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Hi everyone, I am having a little hard time for the moment - In Sweden we have to have a good geometri at the front end to get through the MOT/SFRO - and one of the rules is that scrub radius should not exceed 5% of the width of the rim - about 15-18 mm with 7-8" wide rim. But could I prove that the front end geometri is good, than I could go up to scrub radius to 20-40 mm.
When I bought the setup of my Marlin, I fell for the kit with Cortina uprights and BMW hub - they are clean and talked to my heart.... But to go through our MOT = SFRO I need to find rims with ET over 50 and I want them to be good looking also...?? With ET 52 I get scrubradius at about 33 mm... So my question out to all Marlin builders - what hub do you use, what ET and scrub radius do you end up with then, what type of rim do you have and the most important : how do the car handle with that scrub radius?
So you do not feel any tendence of self steering or different changes in movement on the road when you goes over different holes or bumps?
Good looking rims are with 7 to 20 thin spokes and I am thinking of 17" diameter...but any good ideas could change my opinion
Hi Rickard,
I'm not sure I fully understand this issue but are you allowed to use spacers to achieve the requirement?
Maybe just an idea so you can choose the wheels you like?
...peter
Hi Peter - I adding this pictures, maybe it will explain the problem better... and No - spacers will just add more problem with this scrub radius.. I want it to be as the left picture (fig2) and now I have the middle scenario with Cortina hub and wrong ET at the rim. Green line is the middle of the tyre and the red line is the line in between centrum of upper and lower ball joint. The interesting point is where this lines crosses the level of the tyre, at the ground, see fig 1
So my solution will be like the right picture - find a rim with a ET that are greater than 50 mm to get the distance between red and green line so small as possible - SFRO can let my car pass if it is about 20-40 mm and the front geometri allows so big scrub radius without making the car instable when the suspension works up and down, because when you have too big scrub radius then the car will start turning by it self when you drive into a hole or over a bump - and that is something they(SFRO) wants to provide when you builds your own car. If it is not a good geometri I will need to go down to scrub radius at about 15-18 mm.
If you get it like the right picture the car could feel very dead in the steering - it is neutral and scrub radius is 0 and if you want a more active driving you need to have some scrub radius to tell you whats is happening with the car - so the best is the left picture.
So I am very interested to hear from other Marlin owners - how it looks like for your car...
Choosing a higher profile tyre would help reduce your scrub radius.
I have a Sierra hub, and have had my steering set up by a specialist who advised around 6 degrees of caster - it drives very nicely, but I must admit self centering is very weak.
Where did you find information on the Cortina pivot geometry?
Or did you mean some other geometry?? Not easy to know for sure... I hope you might explain more if I understood wrong.
//Rickard
Hi Rikard
Since you have shown what the KPI/SAI is for a Cortina I was interested to make the same calculation for my Ford Sierra Hubs: as yet I have not found any drawings/photographs with the measurements shown on them to be able to calculate the scrub.
Hi Peter - I adding this pictures, maybe it will explain the problem better... and No - spacers will just add more problem with this scrub radius.. I want it to be as the left picture (fig2) and now I have the middle scenario with Cortina hub and wrong ET at the rim. Green line is the middle of the tyre and the red line is the line in between centrum of upper and lower ball joint. The interesting point is where this lines crosses the level of the tyre, at the ground, see fig 1
So my solution will be like the right picture - find a rim with a ET that are greater than 50 mm to get the distance between red and green line so small as possible - SFRO can let my car pass if it is about 20-40 mm and the front geometri allows so big scrub radius without making the car instable when the suspension works up and down, because when you have too big scrub radius then the car will start turning by it self when you drive into a hole or over a bump - and that is something they(SFRO) wants to provide when you builds your own car. If it is not a good geometri I will need to go down to scrub radius at about 15-18 mm.
If you get it like the right picture the car could feel very dead in the steering - it is neutral and scrub radius is 0 and if you want a more active driving you need to have some scrub radius to tell you whats is happening with the car - so the best is the left picture.
So I am very interested to hear from other Marlin owners - how it looks like for your car...
//Rickard
Rikard
Do these diagrams showing bump steer help?
Sorry about the size!
Out of interest I have calculated my Sierra hub has an SAIof 11 degrees.
I then calculated my scrub radius is 24mm with a -38mmET on a 16" wheel and 215/55/16 tyre using the dimensions shown on Super7thheaven http://www.super7thheaven.co.uk/components/sierra_upright/
My Cabrio drives really well at speed - it does not move around - even at 90mph when it is getting very light due to the air getting under the wings!
I do have some self centering , but at full lock it is not too good.
