Board logo

Tyre width for 900kg tintop, drag racing??
NS Dev - 1/10/08 at 04:29 PM

Got my mate's opel kadett C up and running, now with full 5 link rear end, and he's still struggling for grip.

Best time is now 12.2 sec 1/4 mile, but its still spinning up the wheels in every gear, with 3.44:1 diff ratio and omega v6 manual gearbox!!!

Currently on soft compound Toyo 888's at 195 width.

I think two things are at fault but interested in other opinions.

Firstly, the engine management (modified but still crap std vauxhall LET bosch motronic ecu) is not capable of controlling the boost increase rate and so it comes in like a sledgehammer and breaks the tyres loose.

Secondly, it needs wider tyres!

We have 55 profile tyres so some sidewall give, run low tyre pressures, diff nose approx parallel to the ground at the mo though will be winding it down a bit.

Rear spring rates are 160 lb inch iirc, and dampers set almost full soft.

PS good news is its done lots of runs now and not broken anything since the work was done................apart from him crashing it at Curborough yesterday after going off and finding the hedge!

[Edited on 1/10/08 by NS Dev]

froggy - 1/10/08 at 05:41 PM

ive got some yank books on drag racing and one mod thats worth a go is different spring rates on the back to counter the torque reaction when you launch, slicks are worth a few tenths but boost control is the real answer ive gone all out with my nitrous controller so i can do a proper map for each gear as a fixed 50hp shot lit it up in the first three gears

NS Dev - 1/10/08 at 06:08 PM

quote:
Originally posted by hicost
I run 245x45x16 on the back with a power lock diff and LSD

225x45x16 on the front, I spent a few years getting this mix right then Caterham came along and copied it for the CSR. So they must have looked into it.

Both back wheels will lock to 60 then it clicks off with the bleed off of power.

Hope that helps a bit!!



Ahhh, you might be just the man for another question if you don't mind!!

Ian (the owner) is complaining that the diff isn't working right.

I know nowt about powr-locs but he had the axle build professionally after I'd done all the bracket welding etc (its a scimitar se5 casing with jag independent powr-loc inside it with 3.43:1 ratio)

Apparently at curborough it was spinning up the OUTSIDE wheel on the long corner onto the straight, he could then count to three, then something went "clunk", both rears span up and the tail came out as expected........

He reckons the powr-loc is just a "normal" plate diff, to which I said no way can it do what he described..........

Without taking the diff apart I don't know what its like inside, and I've spent FAR too many hours on the fabrication on the car already!

Any ideas??

NS Dev - 1/10/08 at 08:34 PM

yeah, pretty much as I thought!

Clutch plates were declared "excellent, as new" when it was built up. All bearings and seals etc etc were also done, it wasn't a cheapo bodge build.

Its running normal EP80 oil (which I said to him was too thin and should be EP90 really but he used it anyway) with clutch type LSD additive, make I can't remember but it was from Hauser race cars and the right stuff.

I wonder if its just bedding in or something? The axle builder set the clutch preload up after asking what the car was to be used for so I assume he knew what he was about.

NS Dev - 1/10/08 at 08:39 PM

PS on the bench it did everything you would expect on torque test........

I really need to drive the car myself again to check it all personally.

As you probably know only too well though, with a (too) quick boost curve its very difficult to tell what is going on, gearchanges and wheelspin are just happening constantly and to be honest, even with what I would call a fairly tall (3.43:1) diff ratio, you can't even get it under much load in 5th, its onto the limiter at 7500 rpm in 5th on the public road before you know it........

Thankfully no speedo fitted at present so you can't confirm the scariness of the speed!!!

I really need to finish persuading him to fit proper management to it so we can actually control the boost progression a bit more!

red22 - 6/10/08 at 08:18 AM

I'm not an expert but if you're setting the rear soft to try and get some weight transfer your doing it the wrong way. Should be the other way round, front very soft, rear medium to firm.

If you're interested I've got an Alston Chassis Manual that has shedloads of info on scaling 4 links and effects on traction. You can have it for price of postage or I could try scanning and emailing?