Camber adjustment - top mounts vs camber bolts?

[Gerald81]

Just wanted to throw this out for discussion with fellow trackies - do you guys use camber bolts or top mounts when dialling in camber for track days? For the purpose of this thread, I would like to distinguish between camber adjustment based on the location of the adjustment, i.e. via:

1) Upper - top mounts (i.e. camber plates)
2) Lower - wheel hub (i.e. camber bolts, offset mounting bolts, etc.)

I've observed that a lot of trackies who have installed aftermarket coilovers with adjustable top mounts naturally use those for camber adjustments, perhaps due to its ease and convenience. Adjustments via the wheel hub (eg. camber bolts) take a little more time as you have to raise the vehicle and/or remove the wheel to get to them. Also there appears to be some sentiment that the bolts do not hold their alignment very well under hard driving or on harsh terrain.

However, I have read that camber adjustment for the front axle via the top mounts can sometimes be undesirable, because you will inevitably change the vehicle's steering axis inclination (SAI) angle. The following picture probably helps define SAI angle better than I could possibly explain with words:

As you can visualise from the above picture, most camber plates/top mounts work to increase negative camber at the wheel by tilting the upper location of the suspension strut inwards towards the chassis, which will result in an SAI angle that is higher than OEM-spec.

I'm no expert on wheel alignment angles, but my understanding is that the SAI angle defines the scrub radius for the front tyres when they turn, and is necessary on most passenger vehicles. On the flipside, one drawback of the SAI angle is that in order to ensure that the wheel remains perpendicular to the ground despite having an angled steering axis, the attachment between the wheel hub and the suspension strut will need to be angled to compensate for this. Ignoring other alignment parameters such as caster angle, the net result of having a positive SAI angle is that the front wheel actually gains positive camber as steering angle increases, which is generally undesirable.

On the other hand, camber adjustment done at the hub itself (assuming you are adjusting for more negative camber) actually reduces the angle between the hub and the suspension strut, and therefore reduces positive camber gain as steering angle increases.

At this point in time, based on my understanding of the SAI angle and how it interacts with the wheel, I'm inclined to believe that adjusting for camber via camber bolts and/or using the elongated mounting holes that some coilovers offer might be a superior alternative to making adjustments via the top mounts. Of course I could be missing out on something that I'm unaware of as well, perhaps the difference in suspension design (Macpherson strut vs double wishbone) might play a signficant role as well - what do you guys think?

 

[Shaun]

Re: Camber adjustment - top mounts vs camber bolts?

Motion ratio, ride height, roll centers also change if you move top mount. Eccentrics in the upright usually slip. More proper is to use pre formed and slipfree pills which can be swapped to change camber.

Double wishbone better at separating motion ratio, RC, from camber change even if via changing A arm length. Even then, racecars almost always use shims or pills on the upright.

For a road car that is tracked, typically the range is narrow enough to not matter. The much greater inefficiencies and compromises lay elsewhere in the car and using either top mount or eccentrics at damper bottom won't make a huge difference. Theoretically better to make the change damper bottom or upright though if you want to really split hairs.

 

[TripleM]

Re: Camber adjustment - top mounts vs camber bolts?

when the strut is pushed in to the inner chassis, the wheel becomes less vertical and hence the SAI angle shouldn't be impacted too severely, should it?

 

[pdreams]

Re: Camber adjustment - top mounts vs camber bolts?

So what decides how much camber is to be set?

Faster lap times? Tire temperatures? Tire wear?

I have only come across the amount of camber to be set but to date, no answer to why such a camber is recommended from experienced trackies.

There is a ton of info on the internet on recommended camber but it would be good to know how fellow trackies decide on their camber settings.

 

[Shaun]

Re: Camber adjustment - top mounts vs camber bolts?

So what decides how much camber is to be set?
Faster lap times? Tire temperatures? Tire wear?

Mate, that's like asking "so what decides how much to steer?"
It depends on the vehicle, the nature of the course, of the race, tire. Risks, weather, etc.

I have only come across the amount of camber to be set but to date, no answer to why such a camber is recommended from experienced trackies.
There is a ton of info on the internet on recommended camber but it would be good to know how fellow trackies decide on their camber settings.

This is because it quickly expands to book volume. There can't be any quick and easy answers. There are rules of thumb for extremely specific applications, but they are either too general or if they are really good, they will not be shared at all (based on too much work and experience, too valuable). Even if it were given away, the person blindly following would get completely lost at the next revision of the suspension or tire.

Short version is that you:

a) have a known car spec, or at least known rules that end up defining main spec (min weight, track, wheelbase, CGz, aeroloads, power, tire class and max size, mass and mass distribution) so you know broad loads and transfers.

b) pick best tire that suits the application (knowing broad vehicle spec as in (a). You pick by using manufacturer supplied data (lab rig or track tested) or running your own tests if you have the budget to. Both options have their own strengths and weaknesses and it is extremely complex when getting into fine detail. You are looking at all tire forces and moments, tire spring rates, through a wide range of loads, camber angles, slip ratios, and slip angles, so you end up with a bunch of 2, 3, even 4D plots. Attached is one out of easily 20 plots.

c) you design the suspension system to make the best use the selected tire for the typical course expected. There will be some flexibility in the suspension system via variable mounting points, pills, shims, threaded adjusters. You know fairly accurately what the tires want based on the tire data from (b) and how they will be loaded from (a). Together with broad vehicle layout, kinematics, spring and damper rates, alignment, will all determine load transfer distribution, speed of load transfer (so tire impulse), tire slip angle and camber in turns of varying radii.

===

If there is a lack of tire data or budget to instrument the car and generate data, then you're left with broad rules of thumb, basic and much cheaper tests (or even just feel) for tire vertical and lateral stiffness - simple rig, basic deflection tests, or simple on track tests like chalking sidewalls, video or photos of sidewalls on track, dynamic tire temp IR or static probing. The last 2 are done even at high levels but more as a final check and fine tune rather than relying on it to have a better guess at major tire characteristics.

 

[pdreams]

Re: Camber adjustment - top mounts vs camber bolts?

If there is a lack of tire data or budget to instrument the car and generate data, then you're left with broad rules of thumb, basic and much cheaper tests (or even just feel) for tire vertical and lateral stiffness - simple rig, basic deflection tests, or simple on track tests like chalking sidewalls, video or photos of sidewalls on track, dynamic tire temp IR or static probing. The last 2 are done even at high levels but more as a final check and fine tune rather than relying on it to have a better guess at major tire characteristics.

Thanks. Its good to know (a) and move on to (b) and (c), just that (b) and (c) are not going to be likely accessible to folks like myself.

Thus, perhaps dynamic tire temp IR or static probing will be most easily accessible to the once a month track enthusiast, even if they only function as a final check and not reliance.

From the track days I have been going to, not much has changed with regards to tire setting with the most common phenomenon being the "Ultra sensitive driver - USD" coming into pits after a couple of hot laps and setting his pressures with a tire gauge because he felt or he was told that his pressures are too high or low (normally high). Not forgetting that the USD was operating already on a given set of camber settings which he most likely will declare at the end of the trackday to be some degrees off based on his sensitivity.

Is it done anyway differently for the track sessions you are involved in or because the influence of having done things in even the most budget and structured manner is so small that we should probably ignore it and just get on with the driving?

I apologies to the OP if this seems out of topic but they do lead to the same end, which is the tire.

 

[Shaun]

Re: Camber adjustment - top mounts vs camber bolts?

From the track days I have been going to, not much has changed with regards to tire setting with the most common phenomenon being the "Ultra sensitive driver - USD" coming into pits after a couple of hot laps and setting his pressures with a tire gauge because he felt or he was told that his pressures are too high or low (normally high). Not forgetting that the USD was operating already on a given set of camber settings which he most likely will declare at the end of the trackday to be some degrees off based on his sensitivity.

USDs almost never are USDs. It's always stories, or friend of a friend, some months or years ago, etc. When it is here and now, blind testing, quantified, logged, to completely eliminate variables and the leading of soft minds, then either USD runs away, or the results come back that USD really was UISD (ultra insensitive driver) or worse still OFD (opposite feel driver).


Every manager wants to believe their driver is magic. Many a driver's buddy or mechanic wants to tell the world he is good friends / has worked with, Senna reincarnate, so they go around talking up everything, and legend becomes 'fact'. But it's just not, and it didn't come out of a proper scientific test. I've been fortunate to run some proper tests on some very good drivers far above the mass of 'USDs', and none of them ever was an USD or even close to it.


Some drivers just feel good having a crew to adjust the car for whatever they feel, despite being 2 - 4 % off the pace (line, inputs, aggression). In itself this massive 2 - 4% is enough to stop wasting time messing around with 1 psi here and there to fine tune balance. On top of this driver issue, there is the vehicle - especially roadcars and their weight, fading badly in multiple ways (dampers, brakes, tires, engine, maps) and 4 different corners and asymmetrically, all 4 at different rates, also recovering at different rates when they insert their cool down laps between their short 3 - 5 lap stints, rubber being laid down or collecting offline at different rates depending on car count and activeness, rubber type helping or hurting depending on class of tire most cars are running, where it is laid down dependent on average car and driver class, affecting optimum line, track heating or cooling (moving into or out of peak grip) at different rates depending on weather. It's all even more reason not to mess around with fine tuning to that degree.

Is it done anyway differently for the track sessions you are involved in or because the influence of having done things in even the most budget and structured manner is so small that we should probably ignore it and just get on with the driving?

Unless consistently within 0.5% (time) of reference 3-5 lap average (average to average) after cutting traffic out (data), there's no real use moving around in single psis. It ends up being a waste of time, a distraction - mental issue more than anything. Even if within 0.2% vs sustainable reference(s) (hence 3-5 lap averages and not 1 lap killers), it's much more worth exploring what the reference driver(s) are doing for the advantage. If it is simple trajectory, then it's usually not related to handling unless it is a very difficult get it into that position (turns in close succession).


Better to learn what quicker drivers do (and why)when car's a little loose, a little tight, rather than always change car to suit driving that hasn't been worked hard on at all and therefore is significantly flawed, unadaptive (either through ignorance or inability). Given the variables listed in earlier para, any car, especially a road car, will the majority of the time be handling far from perfectly anyway. Tire pressure changes alone won't be enough to fix it or improve it significantly as it is trading stability for grip.


Get tire manufacturer (not hearsay) recommended hot pressure range for vehicle weight and application, set it right in middle of that recommended range, and then drive. Basic indicators for tire roll, visual on wear, basic static probing, all ok? Drive on!

 

[pdreams]

Re: Camber adjustment - top mounts vs camber bolts?

Get tire manufacturer (not hearsay) recommended hot pressure range for vehicle weight and application, set it right in middle of that recommended range, and then drive. Basic indicators for tire roll, visual on wear, basic static probing, all ok? Drive on!

Couldn't say it any better..

 

[TripleM]

Re: Camber adjustment - top mounts vs camber bolts?

I commute to work by bus, if the bus corners a bit harder and if my two feet isn't far a part enough, I couldn't stand planted.