Self designed top mounts

[campbellju]

I may not convince you on this trip but I will put that to one side and raise the next issue. The problem is not the KPI but the scrub it induces.

To save me typing something in I've copied this below which is a reasonable explaination. The main two ways you effect scrub are KPI and wheel offset.


=======================================================
Scrub radius
The scrub radius is the distance from the centre of the tyre contact patch to the point where it intersects with the extended steering pivot axis (also called the steering axis) on the roadway.
There are positive (+) and negative (-) scrub radii as well as vehicles with a scrub radius of zero (±).



Effect:

• Scrub radius positive: produces stable straight-running stability but requires counter-steering by the driver in case of uneven braking effect
• Scrub radius negative: in case of uneven braking effect, provides automatic counter-steering so that the driver only needs to hold onto the steering
• Scrub radius zero: prevents the transfer of faulty forces to the steering in case of one-sided pulling of the brakes and in case of tyre defect, high steering forces at a standstill.

===================================================

What it implies but doesn't say is scrub is like a lever. At 0 scrub you have no leverage to help turn the wheel with so the steering feels heavy. YOu add scrub so the steering axis acts like a lever against the rotation point of the tyre which is the "wheel centreline".

However, the same lever also acts in reverse with forces from the road which are amplified by scrub. Negative is preferred on FWD and 4wd for the reasons said above.

This article just considers wheel offset though not KPI. The problem with the KPI on a macpherson strut is it changes under compression. This means it becomes more negative under scrub. If you have a little positive scub then under compression it becomes zero and negative which would make the car feel awful.

Scrub will not change the level of grip generated by the tyre but it does change your perception of what the tyre is doing

 

[stumo]

Trip, KPI is the angle between the VERTICAL and the top mount/bottom balljoint.

The first picture is correct, the other two are wrong.

 

[Trip]

Thanks for the compliments Jim.

Lets keep the scrub radius at the side for now . So you are both suggesting that the KPI angle should be calculated from the true verical axis and not the wheel verical axis ?

For this diagram below with exaggerated 7.44 deg negative camber
I am suggesting that KPI should be calculated from the wheel axis and in this diagram its 13.57 deg
While you both suggest that the KPI should be calculated from the true vertical and in this diagram its 21.01deg

 

[Trip]

Hey guys. been reading on this on the net and i can finally conclude that my theory is WRONG :doh:

As both of you said, KPI angle should be calculated from true vertical. I was under the impression that KPI should be calculated from the Wheel axis as mentioned above.

I apologise for the waste of time i have created. I now have a better view of what you said in your first post Jim.


Now........Going back to the original post.

I will see the dimensions and check if its possible to design the topmounts to get only longitudinal adjustment

What tools did you use to calculate KPI and scrub ?

 

[stumo]

thank fuck for that! Finally Trip sees sense! :roll::roll::roll:

yes the KPI will be 21.01deg

The idea is to get the most caster as possible and adjust the camber at the bottom of the strut.


you can calculate KPI by trigonometry....or a program that Jim has :lol:

it will calulate scrub too.

 

[Trip]

Speaking of tools on a side note this what i did so i be able to adjust the heigh and geometry without needing to remove the wheel every time.

Design


Finished product

 

[stumo]

nice design there Trip.

 

[Trip]

thank fuck for that! Finally Trip sees sense! :roll::roll::roll:

yes the KPI will be 21.01deg

The idea is to get the most caster as possible and adjust the camber at the bottom of the strut.

I saw the light 8)

So the end result would be to have top plates to adjust caster without adding KPI..I finally got it

 

[Trip]

nice design there Trip.

I still need to test them out and hopefully the hinges can (or whatever you call them) hold the car's weight.

 

[campbellju]

you can calculate KPI by trigonometry....or a program that Jim has :lol:

I modelled it in excel. I took the standard dimensions to try and model/understand the OEM scrub. I couldn't get the model to work precisely but it helped my understanding of it all. In the end, Stu and just attacked my car and put the top mount to the full outboard position and did some experimentation.

The steering became lighter and took some getting used to. I think it was just +ve rather just -ve scrub so it did give a vague feel on corner entry as it went through zero scrub. Solution don't go full outboard on the topmount unless you also want to test theories.

I like your tracking plates, they are an inspired idea.

 

[Trip]

I modelled it in excel. I took the standard dimensions to try and model/understand the OEM scrub. I couldn't get the model to work precisely but it helped my understanding of it all. In the end, Stu and just attacked my car and put the top mount to the full outboard position and did some experimentation.

The steering became lighter and took some getting used to. I think it was just +ve rather just -ve scrub so it did give a vague feel on corner entry as it went through zero scrub. Solution don't go full outboard on the topmount unless you also want to test theories.

I like your tracking plates, they are an inspired idea.

If the top plates have 0 camber, KPI should be of OE spec (13.5 - 13deg). With age, potholes and bumps our body/chassis will deform and OE figures can be all thrown out of the window. With caster mod my car's KPI was 12.83deg on one side and 9.5 on the other. Caster 2.75deg and 2deg on the other

I have redesigned the topmounts to 0 camber, i just need to check the clearance against the actual shock and go for cutting with a 3 week lead time.

Looking at the whole concept of scrub, KPI and all of that from a different persective, it would be hard to get enough negative camber since the sidewall of the tyre will rub against the coilover (i have 225/45R16), unless going for wheel spacers which will feck up the scrub.


Some food for thought here

Alterations to the wishbone: A slightly shorter wishbone and offsetting the balljoint to the front should lower the KPI and add caster. Downside: Drive shafts and wheel scrubbing on the body on full lock. It must be strong to handle the weight and stresses

Alterations to the hub: Extending inwards the shock mounting holes with some plates should give you a better clearance for camber without the need of spacers. Downside: Stress on the plates since it will be offset

Alterations to the turret: moving the whole thing outwards and backwards to lower the KPI.

I know these are all theory but i saw it done....on TV shows :-D



I have found this good reading with an excel sheet to calculate dynamic camber with different steering angles
http://www.iwsti.com/forums/gd-susp...g-best-alignment-dynamic-camber-analysis.html

 

[Sunny-D]

Is this of any use - http://www.locostusa.com/forums/viewtopic.php?f=5&t=3517&sid=b48b68d2d50bdd2aa09d54f74af1701f

 

[Trip]

So got back to work on this and did another test top mount with 0 camber and full caster.



For starters, since i added 24mm at the top to clear the top turret part, the car came high like a buggy. My Coilovers are Tein HA which do not have a proper ride hight adjustment unless compressing the spring. Second, camber drastically decreased to -0.5 and this is with a the camber mod.

The lack of camber is a concern at this point. I left the upper bolt (between the hub and the coilover) off and pushed everything and tightened the lower bolt, put car on its weight and i only got a max of -1.5deg, I don't think there is more to do without sacrificing KPI.

 

[campbellju]

Drill out your top strut hole to hub mount out some more. You can have as much -ve camber as you want.

If you've raised the body you need to lower the spring to compensate. I'm not sure I understood?

 

[Trip]

Drill out your top strut hole to hub mount out some more. You can have as much -ve camber as you want.

If you've raised the body you need to lower the spring to compensate. I'm not sure I understood?

I wish that was true with my car. As I said, with just the lower bolt in (acting as a pivot) and pushing the hub and strut inwards to create -ve camber, to the point that the hub mount part touches the inner part of the strut. And that gives me only -1.5deg.

The Tein HA i have do not have an independent ride height adjustment, It is adjusted by rotating the perch holding the spring. So ride heigh adjustment is limited.

 

[campbellju]

I'm still surprised :? I run Tein Superstreets and accidentally ran -3degs with the top mount fully outbound. I could have easily ground off a bit more strut hole as well and had more.

You could always try putting a camber bolt on the bottom and moving it out?

For the ride height. If you've raised the car then unless you were already running at the lowest setting then you should have some flexibility.

If you've not then get some shorter springs from Tein, they about £50 each. I did this when I changed to a 5/4 setup so the car didn't ride higher.

 

[Trip]

I'm still surprised :? I run Tein Superstreets and accidentally ran -3degs with the top mount fully outbound. I could have easily ground off a bit more strut hole as well and had more.

You could always try putting a camber bolt on the bottom and moving it out?

For the ride height. If you've raised the car then unless you were already running at the lowest setting then you should have some flexibility.

If you've not then get some shorter springs from Tein, they about £50 each. I did this when I changed to a 5/4 setup so the car didn't ride higher.

its the case of my car.. Next time i get out engine i take the shell to a jig to get it checked for warps and things like that. I can even have bad wishbones. And even caster is different side to side and this was without caster mod.

My aliginment been weired from the first time i did my geometry.:?

 

[Trip]

Got some news..

First of all.. I was doing it wrong yesterday. It was not tightening the bolts enough and when putting the car on its weight the hub was moving out giving me a false camber. Probably i was too tired doing this work at 1:30am and messed up.

Today I re-done the tests.. Left the lower bolt in place and put an M6 bolt on top. Tightened it, put wheel on and lowered car. I got -6 deg or even more (camber gauge reads -6deg max). GREAT...Put in an M8 bolt on top and redone the tests. This time i got around -5.5deg. Done it again with an M10 and got 2.75deg. Camber Sorted

while experimenting, i realised that i would need female spherical bearings (or shorten the tie rod). The male's i have "bottom out" before reaching good toe. Leaving toe as it was and adding -6deg of camber the wheels are now pointing inwards (toe in) . Also with -6deg of camber on full lock the tyre was hitting the inner fender wall and almost touching the oil sump. Probabalby with less camber and adjusting the toe, i "should" be clear...


Max adjustment i can get. Just big enough to put an M6 bolt in



Tyre hitting inner fender . Also note that tow is pointing inwards, 6deg+ of negative camber and having a 5mm spacer between disk brake and wheel.



As above. Wheel just clears the oil sump, infact i could hold an m8 bolt from its head between tyre and sump





Is there anything on the market like a washer with rough surface so it will grip onto the surface. I am afraid that with lots of hard driving, the upper bolt on the hub will somehow move, spoiling camber settings.



Ride height is still a problem with my current coilovers. They consist of two springs. the main one and a small one at the bottom. I am considering removing the bottom one but i think that it is used when the car is on full rebound when wheels come off the ground.. any thoughts ???


These are like mine

 

[Trip]

And for the finale.

-2.75deg using M10bolt

 

[stumo]

Is there anything on the market like a washer with rough surface so it will grip onto the surface. I am afraid that with lots of hard driving, the upper bolt on the hub will somehow move, spoiling camber settings.

you will need to enlarge the hole in the hub carrier so that you can use the original bolts. you won't get enough tension in a M10 bolt.

Ride height is still a problem with my current coilovers. They consist of two springs. the main one and a small one at the bottom. I am considering removing the bottom one but i think that it is used when the car is on full rebound when wheels come off the ground.. any thoughts ???

Do not remove the helper springs, if you do you will end up with a much higher ride height as it is dangerous to have a loose spring when the wheel is dropped.

You might want to get a different set of springs to suit what you want to do.