my bibles for a lot of years, long before the Internet came along.
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"Racing makes heroin addiction look like a vague wish for something salty" - Peter Egan

1973 2Door Slalom/hill climb/road race / canyon carver /Giant Killer 510
1968 Vintage 3HP Mini Bike
1971 Vintage 13' BOLER trailer


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