interesting steering rack ratio Numbers.
- Thread starter Beedub
- Start date
TomK said:
The OEM 224 ///M wheels are 490mm diameter so I'm confused as to what you are describing as the "wheel" from the sizes you've given?
The specs are as below:
M3 CSL
wheelbase 107.4 inches
track/tread (front) 59.8 inches
track/tread (rear) 60 inches
Z4M
wheelbase 98.3 inches
track/tread (front) 58.5 inches
track/tread (rear) 59.7 inches
The Z4M has a wheelbase which is more than 9% shorter than the CSL which means that it will rorate more readily. You can fine tune the steering response by adjusting the toe angles as well as by driving technique.
TomK said:
I refer you back to my earlier link: http://en.wikipedia.org/wiki/Steering_ratio The ratio is related to the angle of the steering wheel not it's diameter.
I was just reading that :wink:
It doesn't quite add yup for me as whilst I understand the rotation of the wheel will be the same regardless, that actual distance traveled by the wheel will be different.
There's some 'interetsing' reading to be had on the subject also on this
http://www.g-w.com/pdf/sampchap/9781605252230_ch09.pdf
page 178
It doesn't quite add yup for me as whilst I understand the rotation of the wheel will be the same regardless, that actual distance traveled by the wheel will be different.
There's some 'interetsing' reading to be had on the subject also on this
http://www.g-w.com/pdf/sampchap/9781605252230_ch09.pdf
page 178
I've just had a "speed read" through that reference and on Page 169 it says: "The steering wheel size has an effect on the effort expended by the driver to turn the vehicle. The larger the wheel, the less effort needed to turn it. This is due to the leverage exerted by the larger wheel."
This is true, but with PAS does it really make that much difference? Then again, as the diameter of the steering wheel gets smaller there is less movement of your hands for the same angular rotation of the steering wheel gives less steering feel. For me, there's an optimum steering wheel size, especially for use on bad public roads, where the road surface gives a lot of input to the steering, whereas on a go-kart on a smooth track a small steering wheel is much better, although it requires more physical effort to turn it. I think the size of the Z4M steering wheel is just about perfect for the car. My Z3MC has an OEM steering wheel which feels like it's come off a bus and it's 375mm: it's far too big.
This is true, but with PAS does it really make that much difference? Then again, as the diameter of the steering wheel gets smaller there is less movement of your hands for the same angular rotation of the steering wheel gives less steering feel. For me, there's an optimum steering wheel size, especially for use on bad public roads, where the road surface gives a lot of input to the steering, whereas on a go-kart on a smooth track a small steering wheel is much better, although it requires more physical effort to turn it. I think the size of the Z4M steering wheel is just about perfect for the car. My Z3MC has an OEM steering wheel which feels like it's come off a bus and it's 375mm: it's far too big.
Agreed, much prefer the size of the z wheel to the m3 wheel (also 375mm) and as you say it is a combination of factors that influence the steering system designs.
This seems to put it quite well... "Steering ratio, as well as overall handling and ease of steering, is determined by many factors. The size of the steering wheel was already mentioned. The relative size of the gears in the steering gear, the size and shape of the steering arms, and the angles formed by the linkage all affect the steering action. The percentage of vehicle weight placed on the front wheels and whether the vehicle has front- or rear-wheel drive are also factors."
This seems to put it quite well... "Steering ratio, as well as overall handling and ease of steering, is determined by many factors. The size of the steering wheel was already mentioned. The relative size of the gears in the steering gear, the size and shape of the steering arms, and the angles formed by the linkage all affect the steering action. The percentage of vehicle weight placed on the front wheels and whether the vehicle has front- or rear-wheel drive are also factors."
exdos said:
That's true of all cars though - you have to compromise somewhere and so you corner weight for your chosen scenario - e.g. mine was done for: driver (me), no passenger, and a full tank of fuel. Don't think the screen wash will change enough to really matter in a 1500kg car ...
tertius said:That's true of all cars though - you have to compromise somewhere and so you corner weight for your chosen scenario - e.g. mine was done for: driver (me), no passenger, and a full tank of fuel. Don't think the screen wash will change enough to really matter in a 1500kg car ...
I agree about it all being a compromise for the best handling/performance in the dynamic situation. Likewise, I can see in your case, where you've replaced the OEM seats with lightweight seats, and possibly done other weight reduction mods such as exhausts and battery, where weight is mostly removed from the rear, that the car may need to be rebalanced and corner weighting is possibly the solution. My scepticism largely relates to a car where corner weighting dictates that the positive rake angle is lost, which would increase aerodynamic lift, which would then completely bugger up all that corner weighting.
Do you notice a difference in handling with your corner weighted car, between a full and nearly empty tank of fuel? Would you mind giving me the heights from floor to the top of the wheel arch at each corner of your car and what is the total weight?
exdos said:tertius said:That's true of all cars though - you have to compromise somewhere and so you corner weight for your chosen scenario - e.g. mine was done for: driver (me), no passenger, and a full tank of fuel. Don't think the screen wash will change enough to really matter in a 1500kg car ...
I agree about it all being a compromise for the best handling/performance in the dynamic situation. Likewise, I can see in your case, where you've replaced the OEM seats with lightweight seats, and possibly done other weight reduction mods such as exhausts and battery, where weight is mostly removed from the rear, that the car may need to be rebalanced and corner weighting is possibly the solution. My scepticism largely relates to a car where corner weighting dictates that the positive rake angle is lost, which would increase aerodynamic lift, which would then completely bugger up all that corner weighting.
Do you notice a difference in handling with your corner weighted car, between a full and nearly empty tank of fuel? Would you mind giving me the heights from floor to the top of the wheel arch at each corner of your car and what is the total weight?
Re. your point about corner weighting meaning you lose the rake - well you just wouldn't unless you slavishly follow the corner weighting to the exclusion of all else. You need to make compromises and it is rare that you can perfectly corner weight a car, exactly because of considerations like that. My understanding is that you normally aim to get the cross weights (i.e LF+RR vs RF+LR) as close as possible and accept that the each corner won't be perfect (for reference my cross weights are within 3kg).
Total weight for my car (with me and a full tank of fuel) is 1535kg
No, I haven't noticed a difference in handling between full and empty, however, I think there are (at least ) two reasons for that - 1) I'm not a particularly sensitive driver; and 2) given where it is, I suspect the weight of fuel is fairly evenly distributed anyway, so while the total weight will obviously go down, the distribution between the corners is probably not significantly changed.
tertius said:
Thanks for that. I've seen cars on the US forums, where they seem particularly keen on corner weighting, which are definitely nose up (-ve rake) as a consequence of following the process to get as close to 50/50 weight distribution. That's fine for their auto-crossing but it seems wrong to me if you intend to drive at higher speeds.
My car will be about 75kgs heavier with me and a full tank of fuel.
I notice the difference in handling of my car at The Ring between having a passenger or not, which will affect the cross weights, but I don't so much notice it as any difficulty in getting the car to turn either way, it's more in the way the dampers perform from the added weight and the car doesn't seem as "planted" on the bumpier parts of the track, particularly with heavier passengers (over 13st). My Z3MC with KWV3 (front coilovers) hops on the entry to Galgenkopf with a 10st passenger but doesn't if I'm on my own.
i added some Rake to my car... forget the exact measurements but did write them down, but i can't say it was any more stable than when it was corner balanced tbh, i noticed no difference, maybe due to the sprinting scene which i love is lots of harsh corners and usually under 100mph max speed.
This is how it looks , you can clearly see the Rake from any of my pics, my car is also set very conservatively in terms of ride height...
This is how it looks , you can clearly see the Rake from any of my pics, my car is also set very conservatively in terms of ride height...
exdos said:
To answer your post (a little late), I agree with your scepticism regarding the benefits of corner weighting our cars. Fortunately, I rarely wash my car (screen included) and I'm an antisocial git so avoid the unnecessary weight of passengers when at all possible
The additional cost of corner weighting when I had my geo set up wasn't much so I thought it worth trying. The car is so much more planted now at all speeds that it is difficult to know the contribution of corner weighting. Certainly, the rear is slightly more lively under power with an empty tank, but it always was. As the others have stated, it isn't necessary to lose rake with corner weighting, it's more about diagonal balance. I haven't any appropriate pictures to show the rake on mine but I'll post one when I have an opportunity. Out of interest, my car with a full tank of fuel and me weighed in at 1533kg, but my relative corner weights are not as close as tertius'.
There's no comparison with the OEM suspension. At extremely high speeds (>130mph) I found the front end floaty and imprecise. I changed to Eibachs with OEM dampers soon after I bought my ///M and this markedly improved things. However, I continued to experience problems with understeer on the track. As you know, I tried an array of things including 15mm spacers, a strut brace and pulling the camber pins. Most effective though was ditching the Contis I had on my track wheels in favour of PS2s. These modifications left me with better turn in and less understeer but on-going problems with wear of the outer shoulder of the front tyres. My current set up is fairly modest compared with the maximum settings available from my Clubsports and camber plates but I will be incrementally cranking up the firmness and negative camber at my next track day. I have no understeer with the current set up and easily provoked and controlled oversteer. I'm not sure how increasing the stiffness of bump and rebound will affect handling but I plan incremental adjustments to each to establish this. I hope that more negative camber will further reduce shoulder wear but without rendering the car too twitchy or being detrimental to stability under braking.
I hadn't appreciated the lack of feedback from the rear end until I fitted the RTABs. I attributed the feeling of inertia under load in comparison with my Westfield to the almost 3x weight difference. However, the RTAB limiters have markedly improved my ability to hold the car on the limit of grip through the corners.
Straight line braking distances seem to have improved as I can brake harder and later, but with the caveat that my relatively increased corner speeds confound my ability to judge this. The car is also more stable if I trail brake into a corner. Whilst I never experienced the baulking when changing down under load, my gear shifts and most especially the smoothness of downshifts with heel and toe have significantly improved with the competition engine mounts (on that note John, did you receive my last PM with the photos of my heel and toe pedal modification?).
I'm glad you're really enjoying your upgraded and sorted Z4MC. 
It's great to experience the transformation isn't it?
The Z3MC, like the Z4MC, in OEM guise is supposed to have almost 50/50 F/R weight distribution, but because it has suspension which is way too soft, the car suffers a lot of pitching, squatting and lateral bodyroll, which in the dynamic situation, throws the 50/50 weight distribution completely out of the window! Consequently, the Z3MC has a deserved reputation of being a real handful, particularly in the wet, and no amount of corner weighting with OEM suspension (if it were possible - but it's not) would ever cure the poor dynamics of the car. However, if you uprate the Z3MC's antiroll bars, you can considerably reduce the lateral bodyroll and diagonal load-shifting which is the feature of the Z3MC's poor handling characteristics and then by fitting better aftermarket suspension (i.e. KW V3 with front linear springs) you can considerably reduce the pitching, squatting, lateral bodyroll and diagonal load-shifting some more, and with proper geometry the car is transformed into something which handles extremely well. As I see it, provided that the ride height of the car was recorded when OEM, then surely after fitting aftermarket suspension parts and lowering the car by an equal drop at each corner, then this preserves the OEM near 50/50 weight distribution.
With specific regard to the Z4MC, in OEM guise, the rear suspension is so damned stiff on 19mm springs, this considerably reduces any tendency to lateral body roll and diagonal load shifting and with massive toe-in at the rear, the car's performance is "contained" (restricted) and is much safer to drive than an OEM Z3MC. However, with the very soft 11mm front springs on the Z4MC, there's a lot of pitching under braking, which can overload the outside front tyre when also turning, which produces the understeer that you hate and could never cure whilst retaining OEM suspension. Now that you've fitted KW Clubsport suspension you've redressed the gross imbalance of F/R spring rates that existed with OEM suspension and so you've considerably reduced the pitching and and diagonal load-shifting on braking whilst turning, thus the understeer is effectively cured. The stiffer ARBs assist further. The RTAB limiters keep the rear trailing arms in a fixed position to prevent lateral "squirming" of the rear trailing arms in the cage which also prevents unwanted load shifting in the dynamic situation. Provided that you've set your ride heights by similar drops at each corner, I doubt that corner weighting has made much difference to the improvement in handling that you've obtained from the actual upgrading of the suspension parts. Do you know the ride heights at each corner, now and OEM?
For me, once you've got the uprated suspension parts limiting the load shifting in the dynamic situation, the damper settings and static geometry can then be adjusted to get the best out of the handling and the tyres. The same approach to the different problems of the OEM Z3MC and OEM Z4MC have transformed both cars into cars with superb handling, because the same principles apply.
P.S. I thought I'd commented about your excellent accelerator mod in a message about that Z3MC you were considering? Does the trick really well!
It's great to experience the transformation isn't it?The Z3MC, like the Z4MC, in OEM guise is supposed to have almost 50/50 F/R weight distribution, but because it has suspension which is way too soft, the car suffers a lot of pitching, squatting and lateral bodyroll, which in the dynamic situation, throws the 50/50 weight distribution completely out of the window! Consequently, the Z3MC has a deserved reputation of being a real handful, particularly in the wet, and no amount of corner weighting with OEM suspension (if it were possible - but it's not) would ever cure the poor dynamics of the car. However, if you uprate the Z3MC's antiroll bars, you can considerably reduce the lateral bodyroll and diagonal load-shifting which is the feature of the Z3MC's poor handling characteristics and then by fitting better aftermarket suspension (i.e. KW V3 with front linear springs) you can considerably reduce the pitching, squatting, lateral bodyroll and diagonal load-shifting some more, and with proper geometry the car is transformed into something which handles extremely well. As I see it, provided that the ride height of the car was recorded when OEM, then surely after fitting aftermarket suspension parts and lowering the car by an equal drop at each corner, then this preserves the OEM near 50/50 weight distribution.
With specific regard to the Z4MC, in OEM guise, the rear suspension is so damned stiff on 19mm springs, this considerably reduces any tendency to lateral body roll and diagonal load shifting and with massive toe-in at the rear, the car's performance is "contained" (restricted) and is much safer to drive than an OEM Z3MC. However, with the very soft 11mm front springs on the Z4MC, there's a lot of pitching under braking, which can overload the outside front tyre when also turning, which produces the understeer that you hate and could never cure whilst retaining OEM suspension. Now that you've fitted KW Clubsport suspension you've redressed the gross imbalance of F/R spring rates that existed with OEM suspension and so you've considerably reduced the pitching and and diagonal load-shifting on braking whilst turning, thus the understeer is effectively cured. The stiffer ARBs assist further. The RTAB limiters keep the rear trailing arms in a fixed position to prevent lateral "squirming" of the rear trailing arms in the cage which also prevents unwanted load shifting in the dynamic situation. Provided that you've set your ride heights by similar drops at each corner, I doubt that corner weighting has made much difference to the improvement in handling that you've obtained from the actual upgrading of the suspension parts. Do you know the ride heights at each corner, now and OEM?
For me, once you've got the uprated suspension parts limiting the load shifting in the dynamic situation, the damper settings and static geometry can then be adjusted to get the best out of the handling and the tyres. The same approach to the different problems of the OEM Z3MC and OEM Z4MC have transformed both cars into cars with superb handling, because the same principles apply.
P.S. I thought I'd commented about your excellent accelerator mod in a message about that Z3MC you were considering? Does the trick really well!