Weight Distribution

[Graham Hutton]

Just had a conversation with the specialist who is going to do a flat floor setup with me on my new 310r tomorrow. He threw some doubt in my mind about weight distribution when I mentioned wanting the front ride height about 10 or 15mm lower than the back. His concern was that it may be better to have more weight at the rear.

SO, what do you all think about front to rear weight distribution? It was my understanding that most sevens are setup with about 15mm rake and you end up with close to 50/50 front to rear? As an aside, they do look a lot better with a little bit of rake too!

Any thougts appreciated

 

[SM25T]

Duplicate deleted

[SM25T]

The set up must be done with the equivalent of your weight in the car, or you and a passenger if that's how you normally travel. And yes ... set front first to get required clearance under sump, then rear 15mm higher.

[David Brown]

Has he set up many sevens before?

[Tazio]

I second what David asks, how many 7s has he done?, does he know how to make them work?, camber, castor, toe, front and rear?

The 15mm rake is as specified by Caterham, over time as you decide how you prefer your car to handle, whether you prefer a lot of oversteer, a little oversteer, or a touch of understeer, this can be achieved by changing the rake, and tweeaking the arbs.

Some set up guys like to set the cars up so they have equal weights across the diagonals ( weight measured on N/S front plus O/S rear equals that of O/S front and N/S rear) with the driver plus normal running accoutrements you carry in the car. Others do it differently. 

[Graham Hutton]

He's a pretty knowledgeable chap. I'll be sitting in it and we'll be setting the diagonals to 50/50. He spends a of of time setting up drifting cars rather than caterhams hence the question about how much weight at the back. I originally set the spring lengths to some dimensions given by Caterham when I was building the car and that has resulted in the ride height at the front and rear being the same. I never noticed any guidance from Caterham on the rake when I was building it hence thinking that the same ride height front and back was right. Anyhow, I'm now going to drop the front 15mm as this will still give about 110mm under the sump and then set the diagonals up.. It'll be interesting to see what the total front and rear weight distribution is going to be. Hopefully not a million miles away from 50/50 too :-)

Thanks for your reassuring comments everyone

[SM25T]

If he sets up drift cars, watch he doesn't increase the tyre pressures to 30 psi !

[Graham Hutton]

Ha! Hopefully he won't put big Nissan stickers on it either although maybe a nitrous kit would be fun.....

[David Brown]

110mm under the sump sounds rather more than most have although would certainly give additional ground clearance. Are track rod arms horizontal with that setting as can result in bump steer if not?

[Graham Hutton]

Thanks David. I think the track rod arms will be nearly horizontal at that setting but I guess the sequence needs to be...

Set front ride height so sump clearance is OK and track rod arms pretty level. Make sure rear is about 15mm higher and the car looks about right. Check weight distribution front to rear is OK and then set corner weights balancing the diagonals. Then I'll make sure we have -2 degrees negative camber on both sides and the toe is straight ahead.

I'll let you know how it goes....

All your comments and advice are really appreciated

[Curly]

Sorry for the slight hijack.

Having fun adjusting ride height to meet Aus rules of >100mm ground clearance -  have adjusted the front to meet that for the sump.

Where do I measure the ground clearance (at back and front) to get the back 15mm higher than the front?

Thanks in advance

Ciurly

[Tazio]

At the rear its where the front of the rear wing wing meets the bottom chassis rail, at the front, some people use the chassis rail underneath the rear of the front lower wishbone, others between the bolts holding the engine mounts.

[Tazio]

Graham,

 

You haven't said why you came up with 110mm under the sump, that's quite a long way from what's considered 'normal, of around 80 to 85mm. If you can just roll a coke can under the sump it's enough.

[7 wonders of the world]

Your guy also needs to be looking at bump steer and castor up front and rear camber, thrust and toe - you cant just pick and choose which you adjust as one impacts on another.

[David Brown]

Sorry, but really think you should be using one of the many companies who are experienced in flat flooring sevens who know what settings to use after you tell them road or track. Drifters and sevens are two completely different things. 

[andy couchman]

Not sure rake will alter the F/R weight distribution much and yes, getting diagonals roughly equal seems to be how most set up a 7. I thought rake was more to do with how the car handled and presumably being able to put the power down quicker (but hey am no engineer!)?

In terms of what weight distribution is best, my 1988 7 started with about 50/50 and BMW ads for years have said that was the perfect result. For a  low powered LR Defender offroad in a field that makes sense too.

However, in competition circles my understanding is engineers are usually looking for more weight on the back. Writing in a classic car mag a while back, Tiff Needell reckoned 40/60 was perfect. I've tried very hard over the years to get away from 50/50 and am up to around 47/53 (all figures with just me on board) and am fairly happy with that. That has included a CF nosecone, ali rad, ali XFlow head, no heater, battery and washer bottle in boot, only one horn and so on. One of the reason the 911 has been so succesfull in motorsport has been its rearward weight bias, despite the engine being in the wrong place and it acting like a pendulum (what engineers call having a high polar moment of inertia - the opposite of what we enjoy in our 7s). 

Having less weight at the front means better traction on starts but also better braking (braking transfers weight to the front so you still have some rear weight so the back brakes can do more), you can trail brake or be off the throttle in corner entry (a bit of deceleration will give you 50/50 balanced cornering) and the the car can get maximum traction by having max weight on the back under acceleration so you can accelerate earlier. All this is only possible due to modern tyre grip (hence 50/50 was fine back in say the 1950s).

At least that's my understanding (stands back while everyone who understands these things proves me wrong... :-)). 

[SM25T]

Have seen Sigma engines cars with 110mm under the sump.

[Tazio]

In the Sigma build manual, there's nothing about sump clearance, simply front and rear ride heights 150mm front and 165 rear.......funny how those measurements are no where near what mine are, after years of setting mine I've settled on 132 fronts lol.

[Graham Hutton]

Right then, I had my flat floor setup done today and the chap was brilliant. Really knowledgeable, happy to let me work with him throughout and a perfectionist. Corner weights now accurate to 0.03% and the whole car feels so much better. Ride height dropped at front so I have about 20mm rake, 95mm under the sump and the bump steer is massively better. The two things that I asked Williams Automobiles to setup at my PBC were out by miles!!! I'd asked for 1.5 degrees negative camber and they were both different but hovering around -0.5. I'd asked for a tiny bit of toe in and it was toeing out quite a bit. I guess this goes to show how important a flat floor setup is. The car also looks a million times better too. It has a good looking stance, if I say so myself. In the end, the front to rear weight distribution was 48% front, 52% rear. In case this is relevant to anyone, my 310R is an SV chassis.

Anyhow, thanks to all of you that gave me advice, I feel I've had a really good job done today. Had to drive home in the rain but tomorrow will be sunny and I'm going for a blat!!

[David Brown]

Glad it's worked out well for you.