Effects of wheel spacers/backspacing changes

So. Trying to learn new stuff here so bear with me. 

Im playing with wheels and tires. Bothe will gain much different offsets. 

First example: stock fwd econobox.  Stock wheels, tires, everything. Just a daily. 

Say i put one or two inch spacers on it to fill up the wheelwell better. What does that actually do? What changes will i feel from behind the wheel, see in maintenance and economy, etc? Parking lot manners or turn radius change?

Second example: track car. Go from a fwd offsed 17x8, to a 15x10 with 3 inches of backspace. Radically different. 

What changes will i feel? Maintenance? Speed? Handling at the limits chhange? 

Im guessing the answers to both scenarios will be the same, however due to extremes will be a difference of magnitude of change.

So, learn me please!

Basically, you'll be increasing the scrub radius- the distance between where the steering axis hits the ground, and the center of the tire contact patch.  The most immediately noticeable effect will be that the tire has more leverage, so the car could get darty and in the case of your giant 15x10s maybe even have too much steering effort or be able to yank the wheel out of your hands.  Depends entirely on geometry though.

There are also other effects that can come into play, although the degree varies depending on the suspension geometry- things like "jacking" where the steering movement is actually playing a huge part in suspension loads, although that's typically at the extreme (think kart) end of the spectrum.

If you add those 2" wheel spacers to the stock FWD econobox, the biggest change you'd see is that your steering would be extremely messed up - super-heavy due to the increased scrub radius, and any tramlining effects would be greatly worsened. Front tire wear would be accelerated mainly due to the scrub radius increase, wheel bearing wear would be slightly accelerated, also the suspension may seem slightly softer since you've effectively changed the motion ratio slightly (sway bars would be affected the same way). You'll also need more clearance in the front wheels wells since the front wheels will sweep more area as they turn. The rear suspension would be OK apart from the bearing wear and motion ratio issues.

For the second scenario, some quick back-of-the-napkin math tells me that the change would be equivalent to adding a several-inch-thick spacer, so it would be the same as above but many times worse. These problems are all a matter of degree, and some things that you could've ignored with the 2" spacer is now a bigger issue: camber affecting ride height. Negative camber will now bring the car meaningfully closer to the ground and positive camber will raise it, so your camber curve now also becomes a bit of a ride height curve. You might also start exposing mechanical weaknesses in the suspension due to the increased leverage the wheels now have, beyond just wheel bearing wear.

Don't some FWD cars start with a bit of negative scrub radius?  If so, it'll likely end up positive after moving the wheels out, which will give a bit more torque steer and depending on how far positive it is, may cause significant tramlining, etc. 

Actually, the motion ratio doesn't change. I used to think that way as well but someone here pointed it out to me. The pivot points that determine the motion ratio are unaffected. Although I wonder how camber gain would change that - probably only enough to be theoretical but not practical. And this will be more true for some types of suspension than others.

You'll get less weight transfer with a wider track as well, which means more cornering traction overall. That's actually a way to adjust handling balance.

I would think motion ratio would change slightly in most suspension layouts.  The distance from the center of the wheel to the inboard suspension pivots gets longer as you push the wheel out, so the wheel will move slightly further for a given amount of suspension travel.  Mind you, it won't have an effect on suspension behavior due to body roll, as nothing relevant to body vs ground movement has changed.  But it should have a slight softening effect when hitting a bump, as the wheel will have slightly more leverage to compress the suspension. 

I did something similar in a mini.  Went from 16x7 ET 45 to 15x9 ET 35 with a 25mm spacer (effective ET10).  The inside of the tire was roughly in the same place and the outside was 2" further out.  It isn't exactly what you are proposing but I think it is close.

Steering effort went way up, the screwy scrub radius made turning in place weird, and tramlining was significantly worse.  I went through a set of wheel bearings pretty fast but it was an autocross car so it is hard to tell if the problem was the wrong wheels or autocross grip levels.  The car was pretty darty on the highway but not undriveable.  I daily drove it for three years until I had kids and couldn't fit them in the back.  I might do it again once all my kids are forward facing.

In reply to rslifkin :

I think the double wishbone is the exception to the rule, and that's where I live my life :) 

Thanks yall!

Lot to think about in here before i do anything.....

So how do we go about measuring and calculating these factors? I've wondered the exact same thing on my E36. I have a wide range of spacers and am not totally sure if I'm doing a good or bad thing by adjusting them.

Generally you want to keep your offset as close to stock as possible...some cars can benefit from small changes one way or another (usually a small increase in scrub radius), but for a sports car when you deviate from stock by more than 0.5" there could be issues, more than 1" and you're asking for trouble. You can get away with much bigger changes on offroad 4x4s.

The offset measurement is independent of wheel width, but backspace is relative to wheel width - including the lip! Online calculator:

http://www.gtsparkplugs.com/Backspace-Offset-Calculator.html

I went from 5.5"x15 wheels to 10"x15 on my Volvo 1800 front suspension. Rear backspacing was reduced by 1/4". With a 13" steering wheel and manual steering, parallel parking is a heck of a workout. On an asphalt highway, the track width is much narrower than the semi truck road ruts and the car veers left or right depending on which rut it is riding in. The faster I drive, the less it is an issue. It corners great.

Regarding the 'motion ratio' effects...

If you do a free body diagram on a swing axle, with the instant center at the inner pivot, you'll see that 100% of any offset change affects the motion ratio.

If you do a free body diagram on a pure trailing arm or equal length parallel double wishbone, with the instant center at infinity, you'll see that 0% of any offset change affects the motion ratio.

If your instant center is somewhere in between, so is the effect from the offset. So how does one factor that in correctly? I'm glad you asked.

In addition to the spring angle correction factor, most common motion ratio equations and calculators use the length from the inner pivot to the spring center line divided by either the length from the inner pivot to the outer pivot or the length from the inner pivot to the wheel center line. The former is incomplete by virtue of not accounting for the instant center, but is a reasonable approximation if the actual instant center location is not actively known. The latter is simply wrong.

The complete answer is the length from the inner pivot to the spring center line divided by the length from the inner pivot to the outer pivot, then multiplied by the length from the instant center to the outer pivot divided by the length from the instant center to the wheel center line. Because most suspensions use an instant center at least 3x-4x longer than the control arm, that means the the wheel center line (offset) effect is relatively minimal... Especially if you're not also getting to the level of factoring the center of pressure location change (moves inboard from wheel center line) on the tire resulting from both static and dynamic camber changes. The rest of the effects from the offset are simply a moment created that loads the control arms axially with no effect on motion ratio or wheel rate.

If you add spacers to one end, that end should generate more cornering grip. So if you are understeering and you add spacers in the front, the car should understeer less or oversteer more. Interestingly, this is different than adjusting swaybars as the latter doesn't affect total weight transfer, just distribution. The wider track from spacers will decrease overall transfer and thus increase grip.

I think I might try and measure my scrub radius with a cheap laser level. I'm curious because I have camber/caster plates maxed out, a shim between the knuckle and bottom of the strut mount and a wide range of spacers to try to use to correct. It's entirely possible I'm way out of whack. This should measure it pretty well right?

• Take the wheel off
• Line laser up with the strut with the light itself at the height of the bottom of the wheel when mounted.
• Put the wheel back on and mark where the laser line hits the bottom of the tire. This is point A.
• Measure the halfway point of the tire width. This is point B.
• Scrub radius is the difference between point A and point B.

In reply to PunchyWrench - Ed Higginbotham :

Kind of?  Technically the line is through the top mount bearing and the lower balljoint, not necessarily the strut body itself.

Ah ok. I think the same method should work using those two points. That's actually good news. I thought adding a shim to the bottom mount of the strut housing would move point A in. Sounds like that's not the case since the lower ball joint is displaced with the knuckle. This is all making my head swim. I should just go start measuring stuff. I'll post what I find in my build thread to not derail Michael's thread any more.

In reply to PunchyWrench - Ed Higginbotham :

Hell no! Post here. Im learning stuff i didn't know I didn't know. 

And it sounds like the daily (macstrut prius) wont be terribly changed. However the rx8 front suspension swap with 15x10 may wind up being a handful. Maybe. Still wrapping my head around it.

As a note, for a car starting out with positive scrub radius, adding camber in some suspension designs (including Mcstruts and a solid axle if you add camber with offset ball joints) will reduce the scrub radius.  So you'd actually need to push the wheels out a bit to get back to where you started. 

In reply to rslifkin :

Yes. Thanks for mentioning this. Ride height changes also result in scrub radius changes on Macpherson struts. If the only changes are wheel/tire and spacers, then scrub radius becomes a concern in a hurry. But if the car is Mac strut and also being lowered, and/or changing offset, it is time to start doing some modeling and/or measuring. After mucking up one of my AE86s by lowering it 2.75" with negative camber roll center adjusters plus a wheel width and offset change, I started researching how to fix the scrub radius for my new ride height, i.e. standard roll center adjusters with longer control arms, longer tie rods to match, and lower offset wheels.

Ok so I got my cheapo laser level and did some probably fairly inaccurate measuring to try and find what my scrub radius situation is. Here's the general setup. Green cube thing is the laser.

I set the laser line on an angle with a piece of cardboard and moved that wedge in and out the get the correct angle to make a straight line from the top bearing of the strut down to the lower ball joint. 

Then I mounted the wheel and marked on the tire where the lie hit at the lowest point of the tire.

Then I took the tire off and measured where that mark fell in relation to center. 

Looks like it's within a couple millimeters of center. And that's with no spacer. 

But then I got to thinking and decided to do the same thing but to try and put the suspension under load. Or at least closer to how it would sit on the ground. Same order of opperations except with a jack involved. 

This time I got a measurement of roughly 5mm of positive scrub radius. Adding spacers will make this even more positive. I ran a 12mm spacer at the first race. I was going to run a 15mm spacer at the next. a 15mm spacer would make the scrub radius like 20mm. Is that like super extreme? I know for a fact that at least one of the fastest Spec3 guys runs 15mm spacers up front and 12mm in the rear.

20mm scrub radius is far from extreme.  It's more than some cars run (plenty are near zero or negative if FWD), but there have also been cars that have come out of the factory with more than 20mm of positive scrub. 

Excellent! Well then I'm going to try the 15mm spacers and not worry about scrub radius. Thanks for the guidance. And thanks for asking this question, Michael. I think next I might try to diagram everything to figure out my motion ratio.

Just an fyi, extreme scrub radius is what my 1800 has. I haven't measured it, but I did draw a diagram of what it would be after I install the front suspension from the 164 sedan's front suspension which is very similar to the 1800's A arm suspension. 4" scrub radius towards the inside tire edge from the tire center line. Is that a positive or negative scrub radius?