trailing arms and 'squatting'
Discussion
I have a scimitar SS1 turbo Mod Prod sprint and hill climb car that has trailing arm rear suspension similar to that of my TVR S2 (and some other TVRs too I beleive) and it suffers from severe squat under launch off the line. The shocks are single adj AVOs near vertical currently running 275lb springs (up from 180lb on std road car). Upping the spring rate over the last half season of last year from 225 to 250, then to current 275 hasnt made a huge improvement in the squatting issue although the ride is spine shattering on big bumps on track. Yesterday at north weald, the rear wheels left the ground on a bump on the run up to the finish! Softening the dampers (most other competitors did the same) yielded a little more compliance but worsened the squatting on launch and made the shocks either hit the bump stops or springs go coil bound on the bigger bumps.
Am i expecting too much on airfield surfaces? (its pretty good on circuits to be honest). Do you think further increasing rate to 300 or 325lb is the way to go? Car weighs in region of 700kg 53%F/47%rear split.
Has anyone tried raising the wishbone mounting on the chassis of a TVR to increase wishbone antisquat angle? if so what were the results?
Your thoughts please gents, sorry for the length of query.
Am i expecting too much on airfield surfaces? (its pretty good on circuits to be honest). Do you think further increasing rate to 300 or 325lb is the way to go? Car weighs in region of 700kg 53%F/47%rear split.
Has anyone tried raising the wishbone mounting on the chassis of a TVR to increase wishbone antisquat angle? if so what were the results?
Your thoughts please gents, sorry for the length of query.
Need to know more about the suspension geometry to understand what's going on. Do you know the full spec? Ultimately what you want to find out is how far the wheel moves when the damper extends by an inch, what angle the pivot axis runs at, how far the rear wheel is from the pivot axis, the height of the pivot relative to the height of the rear axle when the car is static. If you aren't in a position to provide these figures directly, they can be worked out from a schematic of the suspension and half a dozen critical dimensions.
Is it just suspension travel that is the problem, or are you suffering from wheel hop under hard acceleration?
Is it just suspension travel that is the problem, or are you suffering from wheel hop under hard acceleration?
YES!
TVR tried it!
I'm pretty certain the later "S" models had a bit of anti-squat and I'm pretty certain it was done by moving the pivot point on the chassis. Can't remember how much it moved though. I think the original 2.8S rear suspension design was a farily close copy of the Scimitar SS1. Will try to find out more...
TVR tried it!
I'm pretty certain the later "S" models had a bit of anti-squat and I'm pretty certain it was done by moving the pivot point on the chassis. Can't remember how much it moved though. I think the original 2.8S rear suspension design was a farily close copy of the Scimitar SS1. Will try to find out more...
GreenV8s
i will take all the measurements, just havent got round to it as yet. Ive read the usual books (Staniforth etc etc) but none of the ones ive read go into much detail re single trailing arm setups. Its just a fluke i have an S2 and an SS1 which both have almost identical rear susp designs (someone from tamworth holidayed in blackpool or vice versa?!). Watching S series TVRS launch, they too squat dramtically off the line, so i was wondering wether it was more a design trait than anything else. With 4/5link rear setups on live axles, raising the inboard link mountings to the chassis makes a huge difference, but then its usually the lower link which is raised more than the upper relative to one another.
AVocet
cheers for the help, be interested if you turn anything up. I too am sure ive heard something along those lines in the dim and distant past.
i will take all the measurements, just havent got round to it as yet. Ive read the usual books (Staniforth etc etc) but none of the ones ive read go into much detail re single trailing arm setups. Its just a fluke i have an S2 and an SS1 which both have almost identical rear susp designs (someone from tamworth holidayed in blackpool or vice versa?!). Watching S series TVRS launch, they too squat dramtically off the line, so i was wondering wether it was more a design trait than anything else. With 4/5link rear setups on live axles, raising the inboard link mountings to the chassis makes a huge difference, but then its usually the lower link which is raised more than the upper relative to one another.
AVocet
cheers for the help, be interested if you turn anything up. I too am sure ive heard something along those lines in the dim and distant past.
Wheel hop is the achilles heal of the S series trailing arm suspension, so it's good to know that yours doesn't suffer from that. Apart from that the S series suspension works really well, with the right technique it's got excellent traction off the line and the trailing arms give it useful amounts of anti-lift under braking. Hopefully yours will work well too with a little fettling.
I spoke to someone who used to work at TVR and he confirmed that the "S" rear trailing arm pickups were moved upwards on the chassis by about an inch or so in an attempt to reduce squat. If the pivot point is above the rear axle centreline, the car will have "anti-squat" so the tractive force will try to lift the back when you accelerate. Obviously this is countered to an extent by the weight transfer towards the back of the car. If you have it high enough, you can get the back to lift when you floor it!
Has your car been lowered? If so, that's the main cause of squat in cases where it wasn't previously present.
Also, you could try and find some long, squidgy bump stops but that's sort of cheating I guess.
Has your car been lowered? If so, that's the main cause of squat in cases where it wasn't previously present.
Also, you could try and find some long, squidgy bump stops but that's sort of cheating I guess.
any idea how i can find out by how much the S mounting point was changed to the S2s? are there chassis tech drawings anywhere? If not does anyone have a formula for working out the heights? i have Alan Staniforths books and a Fred Puhn one but cant find anything in those (unless i missed it....). Any ideas?
As for the springs, ive worked out the wheel rates again and am convinced that 250lb for the rears is still too low, so have ordered new 300lb springs today which should at least go someway to keeping the tail off the ground off the line....
As for the springs, ive worked out the wheel rates again and am convinced that 250lb for the rears is still too low, so have ordered new 300lb springs today which should at least go someway to keeping the tail off the ground off the line....
TVR should have drawings but they won't be in the public domain. In any case, what worked for them might not be appropriate for you! From memory, it was less than scientific anyway! I can't remember whether the ears on the chassis were extended or the trailing arms were altered - or a bit of both. I think when TVR used a very similar design to the SS1, they forgot tha thtye'd have a bit more torque - so squat was more of a problem!
If it was my car, I'd have a look at where the axle centre was relative to the trailing arm pivot with me sat in it and some fuel - and then extend the chassis mounting until the two were level - just to see how that worked out. If it didn't do what you wanted, you could then always try a bit more.
If it was my car, I'd have a look at where the axle centre was relative to the trailing arm pivot with me sat in it and some fuel - and then extend the chassis mounting until the two were level - just to see how that worked out. If it didn't do what you wanted, you could then always try a bit more.
Ha ha ha, a british sports car company not using a scientific approach?!? how unusual...
As for the geometry, i am going to modify the chassis mounts with a series of holes in say 1/2" increments in an arc to ensure that as i raise the mounting it still keeps the wheel in the correct wheel base position. As for torque, the car weighs around 700kg now, with 240lb/ft of torque and super soft hillclimb List 1B tyres, so a little different than when it left the Tamworth factory. Thanks again for the help and brainstorming, cheers.
As for the geometry, i am going to modify the chassis mounts with a series of holes in say 1/2" increments in an arc to ensure that as i raise the mounting it still keeps the wheel in the correct wheel base position. As for torque, the car weighs around 700kg now, with 240lb/ft of torque and super soft hillclimb List 1B tyres, so a little different than when it left the Tamworth factory. Thanks again for the help and brainstorming, cheers.
CNHSS1 said:You haven't reported any of the basic dimensions of the suspension. Do you have any idea what suspension rate you are getting at the wheel, or how much bump and droop travel you have before you hit the bump stops? I suggest you concentrate on the basics before you start modifying the geometry.
i am going to modify the chassis mounts with a series of holes in say 1/2" increments
Apologies for not filling you in with all the details. I dont have all the details here at work, but i have the wheel rates. Rear is currently 108.23lb (250lb springs) with 170.14 front (400lb springs). The rear trailing arm is 372mm from chassis mounting pivot to axle centreline, with 245mm from chassis mounting pivot to coilover mounting point.
At current ride height, trailing arms are parallel to the ground with approx 60mm droop and 120mm bump at the wheel.
As for modifying the chassis mountings the whole point is to retain the std and have additional points for testing, not burn my boats. Sorry if i confused you.
At current ride height, trailing arms are parallel to the ground with approx 60mm droop and 120mm bump at the wheel.
As for modifying the chassis mountings the whole point is to retain the std and have additional points for testing, not burn my boats. Sorry if i confused you.
Is that current ride height with full fuel and driver? If not, it will be sloping downwards to the front when you get in and will start a vicious circle when you then let the clutch out whereby the harder the car accelerates, the more it tries to push the back of the chassis downwards, I guess.
Are the dampers absolutely vertical? If not, you might have a significantly lower rate at the wheel than you're estimating. But if those figures are accurate then the static deflection at the rear should be very similar to mine - and mine is quite a bit stiffer than standard and doesn't suffer from excessive squat.
Is it fully trailing or semi trailing? This can effect the amount of pro- or anti-squat effect from the trailing arm geometry. The TVR S one is almost fully trailing, I assume the SS1 is similar.
I don't know which AVO dampers you're using but the ones I tried were pretty hopeless, by the time they were stiff enough to provide sufficient damping the ride was totally unacceptable. I'm running stiffer springs now with Nitron dampers and the ride comfort is far better, and I haven't compromised the body control to achieve it.
Another thing worth checking is the bump stops. The TVR runs very long and progressive bump stops which raises the suspension stiffness well before the damper runs out of travel or the spring goes coil bound.
Something else worth trying is to change your technique off the line. If you wind up the transmission against the handbrake before you move off the line, you can get an instantaneous weight transfer which gives loads more traction off the line, it also stops the big pitch transient which interferes with the weight transfer and uses up all your bump travel.
If you're still running out of bump travel under acceleration then your options are either to raise the ride height or increase the spring rate. Both have their draw backs. I'm running about 150 lbs/in at the wheel with 500 Kgs over the back axle and I'd like to go softer for improved traction, but the reason for running this stiff is for roll stiffness not because I have any problems with suspension travel.
Is it fully trailing or semi trailing? This can effect the amount of pro- or anti-squat effect from the trailing arm geometry. The TVR S one is almost fully trailing, I assume the SS1 is similar.
I don't know which AVO dampers you're using but the ones I tried were pretty hopeless, by the time they were stiff enough to provide sufficient damping the ride was totally unacceptable. I'm running stiffer springs now with Nitron dampers and the ride comfort is far better, and I haven't compromised the body control to achieve it.
Another thing worth checking is the bump stops. The TVR runs very long and progressive bump stops which raises the suspension stiffness well before the damper runs out of travel or the spring goes coil bound.
Something else worth trying is to change your technique off the line. If you wind up the transmission against the handbrake before you move off the line, you can get an instantaneous weight transfer which gives loads more traction off the line, it also stops the big pitch transient which interferes with the weight transfer and uses up all your bump travel.
If you're still running out of bump travel under acceleration then your options are either to raise the ride height or increase the spring rate. Both have their draw backs. I'm running about 150 lbs/in at the wheel with 500 Kgs over the back axle and I'd like to go softer for improved traction, but the reason for running this stiff is for roll stiffness not because I have any problems with suspension travel.
i have checked the car again with me sat in it and the arm actually trails down towards the mounting now!(280mm from the ground at the drivshaft centre and 240mm at the chassis mounting bolts). Sounds like thats where to start. Guess i should get my ass down the gym a bit more....
I checked a std road car and they are level as std, mine uses larger rolling radius rear tyres with the spring platforms lowered to return it back to std ride height, so i think im making the problem worse.
How do i know if its trailing or semi? the outer most chassis mount is ahead of the inner by approx 30mm, i assume this is semi? The shocks arent upright but pretty close (pretty similar to my S2). Looking at section in staniforths book, i suspect its only having a very small effect on rate.
Im using AVO pro race spec dampers and to be honest have no problems with them. Ive heard many people discussing different makes re TVRs but ive used koni and spax and the AVOs seem better than both. Horses for courses i guessThe bumpstops arent progressive, but i would have thought that using a progressive bumpstop as part of the suspension is a bit of a 'band aid' to a problem that shouldnt be there?
Take your point on the launch technique tho, will defo try something else, but car uses AP paddle clutch and Nismo cover (read on-off switch!) so not much chance of modulating it! Will try different rpm and boost MAPs.
I checked a std road car and they are level as std, mine uses larger rolling radius rear tyres with the spring platforms lowered to return it back to std ride height, so i think im making the problem worse.
How do i know if its trailing or semi? the outer most chassis mount is ahead of the inner by approx 30mm, i assume this is semi? The shocks arent upright but pretty close (pretty similar to my S2). Looking at section in staniforths book, i suspect its only having a very small effect on rate.
Im using AVO pro race spec dampers and to be honest have no problems with them. Ive heard many people discussing different makes re TVRs but ive used koni and spax and the AVOs seem better than both. Horses for courses i guessThe bumpstops arent progressive, but i would have thought that using a progressive bumpstop as part of the suspension is a bit of a 'band aid' to a problem that shouldnt be there?
Take your point on the launch technique tho, will defo try something else, but car uses AP paddle clutch and Nismo cover (read on-off switch!) so not much chance of modulating it! Will try different rpm and boost MAPs.
CNHSS1 said:
The bumpstops arent progressive, but i would have thought that using a progressive bumpstop as part of the suspension is a bit of a 'band aid' to a problem that shouldnt be there?
The bump stop gives you a rising rate on the suspension, which allows you to run softer and lower suspension for the same overall momentum capacity. Given that you are inevitably going to run out of travel in some situations, it is better to have suspension that gets progressively stiffer and stiffer rather than something that just goes 'clunk' against the stop. Running on the end stops all the time wrecks the handling, but this is really just something to minimise the harm on the odd occasion when you run out of travel.
Gassing Station | Suspension, Brakes & Tyres | Top of Page | What's New | My Stuff