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Old 13 Aug 2020, 20:52 (Ref:3995290)   #1
PePeFIN
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Mk1 Cortina some questions about suspension

Hello to all!


I'm building a mk1 Cortina and my goal is to build a car that will handle good on track. It's going to be a trackday toy, so I don't have to comply to any APP K rules or anything like that, but I also want to end up with a car that will offer an "authentic driving experience". So I have decided to stick with stuff like steering box, instead of upgrading to a steering rack. Engine will be a Zetec with 45 DCOE's and bunch of other goodies. Gearbox type9.



I have been lurking on these forums reading up all I can and it looks like there is quite a lot of experience with these cars here so I'm hoping to get some advice.


To the front end I'm either gonna source a GT / Lotus type steering box with the quicker ratio or shorter steering arms. I already have a set of GAZ coilovers with 225 lbs springs and adjustable dampers, adjustable top mounts and M16 vented Grp1 front brakes.


For the rear I will install anti-tramp bars, Quaife ATB diff, GAZ adjustable shocks and Fiesta Discs with Sierra calipers.


Adjustable pedal box will go inside the car too. I think these cover the most important bits.


Now for the questions. I need to order rear leaf springs. I have been reading that a lot of racing Cortinas seem to run about 140 lbs single leafs. But then again racing Cortinas tend to have about twice stiffer front springs than mine, so I'm wondering if the 140 lbs would be good choice. Mind you I do prefer a car that rotates easily so I would really like to avoid springs that are too soft on rear. Any opinions? Also can you recommend where to buy? I already send some inquiries to Jones Springs ltd and Team Deville but these are simply what I found with google.
I'm leaning towards the 140 lbs leafs, because the front springs would be easier and cheaper to replace for stiffer ones at a later date.


Then the front ARB. The car in question has a 19 mm ARB, is there any way this could work in track use, or am I going to have to swap for a stiffer one? Stiffer or not I'm definitely going to either fab or source an anti-dive kit (with third mount on the middle). If I'm not misinformed, Cortinas tend to have a problem where the ARB moves on it's mounts when driven hard, so that has to be addressed. Can you recommend where I could find a bigger ARB and mounts / bushings for it?


Also tires. These are a big part of what makes Historic competition cars handle the way they do. Am I incorrect in assuming that most Cortinas typically run 450M13 DUNLOP CR65 at front and 525M13 DUNLOP CR65 at rear?


Then again I don't have to follow any particular set of rules. Are there any cheaper alternatives that would handle similarly? I would assume crossply is the only way forward, so maybe the Dunlops are my only option.


Apologies for the long message, but I do have quite a bit of thoughts racing in my head about this!
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Old 26 Aug 2020, 19:20 (Ref:3998158)   #2
PePeFIN
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Well it doesn't seem that my post has found much interest around here. Anyways I'm still gonna update. I was able to source a big pile of parts locally. 24 mm and 28 mm ARB's & aluminium mounting blocks, GT-steering box, mk2 Cortina pitman & idler arms, mk2 center link, mk2 tie rods inner & outer and connecting parts and shorter steering arms.


I'm probably gonna order 140 lbs rear leafs soon unless someone has better suggestions.
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Old 27 Aug 2020, 08:52 (Ref:3998231)   #3
morninggents
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Hi. I raced Lotus Cortinas for about 25 years up until 2015 both to App K spec and to HRSR/HSCC spec which allowed more freedom to engines, brakes and suspension, whilst still using the Lotus twincam engine, standard bodywork and the Dunlop CR65 tyres.


You need to decide which tyres you will use as suspension (and brakes, to a degree) settings differ greatly with the Dunlop crossplies compared to the more modern grippy radial. The Dunlop race tyres are very expensive (around £180 per corner) have very little grip but are a hoot to race on and break away is progressive. Tyre life is reasonable. A modern grippy radial (Yokohama A032/A048 or similar) will cost about £70 per corner, has tons of grip but breakaway is sudden and usually not recoverable. Tyre life not so good as the Dunlop racer.


If you use Dunlop CR65s brakes don't need to be too fancy as the tyres will lock up before the limits of the brake stopping power is reached. With the radials more powerful brakes will be needed for ultimate stopping power. I used vented AP racing discs and 4 pot calipers on the front and non vented 2 pot AP racing rear calipers on my none App K car.


Spring rates for the Dunlops need to be higher than you quote. Around 350/400 lb inch fronts and around 180/200 lbs rears. Spring rates for the radials need to be way higher (I ran 1000lbs inch springs at the front when we could use Yokohama radials in the 1990s before we were mandated to use the Dunlop CR65s). I ran 400 lbs rear coil springs before changing to rear leaf sprigs of around 250 lbs. Dont forget early Lotus Cortinas ran 'A' bracket rear suspensions with coil sprigs in period so that would be a set-up option for you if you want an authentic feel. The coil springs are easier to change than the leaf springs and it's easier to get the correct poundage you need. However, the 'A' bracket needs to be well fixed to the axle case or it will pull the rear diff pan apart and leak oil.


Current thinking for ARBs is the thicker the better. The standard Lotus had a 3/4 inch bar with 7/8 inch bar as a homologated option but 'modern' app K cars run 1 inch bars or even bigger on some I have seen.


As I said at the start - it depends what tyres you opt for.


PS' To make any of it work you will need to seriously strengthen the shell with seam welding and interior roll over bar mountings strut braces and the rear suspension brace bars from turret tops down on to the boot floor and forward towards the door openings are a must fit requirement to stop body flex.


Hope this helps.

Last edited by morninggents; 27 Aug 2020 at 08:58.
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Old 27 Aug 2020, 09:30 (Ref:3998240)   #4
zefarelly
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zefarelly should be qualifying in the top 3 on the gridzefarelly should be qualifying in the top 3 on the gridzefarelly should be qualifying in the top 3 on the grid
I used a load of spare 2nd hand race parts on my road car and have slowly removed them . . . . . any multi purpose car needs some compromises.

UK roads are awful so go as soft as you can and buy good quality dampers.

Koni front inserts.
Quantum or koni rears

I run 300 lb front sporings ( bit stiff) and a lotus 7/8 ARB

std rear leaves . . . . single leaf capri springs decambered are more than hard enough. no need for tramp bars unless you get the geometry spot on.

nylon bushes front of leaf, poly rear

@ the front solid mount ARB/TCA's or use rose joints . . . rose joing drag link

stick with a standard good sterering box and lotus arms for ackerman 2 1/2 turns is horrible on the road.


175/70's all round for the road.

stick with GT/lotus brakes, assuming the car is >800kg . . . . theyre not heavy cars and weight murders them . . . .
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Old 27 Aug 2020, 10:10 (Ref:3998257)   #5
morninggents
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I agree with Joe about compromises for road use but based my previous comments on your car being used solely for track days.


Good dampers are important and adjustable ones can be used to tune in to the spring rates you eventually opt to use. Poly or nylon bushes also a 'must use' for track use to ensure no unwanted movement in the suspension/steering.
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Old 27 Aug 2020, 10:36 (Ref:3998261)   #6
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I would assume a track day car is a road legal car, otherwise its a race car . . . . again, its all about compromises. I don't run my car car anywhere nears as stiff as many, but it's enjoyable to drive and not embarassingly slow.

It's absolutely f***ing awful to drive on the road.
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Old 27 Aug 2020, 11:07 (Ref:3998267)   #7
Gerard C
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Just take care with the rear spring leaves you chose, the distance between the middle of the front eye and the axle location pin must not affect the wheel base. Well, must stay within the tolerance. Depending on the targeted category, the ROPS must not be welded directly to the front turret tops, check the FIA appendix for clear info.

Joe please, in what the Konis are better than Bilsteins for instance? I remember you're using Quantum at rear, I assume its a must.
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Old 27 Aug 2020, 11:22 (Ref:3998270)   #8
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Thanks for the replies guys! I'm situated in Finland and our roads are pretty horrible too. Nice windy roads everywhere, but a lot of not so good patches on them.


Car will be built for trackday's, but I will drive on the street occasionally too so sadly I have to compromise on the spring rates. Then again I don't need the car to be quite as fast as the top competitors on actual races, just needs to handle good and be a joy on the track. That's why I'm leaning towards the 140 lbs rear spring. They would stiffen the rear a lot from stock, but still be somewhat drivable on the road. No A-frame for me, I want to keep things simple, reliable and on budget and my understading is that a leaf sprung Cortina will be joy to drive too when setup well.

What would be the spring rate for the Capri single leafs?

And yes my trackday tire will have to be the expensive CR65, I don't want to ruin all the fun with chasing maximum grip and fast times.
Road tires will be something not too wide and grippy too.



I have a safety cages bolt on cage waiting to be installed on the car too. Good point about the extra strengthening on the shell. I'll have to think about that too.


All the bushes on the car will be swapped to poly or nylon, I just haven't ordered them yet because I have to figure out bunch of stuff first.

Last edited by PePeFIN; 27 Aug 2020 at 11:29.
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Old 27 Aug 2020, 13:24 (Ref:3998291)   #9
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nothing wrong with a simple rear end

https://www.youtube.com/watch?v=M8x0PlEfspo

chassis reinforcements are advisable . . . . proper ARB mountings are a must. steering areas also, very flexible.

Koni inserts are all that was available at the time, and I was advised by quantum they're probably the best readily available 'off the shelf'

MxPherson struts are somewhat compromised drom a damping point of view. But thatd from an expert. my driving isn't good enough to notice the difference !!!
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Old 27 Aug 2020, 15:53 (Ref:3998332)   #10
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Polybushes in black is a good choice imo, I see no inconvenient using them. I'm but an engineer and certainly a worse driver than Joe and others here! I find the Cortina Mac Pherson very well designed compared to a Capri/ Escort ones.
You'll see what a joy to drive can be a Cortina when everything is well put together. Watta car!
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Old 28 Aug 2020, 07:32 (Ref:3998449)   #11
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I think most agree that the old wooden dunlops are very forgiving, and probably flattering to most of us part time amateurs. I did drive the ex Broadspeed Anglia at Brands a few years ago which was prepped for Modsports with a crossflow and on slicks . . . . understandably I was a bit cautious, and subsequently posted exactly the same laps times as I do in my Cortina!
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