Suspension set up and handling

[yellowss7]

I'm currently running stock Caterham springs and shocks with a wide track front end. After having George Alderman at Midlantic Caterham cornerweight it and adjust the toe and camber, the car handled really well, i.e. no understeer, and just a tad of power on oversteer. However, on Slicks, I find that the car does have a significant roll to it on hard turns. So much so that the side exhaust pipe actually scrapes the ground on hard right handers. http://www.usa7s.com/aspnetforum/images/emoticons/ack2.gif (See attached photo)

 

 

 

I bought a set of 250 lb front springs to try out, but I know that if I stiffen the front up I will probably get more understeer. I could go to a smaller front anti roll bar but it's the roll I'm trying to limit. Any suggestions? Tom

 

 

 

http://www.usa7s.com/forum/uploads/20070219_165859_Copy_of_philly_.jpgyellowss72007-02-20 05:50:09

[Boxologist]

have the sway bars been changed? also, the exhaust tip seem to be angled slightly downwards in its initial orientation.

[blami]

A little more information may shed a little light:

 

 

 

Were the original tires bias or radial design and what design are the slicks?

 

 

 

When you switched to slicks did you change tire size including the overall diam. of the wheel tire combo?

 

 

 

What tire pressures are you running?

 

 

 

Bart

[yellowss7]

The original tires were Avon CR500's on 13 inch wheels. They are Radial design. The Slicks are 13 inch Bias design, that I run at 12-12.5 PSI. I seem to recall that the overall diameter stayed pretty much the same. Never had the problem with the AVONS, and the tail pipe was always oriented that way. The swaybars are the stock superlight set up. Green bushed 18mm up front and I believe 1/2 in the back on the front most setting.yellowss72007-02-20 05:16:09

[Birkin42]

What springs are you currently running front & rear?

 

Are these stock? If not, what were the stock rates?

 

Have either/both the front/rear sway bars been changed?

 

Does your car have a rear sway bar?

 

All of this would be helpful to know.

 

 

 

In the end, if you have more roll then you like, you either need to increase roll stiffness, lower the center of gravity or lower the roll center of the car. The last 2 are major changes so increasing the roll stiffness is likely the best option.

 

To increase roll stiffness you can either increase the spring rate, install a stiffer sway bar or increase the leverage on the sway bar. The last 2 will have the same effect.

 

In making any of these changes you will need to maintain the same front to rear ratio of roll stiffness to keep the same understeer/oversteer.

 

In deciding whether to add more roll stiffness through springs or the sway bar will depend on what your thoughts are on compliance of the car driving in a straight line including brake dive, squat under accel and compliance over bumps. If the car moves too much under these situations, add spring stiffness. If you are happy with these, add sway bar stiffness.

 

 

 

Hope this helps.

[yellowss7]

Birkin42, stock springs are 170 front 150 progressive rear. Yes currently stock. Front and rear swaybars are stock. Car handles great other than the roll. I could lower the ride height but since I'm only running 75mm clearance under the sump, I'm hesitant to go too low, since I do drive it on the street too. Tom

 

 

 

yellowss72007-02-20 06:01:25

[Birkin42]

I'm surprised the front spring rates are so low though I am more familiar with Birkin geometry then Caterham.

 

Your car has double wishbone front with the shocks canted at close to 45 degrees, and the rear is a DeDion axle with the shocks nearly vertical?

 

Do you know your corner weights?

 

If you are close to 50/50, I would expect the front to have stiffer springs then either the 170 that you say is stock or event the 250's you bought to try out. A Birkin runs stiffer then this on the front and has the engine mounted a bit further rearward then a Caterham as I understand it.

 

You say your rears are progressive 150. Is the 150 the start rate or the finish rate? Normally progressive springs list 2 rates like 150/200 for example.

 

 

 

Lowering the car may be an option for when you are running slicks and then bump it back up when you run on the street when lean is less of an issue. This may be more of a pain then it is worth though. Also, lowering the car will reduce the roll somewhat, but likely the amount of roll you lose will be less then how much you lower it by so your exhaust grinding issue will likely be worse, not better.

 

 

 

If you want to do some analysis on your spring rates, you can download a catalog from Eibach. The catalog you want is complete Motorsports catalog. In the back it tells you how to calculate wheel rate for your front suspension as well as for your rear suspension. This isn't the end all be all, but it might prove helpful in understanding where you're at and where to go.

 

 

 

Hope this helps.

[blami]

From my experience (and mine only) I would say that your tire pressures seem to be low and I would suggest 15 to 16 PSI as a starting point.

 

 

 

This suggestion is based on my experience with Hoosier and Goodyear bias belted slicks on 13 inch wheels.

 

 

 

Good luck--at least this suggestion does not cost any money to try out.

 

 

 

Bart

[yellowss7]

Thanks Bart. That's easy enough to test. I got that pressure from Michael Dougherty, he's using the same. I'll switch the springs once it warms up a bit. Anyone have experience with an adjustable front and or rear anti sway bar? I know Freestyle in the UK sells them.

 

 

 

Thanks guys for all your suggestions, it gives me somethings to check at the next test and tune.

 

Come on Spring.           Tomyellowss72007-02-20 17:12:36