Technical Buick drum/scrub radius question

I would think that should read "The further the tire center line contacts the pavement outside the king pin center line, the harder the steering will become." It is a proporsional thing.

 

ooh - and don't forget....
a 6j wheel with a 3 and 5/8ths back space does not equal a 5/8ths positive offset / ET
as a 6J steel wheel is actually 7 inches wide to the outer rim (where you would measure the backspace from)
so in fact has an ET of 1/8th inch - or 3.25mm in new money


so what are the known numbers for a 1940 spindle, hub, wheel offset and tyre diameter
or if you have a 40 spindle with a 70-77 gm rotor
what wheel offset and tyre diameter do you aim for.
or if you have a 6j by 15'' wheel with a 1/8th positive offset
what diameter tyres should I buy

 

Absolutely correct.
Wheel offset (or ET) can be hard a little hard for some to understand, but it is a good idea to know what it means.

Back space : measurement from the wheel mounting surface to the inside edge of the wheel.

Offset/ET : measurement from the wheel mounting surface to the center line of the wheel. Positive (+) if the center line is inboard of the mounting surface. Negative (-) if the center line is outboard of the mounting surface.

 

Found it. This is a post on scrub radius by @Ned Ludd in another thread last year. For me, this describes it beautifully.

"Loquacious" means tending to talk a lot. A great way to describe road feel through the steering....

 

Question for the educated suspension experts here. I just want to check my logic.
If I have a truck with a beam front end and put smaller diameter wheels without changing the rim offset, will I increase the positive scrub radius?

Or if on the same truck I put a much larger diameter tyre/wheel, I may end up with negative scrub radius unless I widen the track by moving the wheels outboard or a more positive ET rim.

Steve

 

what I have found is.
using a 37-41 spindle hub widths are as follows
stock the 40 hub is 1/8 under 2 3/4''
an f1 hub is 2 3/4''
an f100 hub is 2 1/4''
when using the 70-77 chevy rotor its 2 3/4
using a wheel with a 1/8 positive offset will bring the wheel centre to stock 1940
do 1940 ford wheels have a zero offset? @16''
what is the diameter of the tyre

do I go for a 195 80 15 tyre - I need to use commercial tyres and want to keep them the same front and rear.

 

1: I am not a suspension expert by any stretch of the imagination.
2: Smaller rolling diameter will increase positive scrub radius.
3: Larger rolling diameter will decrease positive scrub radius.
4: It is unlikely you will get into negative figures. Negative scrub radius is undesirable on rear wheel drive vehicles.
5: Widening the track width is acheived by using wheels with less positive offset or negative offset.

 

Ok @nobby , Let's see if I have this correct.

By using an F100 hub and the 70-77 Chevy rotor, you end up with the wheel mounting surface in the exact same place as the standard 1940 Ford setup. Is that right?

1940 Ford has a king pin inclination of 7 degrees (as far as I can determine). This would mean that for every 1" you change the height of the spindle, you move the scrub radius by 1/8". Raising the spindle moves the scrub radius in (towards negative). Lowering the spindle moves it out (more positive). Now, to change the spindle height by 1" you would need to fit tyres that are 2" taller. As you can see, tyre diameter really has little effect on the scrub radius.

The standard tyre size on the 1940 Ford was 6.00-16. This gives a rolling diameter of 28.9". If you were to use a tyre with the same diameter, it would not shift the scrub radius from Henry's original setting. A 195/80r15c tyre will have a diameter of 27.3" This is 1.6" less than standard diameter and will drop the spindle height by 0.8". That will add 3/32 of an inch positive scrub radius (2.38mm), and should not make any noticable difference at all.


Wheel offset is a different kettle of fish. At a given tyre diameter, any change in offset changes the scrub radius by the same amount. This is where it can get out of hand quickly, and you should be careful. Big offset changes are not needed and not generally recommended.

So, as an example, reducing the tyre diameter by 2" will lower the spindle by 1". This will give you 1/8" more positive scrub radius over standard, and would not be a concern. If you wanted to correct this back to standard, you would need a wheel with 1/8" (or about 3mm) more positive offset, which moves the wheel slightly inwards.
Please note: these figures only work for 7 degrees king pin inclination.

I can't find a wheel offset figure for standard 1940 Ford wheels anywhere. Maybe someone can let us know and you will have your starting point.

Hope this helps...........

EDIT
1940 Ford standard wheel specs (as far as I can find)
16 x 4
5 x 5.5" bolt pattern
3.25" backspacing
This means your starting point for any wheel should be an offset/ET of around 19mm positive.

 

btt

 

@nobby

 

/\ i think this needs varification.......
is a 16 by 4J - J being where the tyre sits actually 4'' wide
or is it 5 inches outer rim to outer rim ? from where you measure the back space?
which would be 1/2'' from 3/4''
just saying, then its only 1/4'' 6mm +ET
plus of course, fitting the wider rim - a 6j, which is actually 7 inch wide

and plus the kpi = 7 great, but the spindles acutal axle isn't leve;l to the ground, I recall a camber lean of 5mm on a 26'' tall tyre.

nice to have the figures
just so there is a datum from where to begin.
as there is nothing like buying a new axle, spindles, rotors, bolt on steering arms, wheels and tyres then stringing it all out for suspension geometry and 'the basics' and it being a bit poor.

 

46 hubs and backing plates. F1 15x5 wheels

 

First up, backspace and offset are two different animals. They are related though, and you can work one out from the other. Backspace is the dimension from wheel mounting surface to the inner edge of the rim , usually in inches. Offset is the dimension from the wheel mounting surface to the center-line of the rim, usually in millimetres.

Rim width is measured at the tyre mounting collar. A 16" x 4" wheel will have a nominal total outer width of 5". Center-line of this wheel will be 2.5" in from the inner rim edge. Next we add the positive offset (+19mm) which is .75". So, 2.5" + .75" gives a backspacing of 3.25" from the inner edge to the mounting surface. These are the specs for the original 1940 Ford wheels.

An important thing to realize is that, with a certain offset, no matter what the wheel width, the center-line of the rim/tyre does not change. Any added wheel width is shared equally inwards and outwards. This does, however, change the backspace. For every inch of extra width, the backspace increases by 1/2".

Because scrub radius relates to tyre center-line, it is advisable to work out the offset you need first. You can then work out the backspace you need with different width wheels.

1940 Ford alignment specs give a camber setting of a minimum of +1/4 degree to a maximum of +1 degree. Positive camber will lean the top of the tyre out and the bottom in. With a 29" (standard original diameter) tyre, the minimum 1/4 degree will shift the tyre center-line 1/16" at the road contact patch.The reason I have used the minimum 1/4 degree is because positive camber will actually move the bottom of the tyre inwards, which works in your favour if you are fitting a smaller diameter, but it really is not worth including in your calculations. Besides, this should be a constant with the correct spindles.

 

Let's see if this sounds familiar with some of the builds we see.

1: Want "big and littles", so puts smaller diameter wheels/tyres on the front - Increases scrub radius
2: Want wider front wheels/tyres, but this makes them foul on the split wishbones/4 bars/etc.
3: To accomodate wider tyres, fit wheels with less backspace - Increases scrub radius
4: Want Buick finned drums or similar, and want them to be 'seen'.
5: To accomodate the 'seen' brakes, fit wheels with even less backspace - Increases scrub radius
6: Wonders why the handling is not so good.
7: "It's a hot rod. Don't they all handle like this???" or "Mine handles fine, no problem."

And that's just scrub radius. There is lots more to consider.

 

Well, it seems my innocent questions have led us not down a rabbit hole but into a goldmine of information! A big thank you to twenty8 and nobby for all of the priceless tech. This is the train of thought that I was on as I choose which parts to use for the front end of the pickup. Often we just bolt on the usual parts and hope for the best but the above information helps us visualize, compare and select the best parts from the start. Thanks guys!

 

Thanks @bchctybob
In answer to one of the questions in your original post, the width of the axle will have no effect on scrub radius. It will simply move everything in or out 'as a set', if that makes sense. Use the axle width to get the width you need for your wheels/tyres (track width) while using the brakes/etc you want. Fine tune for scrub radius with wheel offset/backspace.
And remember, be sensible with your objectives (see post #44).........

PM me if you would like me to do some calculating for you.

 

So, is a 195 80 15. 2021 Suzuki Jimny tyre 29 inches in diameter
I looked they are 27.3 inch
So they have the load rating on them which is great, as 4 X 4 tyres
Add power steering and even with a 6 to 7 in nches of tyre, will be like a stock modern beam axle or solid 4x4

 

X=?-4