Rear diffuser & engine placement question

[s2k7]

xxx

Edited January 24, 2014 by s2k7

[rnr]

I complained that my car seems to light and very scary at highway speed. I felt that the rear likes to airborn especially when the Vtec kicks in.

 

What kind of Seven do you have? What kind of tires are on it - sounds more like they are breaking loose rather than getting airborne

 

My question: Do you think the placement of the engine almost pushed at the middle created unstability at hiway speed?

The steering is very, very light, a simple steering mistake perhaps will

kill me.

 

Mid-engine caars intentionally have a low polar moment of inertia which makes them less resistant to direction change and easier to maneuver: http://www.autozine.org/technical_school/handling/tech_handling_4.htm

 

My steering does get lighter as the speeds pass about 70 mph. While it was a bit disconcerting initially, I no longer even notice it.

[Boxologist]

a rear diffuser works if the air coming off the top of the backside is directed properly as well. that's pretty difficult with the typical shaped se7en and its rollbar. Since u have an Ultralight presumably, check w/ Loren and Kevin to see if they have used any underside aero in addition to the parkbench spoilers that is currently on Mojito for high speed stability.

 

I too am thinking about a rear lower diffuser, but i know a need a work around for the upper airflow to see any tangible benefit beside lightening my wallet.

[s2k7]

xxx

Edited January 24, 2014 by s2k7

[supersportsp]

http://www.fluke-motorsport.co.uk/newImages/182_2.jpg

 

http://www.fluke-motorsport.co.uk/product_thumb.php?img=newImages/182_8.jpg&w=800&h=600

 

http://www.fluke-motorsport.co.uk/carbon-fibre-multiplane-contoured-diffuser-p-182.html

 

Having the rear end feel light is not typical in Se7en. The front, absolutely, the rear, unusual. As you can see, there are diffuser options out there for the Westfield.

 

Here is what I would start with:

 

Different tires. Those appear to be drag compound radials. Good for straight line hook-up, not so good for lateral movement.

 

Then, I would make sure the alignment is adjusted optimally.

 

Moving the motor rearward would potentially make the front of the car lighter (and lighter feeling at speed).

 

You could also try some of the little carbon nose canards that Caterham uses on the R500. A local guy I know said they actually had a noticeable impact on front end stability at the track on his car.

 

Also, I am assuming that the dark red car in that link is yours? It is tough to tell from the photo, but it looks like your car has the narrow track front suspension on it. Going to the wide-track front-end would likely improve stability as well.

Edited October 26, 2010 by supersportsp

[s2k7]

xxx

Edited January 24, 2014 by s2k7

[rnr]

As the rpm & speed rises, the steering is like a jello & my fear is growing quickly. Sometimes I'm guessing the rear wheels spinning in the air.

 

I am confused - how can a "jello" steering be caused by spinning rear wheels? Do you have any confirmation that the rear wheels are spinning - have you heard wheel spin or has anyone else seen tire slip?

 

Perhaps it is just the steering going light due to cycle fenders in which case you try venting the fenders and/or going to the CSR style "spoiler" fenders.

[JohnCh]

As others have said, there is something wrong with your setup. You should fix this before you begin experimenting with add-ons like a rear diffuser. Are you sure you are feeling lightness in the front end and not bad tracking/tramlining? This can be pretty disconcerting at speed. Assuming that is the problem, in addition to checking the alignment settings, also check your front & rear wheel bearings for excess play. As for alignment, below are the Westfield factory settings and what I am running on my car (SEiW with widetrack, no ARBs). The latter settings came from some racers in the UK who have experimented a lot both on and off track and they have transformed the handling of my car. They are a little aggressive for the street and do have some wear problems in the front, but the car feels far more planted, with much better turn in than stock. A few Westfield-specific things to consider:

 

• Front ride height should be set so the imaginary line between the inner and outer pivot points of the lower control arm is parallel to the ground
  
• Rake is critical. The more you stray from the recommended range of 15-20mm higher in the rear, the more problems you’ll have. The factory points for these measurements are in the build manual. Let me know if you don’t have this.
  

Factory (Front/Rear):

Camber: -0.25 to -0.5/-0.25

Toe: 10 min toe in/30 min toe in

Rake (Rear-Front): 14mm

 

My settings

Camber: -2.5/-1.5

Toe: 15 min toe out/30 min toe in

Rake: 20mm

 

-John

[Kitcat]

If your steering is so scary you think it might kill you, something is seriously wrong. Have you tightened all suspension components (not just the ones that are easy to reach, but all?). I had a huge steering stability problem at LOG 30 and Tom (YellowSS) Dx'd loose front upper A-arm bolts. They were nearly impossible to reach, so I had skipped them. When I finally tightened them, handling nirvana was restored.

 

Ditto engine mounts. Checked air pressure (a little goes a long way in a 7, I run 16-18)?

 

An alignment (with you in the driver's seat-or a comparable amt of weight substituted)? I think I have max caster dialed in-wh/helps with steering stability, a touch of toe-out (some toe-in wld be better for stability tho) and abt a degree of negative camber up front. Everyone has their own alignment preferences.

 

Better tires (are yours damaged or do they have uneven wear).

 

But something's out of whack. I cant believe a small change in engine placement wld have such an adverse effect on steering.

 

Mike

[slngsht]

as others said, something much more basic is wrong... aero aids hardly have any impact at normal highway speeds. Check for play in suspension, alignment, etc...

[s2k7]

xxx

Edited January 24, 2014 by s2k7

[supersportsp]

Not to be too direct, but the description of your issue(s) could use a little refinement. I think many of the folks that have wandered into this post are chasing around the various issues you have described pretty effectively, but you seem to be unclear as to what the actual issue really is.

 

What you describe in your last post referencing the master/slave change sounds like an issue with your clutch. Are you breaking traction or is the clutch slipping? That is a huge difference when it comes to providing a solution. Either way, aero apendages are highly unlikely to solve this issue.

 

What does the light steering have to do with the clutch slipping or traction issues? Why is steering light only above 8k rpm? I have not heard of steering issues related to RPM. Does your steering rack/column sit very close to the motor and experiences significant vibration at that RPM?

 

My revised recommendations:

- Figure out if your clutch is slipping or if you are breaking traction

- If clutch is slipping, fix

- Try running 16-18 lbs pressure in your tires

- Check all suspension components for free play and tighten accordingly

- Get your alignment and rake set to the specs that JohnCH's posted

[JohnCh]

Adding to supersportsp's post, are you saying that the issue is acceleration based, not speed based? i.e. If you are cruising in 6th gear at 70mph does the steering still feel light, or do you need to be accelerating hard in 2nd or 3rd to experience this at that speed?

 

-John

[s2k7]

xxx

Edited January 24, 2014 by s2k7

[Kitcat]

Light steering under hard acceleration wld not be surprising. I heard of an instructor who had a unique way of showing this at trackdays w/newbies.

 

He'd explain the phenomena, then reach over at a high acceleration straight, grab the steering wheel and yank it downwards. When the car continued along straight ahead, he'd make his point again. (I don't recommend trying this at home kids).

 

Anyhow, if your steering feel returns on deceleration, it sort of suports this.

 

Exactly how much HP does your engine make?

 

Mike

[s2k7]

[xx

Edited January 24, 2014 by s2k7

[Mondo]

With a custom made location for the engine you might want to check the 4 wheel balance of the car too (preferrably with you in it). Have the castor checked when you get the alignment done. The more castor the more the car wants to stay straight (a little heavier steering too).

 

I can cruise the freeways well past the speed limit... though with the really sensitive steering, approaching triple digits on country roads, with their dips, imprefections, is too scary for me.

 

Disclaimer: I would never intentially go over the speed limit;)

[gjslutz]

When I run 0 toe in for crisp turn in, it is a hand full at high speed straight line.

 

If I add a little toe in it feels very stable at speed. It is like installing feathers on an arrow type of difference. I also run 2.5' of camber at all times.

[MichaelD]

My car is the first Caterham with a factory installed Zetec in the US. The engine is back as far as it can go. My cross weights ae 50 x 50.

 

The Clam Shells would make the front very light over 110. To a point that I felt more than a little suseptable to cross winds. This photo shows the crude yet very effective splitter/large canard combo now on my nose cone. With it, the car is rock solid at 135+ with plenty of front and rear grip.

 

One of the Birkins I run with seems to be able to beat me on the long straights. He also has a diffuser with similar HP. Maybe they do help ;-)

 

Can't upload photo??

[pureadrenalin]

Check the alignment, and then close up the floor. Diffuser or not, eliminating the lift from the air packing in to the gap under the fuel cell will make a difference.