Roger440 Posted July 12, 2006 Report Share Posted July 12, 2006 Recently did some adjustments on a Porsche 996 after the customer had changed the suspension and lowered it a touch. Camber and toe front and back was wrong. Said customer had some Very nice expensive wheels, and as our equipment is of the rim clamp variety -(is that the right description?) i elected to change the wheels for some "less good" ones we have lying around to avoid any possilbility of marking them. The customer posted on a forum saying how happy he was only for someone to come along and say that his garage must be daft changing the wheels as the alignment would then be wrong. Having thought about it at some length, i am still of the opinion this is nonsense. Am i missing something. Yes, i did check the run out when i mounted the equipment, and yes the wheels fit correctly (they are off another porsche) Any views? Quote Link to comment Share on other sites More sharing options...
discount tyres dan Posted July 12, 2006 Report Share Posted July 12, 2006 sorry dude but rim diameter does affect the readings, also the offset would need to be the same. Quote Link to comment Share on other sites More sharing options...
scorps Posted July 12, 2006 Report Share Posted July 12, 2006 I cant help you im afraid.......but to more important things.....Is that a Holden Maloo R8 6ltr V8? Yum.....yum Quote Link to comment Share on other sites More sharing options...
Tony Posted July 12, 2006 Report Share Posted July 12, 2006 Recently did some adjustments on a Porsche 996 after the customer had changed the suspension and lowered it a touch. Camber and toe front and back was wrong. Said customer had some Very nice expensive wheels, and as our equipment is of the rim clamp variety -(is that the right description?) i elected to change the wheels for some "less good" ones we have lying around to avoid any possilbility of marking them. The customer posted on a forum saying how happy he was only for someone to come along and say that his garage must be daft changing the wheels as the alignment would then be wrong. Having thought about it at some length, i am still of the opinion this is nonsense. Am i missing something. Yes, i did check the run out when i mounted the equipment, and yes the wheels fit correctly (they are off another porsche) Any views? As dan said the wheels diameter and off-set will become the cameras reference scale, if there is a deviation between the measured rims and the driven rims then the set-up would be redundant... If you want to continue a more in depth explanation please say so i would love to oblige. Quote Link to comment Share on other sites More sharing options...
Roger440 Posted July 12, 2006 Author Report Share Posted July 12, 2006 The wheels were the same offset, but were 1 inch smaller in diameter. Could you explain where/what the difference would be? Im not quite sure how that would be. Yes, that is a holden SS ute (sadly not a maloo) with 5.7 V8 - oh and a turbo on it too! OOps, sorry tony, was writing my post at same time as you did. Yes yes yes, please explain. Not sure i entirely follow your first stab at an explanation either. Quote Link to comment Share on other sites More sharing options...
discount tyres dan Posted July 12, 2006 Report Share Posted July 12, 2006 we set up a civic type-r once, it had 17" rims on, after doing the geom we set the same car up again but told the computer it had 24" rims on, all the reading were totaly wrong.. if i remeber right the front toe was well over 10mm out of spec! a good experimiment if you have the time to do it. Quote Link to comment Share on other sites More sharing options...
Tony Posted July 12, 2006 Report Share Posted July 12, 2006 The wheels were the same offset, but were 1 inch smaller in diameter. Could you explain where/what the difference would be? Im not quite sure how that would be. Yes, that is a holden SS ute (sadly not a maloo) with 5.7 V8 - oh and a turbo on it too! All machines need information to generate their position in a 3D environment. The wheels diameter forms the distance calculations for the Castor/kpi swing at (20 degrees) most times. If the diameter is incorrect this allows a lower than actual position for both angles since the actual comparison by calculation is incorrect.. A 20" diameter wheel/swing must generate a different reading from a 18" diameter/wheel swing... as an example. Quote Link to comment Share on other sites More sharing options...
Roger440 Posted July 12, 2006 Author Report Share Posted July 12, 2006 I think i follow that. Firstly though, surely it is the overall wheel diameter that is critical, or is it the diameter at which the equipment is mounted? I don't know how important this is but i am not using electronic machine. Camber and caster checks done with a seperate gauge and toe done with some old but quite accurate optical gear. Yeah, i know, takes forever, but it is all we have Quote Link to comment Share on other sites More sharing options...
Tony Posted July 12, 2006 Report Share Posted July 12, 2006 I think i follow that. Firstly though, surely it is the overall wheel diameter that is critical, or is it the diameter at which the equipment is mounted? I don't know how important this is but i am not using electronic machine. Camber and caster checks done with a seperate gauge and toe done with some old but quite accurate optical gear. Yeah, i know, takes forever, but it is all we have Same result sorry... If you reference any measurements from a smaller diameter then the project results could not be the position destined for a wheel of a different diameter.... Lets try some procedure. I assume to obtain a castor measurement you need to do a swing? if yes what increment is this 10 or 20 degrees? Quote Link to comment Share on other sites More sharing options...
Roger440 Posted July 12, 2006 Author Report Share Posted July 12, 2006 20 degrees Quote Link to comment Share on other sites More sharing options...
Tony Posted July 12, 2006 Report Share Posted July 12, 2006 20 degrees To obtain the measurement the swing rotates the camera in a 'longitudinal/diagonal' curve. The amount of displacement would vary if the wheel had a different diameter... a bigger wheel would rotate the camera less than a smaller wheel both taken at 20 degrees. IF then from results the cars Geometry was set, then the addition of bigger wheels would not be consistent with the previous set-up. Quote Link to comment Share on other sites More sharing options...
Roger440 Posted July 12, 2006 Author Report Share Posted July 12, 2006 Can you clarify the "wheel diameter" Do you mean the overall wheel diameter including the tyre, or the actual rim diameter. If you mean wheel diameter, then yes i follow you. If rim diameter, then no - im still not clear. Fortunately, the castor angle was the same both sides and within spec and is non-adjustable on this car, so at least i'm semi in the clear on that measurement. Might have to continue this tommorow as i need some shut eye. Very interesting though. Quote Link to comment Share on other sites More sharing options...
Tony Posted July 12, 2006 Report Share Posted July 12, 2006 Can you clarify the "wheel diameter" Do you mean the overall wheel diameter including the tyre, or the actual rim diameter. If you mean wheel diameter, then yes i follow you. If rim diameter, then no - im still not clear. Fortunately, the castor angle was the same both sides and within spec and is non-adjustable on this car, so at least i'm semi in the clear on that measurement. Might have to continue this tommorow as i need some shut eye. Very interesting though. All machines ultimately mount to the rim. Either directly by clamping (four point position) or by resting (three point position) so it's the rim diameter by increment.... the tyre size owns the consequence of the designed Geometry positions, so the tyre is a known factor within the overall package. Quote Link to comment Share on other sites More sharing options...
Roger440 Posted July 13, 2006 Author Report Share Posted July 13, 2006 OK, so im still not sure i understand. At the risk of going off at a tangent, the porsche factory equipment mounts in the centre of the wheel so how does that affect it? Bear with me hear, Im sure you are right that rim diameter is critical, but im trying to understand exactly why rather than just take it as a given. Quote Link to comment Share on other sites More sharing options...
Tony Posted July 13, 2006 Report Share Posted July 13, 2006 OK, so im still not sure i understand. At the risk of going off at a tangent, the porsche factory equipment mounts in the centre of the wheel so how does that affect it? Bear with me hear, Im sure you are right that rim diameter is critical, but im trying to understand exactly why rather than just take it as a given. It depends on the equipment albeit mechanical or electronic by design. If the equipment is mechanical the the wheel centre would need to be manufactured, if it's electronic then the centre would only need a place in space within the cars environment. Linear positions have no need for any centres since their plane is at right angles, so it's easy to 'box' the four wheels by measurement. The camber/castor/kpi evolve within a plane not dependent of the wheel centre but the position expressed be the dimensions given... as a marker this environment is within an 'X' from this we have a imputed centre and a position for the 'swing' at a known factor of 20 degrees, after the swing the results will conclude the angles. In truth there are many ways to explain how machines gather the 3rd dimension, this i feel is of little value to your initial post (wheel diameter) this explanation needs only an understanding of amplification by reference. I welcome your reply, since i am told i ramble on, and loose the initial thread by saturation Quote Link to comment Share on other sites More sharing options...
Roger440 Posted July 14, 2006 Author Report Share Posted July 14, 2006 Errr, i'm not sure that helped a great deal!!!!!!! Im still trying to get clear in my head how mounting the (mechanical-ish) equipment on a smaller rim alters the castor reading i obtain from the 20degree swing. I tried it with 2 different wheels on the same car and got the same result! That said, only an inch difference in rim diameter, same overall wheel diameter. If i could put, say, a 10 inch wheel on it as well, maybe i would grasp it. Sadly i cant do that ! In really simplistic terms the castor angle is what it is surely, regardless of wheel size. Is it not a function of the suspension design? Yes i have re-read the castor page! The method would we use to read it is done because it is far easier to achive in the real world on a car. In reality could you not measure the angle off a suitable part of the strut (if fitted) with a gauge? As i say not very practical but is this right.? Can you simplyify the reply a bit? Quote Link to comment Share on other sites More sharing options...
Tony Posted July 14, 2006 Report Share Posted July 14, 2006 Errr, i'm not sure that helped a great deal!!!!!!! Im still trying to get clear in my head how mounting the (mechanical-ish) equipment on a smaller rim alters the castor reading i obtain from the 20degree swing. I tried it with 2 different wheels on the same car and got the same result! That said, only an inch difference in rim diameter, same overall wheel diameter. If i could put, say, a 10 inch wheel on it as well, maybe i would grasp it. Sadly i cant do that ! In really simplistic terms the castor angle is what it is surely, regardless of wheel size. Is it not a function of the suspension design? Yes i have re-read the castor page! The method would we use to read it is done because it is far easier to achieve in the real world on a car. In reality could you not measure the angle off a suitable part of the strut (if fitted) with a gauge? As i say not very practical but is this right.? Can you simplyify the reply a bit? All angles are simply placed and measured by whatever means... mechanical/electronic this is without dispute, but the consequences by manipulation are..... We can measure the camber and set it as a package for a given diameter, but if the ride has a different diameter then the absolute positions must be in dispute. The castor angle in measurement is a mechanical trail... followed by the pneumatic trail, this is the 'constant' position at 0 degrees. If the increment of measurement by wheel diameter is not consistent then there will be an element of error however small... do you agree? Quote Link to comment Share on other sites More sharing options...
Roger440 Posted July 14, 2006 Author Report Share Posted July 14, 2006 I dont disagree, because i don't follow it Let me theorise if i may? In order to calculate the castor angle, we use the usual technique of 20 degree swing etc. As i understand it we are effectively measuring an angle at 2 different points, each end of the swing, and as a result calculating what the castor angle actually is from these results. The figures obtained at each end of the swing would indeed be different for different size wheels and offsets, but surely however, the calculated figure would be the same? The castor angle imaginary or otherwise remains the same. I do follow that if the TYRE is a different diameter than the point at which the caster angle line, (if i can call it that) intersects with the ground will be different. Indeed, the bigger the wheel the further forward that imaginary intersect will be. But the actual angle is still what it is surely? Quote Link to comment Share on other sites More sharing options...
Tony Posted July 14, 2006 Report Share Posted July 14, 2006 I dont disagree, because i don't follow it Let me theorise if i may? In order to calculate the castor angle, we use the usual technique of 20 degree swing etc. As i understand it we are effectively measuring an angle at 2 different points, each end of the swing, and as a result calculating what the castor angle actually is from these results. The figures obtained at each end of the swing would indeed be different for different size wheels and offsets, but surely however, the calculated figure would be the same? The castor angle imaginary or otherwise remains the same. I do follow that if the TYRE is a different diameter than the point at which the caster angle line, (if i can call it that) intersects with the ground will be different. Indeed, the bigger the wheel the further forward that imaginary intersect will be. But the actual angle is still what it is surely? Your question is an answer... Quote Link to comment Share on other sites More sharing options...
Roger440 Posted July 14, 2006 Author Report Share Posted July 14, 2006 If i read that correctly then, the castor angle as set by the design of the suspension is a figure, lets say it is 4 degrees. This will give an intersect point with the ground a certain distance ahead of the tyre contact point with a given TYRE size. If the TYRE were bigger then the "effective" castor angle would be greater, giving the same effect as a greater castor angle, ie more than 4 degrees. Assuming the above is correct, when we measure the castor angle on a car, are we measuring the true angle or an effective angle? Quote Link to comment Share on other sites More sharing options...
Tony Posted July 14, 2006 Report Share Posted July 14, 2006 If i read that correctly then, the castor angle as set by the design of the suspension is a figure, lets say it is 4 degrees. This will give an intersect point with the ground a certain distance ahead of the tyre contact point with a given TYRE size. If the TYRE were bigger then the "effective" castor angle would be greater, giving the same effect as a greater castor angle, ie more than 4 degrees. Assuming the above is correct, when we measure the castor angle on a car, are we measuring the true angle or an effective angle? I disagree, although the mechanical trail is the same regardless of the wheel size, the effect is reduced as the diameter increases... remember the projected trail (mechanical) is formed within a projected wheel diameter -ve- ground... increase the pneumatic trail then the actual mechanical trail can be distorted. Assume the tyre size is a contestant within the parameters that are measured.... i advise you remove thoughts of tyre size from the calculations of the castor angle. Quote Link to comment Share on other sites More sharing options...
Roger440 Posted July 14, 2006 Author Report Share Posted July 14, 2006 Hmmm, i was getting carried away there. Lets forget about the actual effect, trail etc for now. Confusing enough as it is Back to the last bit of my previous post, are we measuring the true angle or an effective angle? I "think" the answer to this will steer my thoughts in the right direction. Quote Link to comment Share on other sites More sharing options...
Tony Posted July 15, 2006 Report Share Posted July 15, 2006 Hmmm, i was getting carried away there. Lets forget about the actual effect, trail etc for now. Confusing enough as it is Back to the last bit of my previous post, are we measuring the true angle or an effective angle? I "think" the answer to this will steer my thoughts in the right direction. Can you define 'true and effective' please. Quote Link to comment Share on other sites More sharing options...
Roger440 Posted July 15, 2006 Author Report Share Posted July 15, 2006 Yes indeed. By true, i mean the angle you would obtain by placing an angle finder against the front strut/imaginary line if no strut fitted. By effective, i mean the effective angle that actually influences the characteristics of the car. Let me explain what i mean. In answer to my original post, it seems that changing the wheel size could influence the castor angle. This i didn't quite understand. To my mind if the car were placed on 4 stands connected to the hubs, and the angle of the strut measured the angle will be what it is. Nothing will change it other than adjusting the suspension components themselves. So, therefore, if changing wheel size can have an influence i can only conclude that we are talking about an effective angle. Hence my query of actual and effective. Make sense? Probably got the wrong end of this, but i hope you can bear with me. Quote Link to comment Share on other sites More sharing options...
Tony Posted July 15, 2006 Report Share Posted July 15, 2006 Yes indeed. By true, i mean the angle you would obtain by placing an angle finder against the front strut/imaginary line if no strut fitted. By effective, i mean the effective angle that actually influences the characteristics of the car. Let me explain what i mean. In answer to my original post, it seems that changing the wheel size could influence the castor angle. This i didn't quite understand. To my mind if the car were placed on 4 stands connected to the hubs, and the angle of the strut measured the angle will be what it is. Nothing will change it other than adjusting the suspension components themselves. So, therefore, if changing wheel size can have an influence i can only conclude that we are talking about an effective angle. Hence my query of actual and effective. Make sense? Probably got the wrong end of this, but i hope you can bear with me. I think our barrier is the machinery and the formulation of measurements.. can you post a picture of your machinery so i can understand how you evolve the figures.... and at any point does the machine require you to impute the wheel diameter? The 'true' is the mechanical trail and is factual by design as you say. The 'effective' is surly the consequence of the final positions generated from whatever means. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.