Start line grip (lack of) - revisited

[Paul Deslandes]

Even with 205 60 13 Toyo 888 GG tyres on the back on 8 inch rims I'm very short of start line grip. I expect the rears to spin up with a 245bhp Vx when cold but I reckon I ought to be able to do better off the line. 64ft times at Wiscombe etc usually 3.2 secs, others with comparable power/tyres are doing sub 3 secs. Maybe I'm just carp at getting off the line, but I am consistantly carp.

 

Do any of the suspension experts out there have any tips on improving things without necessarily spoiling cornering performance which is reasonably good, although the back end tends to be a bit too lively and gets out pretty readily as well on damp hairpins? Tyre pressures are 16psi all round and rake is about 15mm. Rear springs are 175lb/in and I tend to have my Leda shocks on 6 - 8 clicks (out of 24).

 

Fronts are also 205 60 13 on 7 inch rims and there is no tendency to understeer.

 

My thought is to increase rake but I'm not sure why this would make any difference.

 

With Brighton Speed Trials on Saturday a quick fix would be welcome!

 

Paul

 

 

 

Edited by - paul deslandes on 27 Jan 2009 11:20:41

[rgrigsby]

I've not been to Wiscombe but 3seconds seems like a very slow-64ft time. On the soft Kumho's we use in the L7Club championship people regularly see low 2s times at most venues.

 

I think relatively soft rear suspension can help with getaways but ultimatly it's down to how well the wheel spin/power is controlled of the line that makes the biggest difference.

 

Rob G

www.SpeedySeven.com

[peterg]

It sounds like your rear end is too hard

 

For the Brighton Speed Trials startline, I would reduce the rear tyre pressures to 10psi and soften the rear suspension as much as possible - just remember to put it back (but maybe leave the back a bit softer) before you drive home

[Jason Plato]

I may be wrong - but from when me and you have done the brighton trials together Paul I have noticed that you tend to use far more revs than me off the line ?

 

maybe you are simply sat there wheelspining with no traction not as a result of setup just too many revs too soon ?

 

I tend to go for the 3200 rpm steady throttle and then release the clutch moderatly and then control spin with the throttle fro the first few yards and then hook second with full traction. The same method has applied to all my various K engines upto 247bhp and the present duratec.

 

wiscombe if not very grippy at the best of times and I tend to tickle it off the line with very few revs .

 

Taffia rear gunner

 

 

Edited by - Dave J on 12 Sep 2008 08:22:45

[Jason Plato]

also to add

 

I would be chuffed with a 2.3 64foot time on R888's, even with tyre warming and very good startline grip.

 

regular 2.1 for acb's and sticky slicks etc

 

Taffia rear gunner

 

 

Edited by - Dave J on 12 Sep 2008 08:24:33

[Shaun_E]

GG is the medium compound for R888s - you could switch to the SG compound which is soft. Even better switch to Kumho V70A in soft compound.

It does sound like you have too hard rear suspension. Most of the L7 competitors are running somewhere in the 135lbft to 150 lbft range. I am using 150lbft, relatively stiff damping, Kumho V70 in soft 235/13 and get consistent 2.2s times.

The other variable is gearing. One of our competitors has a very tall 1st gear and the best he can get is a high 2.4s launch.

 

Yellow SL #32 - member of Drowned Rat Racing

[Shaun_E]

Agree with Dave on startline technique. I use between 2500 and 3500 rpm depending on conditions and bring the clutch up fairly quickly but smoothly, control wheelspin with throttle until full traction achieved and then nail it.

I switched from R888 SG to Kumho soft part way through this season and gained a tenth to 64 feet.

 

Yellow SL #32 - member of Drowned Rat Racing

[Mark Durrant]

I agree with Dave and Shaun regarding technique and even with a medium compound tyre you should be able to achieve 2.5 secs for 64ft as I could easily achieve this on list 1a tyres. Do you use a rear ARB, if so (for Speed trials only) disconnect the drop link on one side and run the rear dampers fully soft.

 

Mark D

Comp Sec

 

[Paul Deslandes]

that' the trouble with doing things late, my tyres are SG not 2G.

 

Dave, you're right about me using (too) high revs at the start. My throttle is very fierce and I've tried to tame it and make it more progressive by increasing the radius of the TB end of the linkage which has improved it but it could still be a lot better. Even with a new cable its a bit sticky and I think it needs re-routing to increase the bend radius.

 

My recent times at Wiscombe, apart from last weekend when it was wet and muddy, have been been in the low 43s so overall the car is going pretty well. But I reckon there's at least another half second to be had on the start if I can only tame the throttle.

 

10 psi seems very low but I know the dragster boys run almost flat so they get tyre wind-up. I've only got to get it back on the trailer so no probs going home!

 

Not sure why the rear ARB would make any difference in a straight line but mine's on softest setting as will be the shocks. If I had a set of softer springs I'd try those.

 

Anyone else at Brighton tomorrow? Supposed to be usual good Speed Trials weather.

 

 

[Richard Price]

Although not a quick fix, I'd have thought a reduction in rear spring rate would be of benefit.

Standard roadsport progressive springs are rated 110lbs/in with just the weight of car and driver, rising to 130lbs/in when in compression (the last part of the spring is rated at 200lbs/in, but you only get there in extreme bump).

I'd guess the quickest hillclimb cars are all running 130lbs/in or lower.

 

[Richard Price]

Paul,

I'd guess you'd gain more with better traction at Wiscombe.

With three very slow corners, traction out of them is everything!

Removing the rear ARB is likely to improve traction in the slow corners.

 

 

Edited by - Richard Price on 12 Sep 2008 09:06:35

[Jason Plato]

I would practise some starts on a quiet industrial estate - gaining part throttle control would benifit you everywhere, off the line and on all the hillclimbs. Or strip and refit the throttle cable to get a smoother run .

 

I havnt found that the spring rates between 200lb and 130 lb affect the startline times over the 64ft . I have found the dampers have a far greater effect, but tyres grip from warming , and part throttle control are the greatest gains.

 

for brighton try simply lowering the rear ride height by 2" on the platforms . I whizzed off the line when I did that 😬

 

 

 

Taffia rear gunner

 

[Jason Plato]

the arb will only affect traction where the ground is not level or the chassis has yaw / lean. Then it will attempt to lift the unladen wheel.

 

but if you take it off and dont compensate with increased spring rates you will get a large degree of body roll which will affect performance too.

 

Many hillclimb and sprint venues are bumpy and higher spring rates (circa 215lb rear + as used on the R/C400 race cars) are unusable to compensate for the lack of body control from the removal of the anti roll bar.

 

At Castle combe I cant run the spring rates I would like to use as the track is very undualting and bumpy and the car actually skips across the bumps loosing traction. So I use quiet stiff arbs to control body roll instead of higher spring rates.

 

its the same ole problem - do you prop the car up on dampers or springs or antiroll bars

 

 

Taffia rear gunner

 

[peterg]

10 psi seems very low but I know the dragster boys run almost flat so they get tyre wind-up

 

I ran my single seater on slicks at 8psi rear (usually 12psi) - it's not so much to wind up the tyres like the top fuellers do but to get the rubber dug into the rather patchy tarmac on the seafront

[Paul Deslandes]

Apart from going to the SG Toyos my biggest improvement was moving the torque curve down by changing cams and exhaust. Through the Wiscombe esses, 1st was too low and 2nd too high, now its 2nd all the way through with all the grunt I can use. I took the rear arb to its softest setting last weekend and it made a perceptible difference, especially out of Sawbench where the bottom of the hill was pretty green.

 

No time to get new springs for tomorrow so flat tyres, no shocks (well, minimum setting anyway) or rear arb and 🙆🏻 on the deck should do it. 12.56 to beat.

 

No ones mentioned rake to help the overall grip issue, apart from Brighton which is a special case as you don't go round any corners, at least not on purpose 😬

 

Thanks for your comments so far guys. Keep 'em coming.

 

 

[Jason Plato]

another alternative is to get another driver 😬

 

Taffia rear gunner

 

[Paul Deslandes]

Thought it might be worth an update on this.

 

It turns out that what I thought was a problem of a sticky and jerky throttle linkage was a bit more subtle.

 

It was, in fact, impossible to set the engine revs between 4200 and 5200 rpm off-load. If the throttle was pressed slowly down, revs would rise to 4200 and, any throttle increment more, go straight up to 5200. This was due to excessive ignition advance and has been tamed by retarding the ignition in the off-load sites over that rev range so I've now got some throttle control back. It won't make any difference to the power once the clutch is released and the throttle nailed as we'll be onto other load sites by then. Haven't tried it yet in earnest but it works in the garage.

 

In the process I copied Bob Simon's throttle linkage mod which makes the operation a lot smoother.

 

Now looking at softer springs to get some weight transfer off the line. 130 or 150 rear and 300 front on wide track seem to work out from the sums.

 

 

 

Edited by - Paul Deslandes on 27 Jan 2009 11:27:58

[Gambo]

Paul, not wishing to bang on about what has been mentioned, but 4K plus revs off the line would have seen me sitting in a great pile of smoke in my Vx. I like dave used to use about 2.5 to 3K to get mine up and running.

[Jason Plato]

take care with the springs - too soft on the rear with sticky tyres and you will end up with a understeering monster at the corners

 

 

here is C7 TOP

Taffia joint AO with Al

 

[Paul Deslandes]

Hi Richard, the throttle was pretty impossible all the way up really - almost like an on/off switch - so I'm hoping to be able to regulate the start line revs a lot more easily. 2500 - 3000 never worked for me but again that might be the lack of throttle control available. It'll be interesting to see what happens once that's sorted.

 

Dave, I'm going to try 150s and see what happens plus take the front down a bit as well in an attempt to keep some sort of balance. If you don't experiment you never find out 😳 and springs aren't expensive.

 

Paul