100mph plus

[Red SLR]

On the track I struggle to get over 100mph. At Elvington the main straight is quite long, I would say about 2/3 the length of Hangar straight.

 

Other 7s were reporting about 110 to 115 mph. I managed 101.4mph but that was breaking so late it was getting on the limit. I am by no means a slow driver but I am just wondering why I am that much slower on the straight. (I am getting a good drive onto it also)

 

I am running about 150bhp at the moment. The other 7s there were at 170 - 230 bhp.

 

Is my power the main factor here?

 

I am going to take the power to 180bhp over the winter and I hope this will help.

 

Would a different diff help?

 

Ta.

 

Simon.

 

X777CAT

[Dave M]

Simon,

 

Lose the screen and fit a JPE or aero. Made a real difference to my car in that it gets to 100 quicker on the straightsand keeps pulling. Also saves a bit of weight wich will help with your power disadvantage.

 

Dave

PRB87

 

Edited by - Dave M on 16 Aug 2001 11:47:14

[Red SLR]

I have one on the way teeth.gif

 

X777CAT

[rynicolson]

Simon... I think I remember we have similar cars, what aeroscreen are you going for? Are you keeping the screen and planning to be able to swop? Also remind me what you have done to upgrade to date... mine is 1.6 lightened flywheel and SS upgrade, what should I do first?

[Tony Martyr]

There are two considerations here, First point is that the track section of which you speak might not be long enough for you to have reached maximum possible velocity. In which case you were still accelerating. Rate of acceleration is a function of engine power and effective mass. More power, lighter car = better acceleration. The role of engine to wheel ratios is a bit complacated but effect acceleration more than maximum speed - unless you completely run out of engine revs

The second point is that maximum speed of any vehicle is determined by its Road Load Equation. The first term relates to vehicle mass, so you should lose weight.

The second term is concerned with rolling losses and in practice is mainly the energy absorbed by tyres, so you should run with the maximum pressure commensurate with other performance requirements.

The third term is the killer because it relates to air resistance that increases exponentially with speed and characterised by a function peculiar to the vehicle shape, commonly known as the cd factor. When you reach terminal steady speed you have run up the curve of the RLE so that the force available at the wheels is exactly balanced with the resisting forces of which the air resistance is greatest. If you have more power you will not get a directly proportional higher speed because of the gradient of the curve.

Theory therefore dictates that you take off your window screen and replace it with something that improves air flow over the whole width of the car, take off the front number-plate and keep on your side screens.

 

[Peter Carmichael]

Higher speeds are all about power and aerodynamics. If you have identical aerodynamics between two sevens and one has 150bhp and the other has 250bhp, when the 150bhp car is at top speed the aerodynamic drag is soaking up all of the power. At this stage the 250bhp car still has 100bhp left and keeps accelerating.

 

Drag power is a function of the cube of the speed, so if the 150bhp car is all out at 120mph then the 250bhp car will be all out at 142mph (120 *(250/150)^-3).

 

[Please note that none of this allows for mechanical losses]

 

The essence of this game is that there is never an amount of power that is too much, which makes it a rather silly game of diminishing returns (been there, doing that).

[Red SLR]

ok...... well up to date I have...

 

Replaced the number plate with a plastic sticky one that is on the top of the nose cone.

 

I am getting an aero screen with the idea of being able to swap over from full glass to JPE in the summer / when on track.

 

So after fitting the JPE I should reduce my drag a bit and see better top end acceleration. Then once I have more power the top speed should be attained much quicker. (133mph) I take it this is governed by the diff? As an SLR has a top end of 140mph...

 

 

 

X777CAT

[Jason Plato]

Simon ,

Two points , I assume the other drivers were not quoting their speeds from the caterham speedo ?? as this is often 10-15% fast ,

Secondly the speed you reach down a straight is affected by your exit speed out of the last corner .

 

You should remember that a TVR would quite often pass me on a longish straight but I would pull out huge gaps in the corners and under braking .

 

Ps : Take your headlamps off for trackdays also .

 

Dave

[domster]

Hang on a minute... 140 mph out of an SLR?

 

Now, that may be possible, but would anyone really go that fast in a Seven (wind noise, skittishness, sheer terror)

 

So... answers please: what is the fastest speed anyone has ever done in a Seven???

 

I never did much more than a 110 on the speedo on my Supersprints - it was a bit lively and I didn't even have the quick rack and smallest Momo!

 

Rgds

domster

 

 

[V7 SLR]

Rev limiter in 6th in V7. Still not fast enough though. Vinnie beat me to the fastest lap on the Scandi circuits last year. Power upgrade over this coming Winter won't help either unless I raise the rev limiter.... not keen. Scared I might destroy V7's motor.

 

If you want to know how fast that was, use the gearing program with the following info:

 

6th = 1:1

Diff = 3.62:1

Outside diameter of driven wheel = 22 inches

Rev limit = 8000rpm

 

Caterham speedo indicated a shade under what would have been 160mph had there been any indication past 140mph on the clock. Bit of a work of fiction eh?

[domster]

Errr, schoolboy maths says that's about 145 mph.

 

What speed did you get - I was chucking pi in from memory (3.159?).

 

PS My car is geared to do 44.1 mph per 1000 rpm in top gear. It ain't a seven, but can you guess what it is?

 

[Peter Carmichael]

A common misunderstanding, domster.

 

It isn't the quick rack or the small wheel that makes the car lively. The chassis may be lively or stable, depending on set up. The steering link is just the size of the lever you have to winkle the car away from its intended course. If the car is unstable, then throwing it into a loop with an overgeared steering input will make things unmanagable quickly. If the car is stable, the overgeared steering just means you steer by larger pressure/smaller movement rather than arm twirling - it is actually more stable.

 

FWIW, I think at 140mph you should be quite happy driving with your hands off the steering wheel, otherwise your car is unstable so what are you doing at such speeds... (OK, maybe you are exploring into an area where the car becomes unstable and that is a suitable trade off for a handling gain elsewhere, but you know what I mean).

 

According to the Stack, 136.7mph, so not quite as fast as V7's ~145mph. That was 8000rpm in 6th which is explained by my 6inch tyres having a 21inch diameter to V7's 22inch. I was still accelerating at 0.12G. Don't remember if I had white knuckles or not...

[CHRIS CLARK]

...................probably not if you didn't have your hands on the wheel, but the buttocks were probably in full tension mode!!!

[EFA]

If a std SuperlightR with aeroscreen can do 145mph I'll eat my hat.

 

My VX when newish would hit about 125mph on the Lavant straight at Goodwood when it had what was actually dyno'ed at 197bhp. Lavant straight is almost a mile long.

 

The effects of aerodynamics on any Seven are such that at around 125 -130 mph on a still day the level of acceleration will dramatically reduce. This is due to the large frontal area relative to the slipperyness of the shape.

 

Removing the headlamps makes only a very marginal difference - too small to figure if its just a lap to lap improvement.

 

I have experienced this 130mph brick wall for years with a much varied power output as I have developed my car. 145mph needs at least 230 - 250bhp IMHO. JPE's which had around 230bhp in reality could just top 148mph 2 way average. The extra 40-50 odd horsepower I have in excess of this adds little to the top end - my car tops out at a GPS calibrated 153mph when it had gearing that permitted this speed. Currently is geared for 151mph at 9250rpm - a speed I would not attempt to hold - read driver restricted max is about 145mph.

 

Where the effect is very noticeable is between 100 and 130mph, to the entent that I can rapidly reach 130mph going uphill (up the Honfleuer bridge for ex) then the rate of accel drops right off.

 

Do you get the brick wall effect Peter?

 

 

 

 

 

Fat Arn

See another FAT ARNIE here

See a meaty Vauxhall car here

See the Le Mans Trip Website here

See the Lotus Seven Club North Kent Website here

 

[Red SLR]

I am exiting the corner ok.

 

I think the speedo suggestion is a good one! I was using my cycle computer which is spot on.

 

I think it would be interesting to see just how fast a 7 could go,,,, before it took off!!!

 

I could see it doing a NASCAR style flip at about 200mph.

 

I am at Anglesey tomorrow so I wont be able to see if I can break the 100mph, but I will be back at Elvington by the end off the summer.

 

 

 

X777CAT

[Martin H]

Another possible thought about the speed differentials - were they slipstreaming other cars (could make up to 10mph difference)?

[kenny.]

I think we should erase this thread in case any wallowing 4.2 litre Cerbera owners are looking in.....they'd piss themselves.

 

Kenny HPC

 

 

[V7 SLR]

What's the point in having a fast TVR if it won't start or breaks down before the end of the journey?

 

Anyway, I understand that my simplistic approach to calculating speed may in fact be erroneous. For a start, the 22 inch diamter I quoted for my driven tyres is actually what is written on the side of them. I have no idea how accurate that is. Also, I have a screen, so even at 197bhp would doubt it's ability to reach beyond 135mph, and wouldnt' be surprised if it was limited to 130mph, however, the facts are that I have felt the rev limiter in 6th, so how is this so?

 

I would have thought that the calculated speed based upon gearing/diff and tyre diameter against a known rev limiter would be accurate, but I wonder whether there are other forces at work. Is it possible that due to increasing mechanical losses (friction and wind resistance) the car could actually slow its rate of acceleration compared to the rate of increase in revs and therefore the calculated top speed becomes inaccurate?

 

Anyway, I stand by my observations of the rev limiter in 6th. It got there pretty easily and the hard cut out is quite ovbious. It's possible the gearing isn't what I claimed as I've never verified it, but it is what is supposed to be standard.

[pfreeman]

Managed a couple of 137 mph stints in my R500 using GPS for actual speed, speedo was showing 143

[EFA]

Paul,

 

You can calibrate the Stack unit to be 99.9+ % accurate using a handheld GPS.

 

The linearity of the tyre dimension is suprisingly constant so acuracy remains across a wide range of speeds - I tested this at 30, 70 and 120mph a few months ago.

 

V7 - The ACB10's are exactly 22.1 inch dia. If you used 22 inches this would not make a huge difference. Perhaps you have a 3.92 diff instead of a 3.62 - this would make your speed calc more appropiate to the hp of your car, and also explain how you can easily hit the limiter in 6th.

 

 

 

Fat Arn

See another FAT ARNIE here

See a meaty Vauxhall car here

See the Le Mans Trip Website here

See the Lotus Seven Club North Kent Website here

 

[Nick M]

That's assuming you can rely on the GPS set.

 

Most of them use a simple calculation to work out speed based on the time taken between two position fixes. If those position fixes are perfectly accurate then so should the speed reading, however, there is still a small error in the GPS signals to the tune of about 10 to 25 metres. OK, so it's not much, but it does mean you can't rely on a GPS for a perfectly accurate speed measurement.

 

We proved this on my father's boat where we calibrated the log using a measured mile and a stopwatch. As a result, the speed reading is pretty accurate and, on flat water and with a constant throttle opening, the speed only varies by about 0.2 of a knot over a mile or so. The GPS sets (two of them), however, fluctuate by as much as a knot which, at 20 knots, is a 5% error.

 

The faster you go, the harder it is for a simple GPS set to maintain accurate position fixes so the speed calculation suffers as a result.

[V7 SLR]

It's possible.... but as I can't rely on the speedo at all I can't even tell whether my rev figures at 100mph match other people's.

 

It would be odd though, as it was a standard purchased SLR in late '99.

[Peter Carmichael]

Arnie,

 

I wouldn't know whether to call it a *wall* because it still felt astonishing to go from cruising @96mph to 136mph in 9.3 seconds. A foray into these silly speeds was happening just inside the attention span of three goldfish, which is about all I can manage.

 

1. I wonder what happens when I press that pedal on the right.

2. Coo.

3. Look at that pretty reflective car with the two burly front seat occupants.

4. I wonder when my head is going to fall off.

5. Wow. Wouldn't it be crazy if my tyres exploded right now.

6. What's that noise? Ahh that will be the engine.

7. Nice day for it. (going)

8. ! ...(gone)

9. Now what was I just doing?

 

Now your point was about how nothing much happened above 130mph (the wall). I changed into 6th at 132, and the thing picked up and accelerated some more, albeit less than before and in line with the maths. I certainly didn't deploy the patience I used to max a Fiat Coupe Turbo on some downhill section of autoroute or the ruthlessness I used to autobahn a BMW 323i at a trip computer calculated average of 140mph for three hours.

 

There is always that bit of maxing a car when the asymptotic approach to the max speed results in a crawl where the speed does not appear to change at all for a period of seconds until the speedo creeps another notch around and maybe there are only 1 or 2 mph to go before you call it a day. I haven't reached that yet at 136mph so the max speed must still be a few thankless mph away. The gearing of 6th would pitch me back on max power at 148mph, so I think I might end up just short of that. Say, 143-145. I assume that 140 upward would be a drag.

[Red SLR]

It seems that my car with only 150bhp will struggle once over 100mph. The acceleration really slows down at about 95 and then it starts to crawl into plus 100mph.

 

Over the winter I palan to bring the power up to around the 190bhp mark. I hope this will give the car the extra power it needs.

 

At Anglesey on Friday the Exiges were puling about 105mph on the straight and a W*******d Blade was doing over 110. My 7 was only managing 94mph. I know I am getting a good run down the straight as I was following the blade onto it at the same pace, then he just pulls away.

 

sad.gif

 

 

 

X777CAT

[Alex Wong1697456877]

I saw an indicated 135mph when my 200BHP engine went pop at Ceram. This year with 40BHP more, I saw an indicated 150mph at nurburgring at the fast section just before the Foxhole.

 

I think my speedo is pretty optimistic though!

 

Alex