Upgrade path for a 'grand touring' focused 135

[gjm120]

Wonder why I've been told T2 / T3? Will probably end up with rake which appearance wise doesn't bother me if it's not extreme. But I don't want to end up scraping the front on speed bumps and inclines.

[gjm120]

Originally Posted by gjm120

Wonder why I've been told T2 / T3? Will probably end up with rake which appearance wise doesn't bother me if it's not extreme. But I don't want to end up scraping the front on speed bumps and inclines.

Correction to this: the recommended springs for my car are T2/T2. I had the part number correct but the wrong associated T_ for the rear spring.

[atr_hugo]

I think I'm getting closer to a decision on suspension upgrades (thanks to a lot of youse guys input). I'm leaning to the Koni/Eibach 1145-1018 kit (Yellows & Eibach Prokits). I've also been doing a bunch of bad math trying to figure out how I can achieve the 'agile AND comfortable' requirements. I'll explain the math in a later post.

But the math part went a bit like this YouTube vid:

[atr_hugo]

Here's what math has been perplexing me (and still is), determining the natural frequency of the sprung mass, given spring rates and mass.

I want to keep it between 1 Hz and 1.5 Hz which given the mass of the corner weights will determine the max spring rates I want to use. The M Sport frequency is about 1.1 overall, I think, (with the rear frequency being about 10% higher than the front - since the rears go over bumps after the fronts they need to resonate at a slightly higher frequency to 'sync up').

The Eibach ProKit and the BMW Performance Suspension springs are about the same for the 135i Heavy - the Prokits are 148 ib/in front and 456 lb/in rear.

So why do the rears have a higher number than the fronts? That has to do with the motion ratio (or leverage) of the spring. The strut has a motion ratio of somewhere between .94 and .98 while the rears are somewhere between .3 and .4. So the rear spring rate has to be multiplied by 3 to match the rate delivered by the springs hung on the struts.

It would appear that the Eibach Prokit will lower the car about 9/16ths of an inch over the M Sport suspension - something I can live with. And, if I've been doing the math correctly - which is a BIG if - I will have raised the natural frequency of the suspension by a tenth of a Herz (or thereabouts).

Then there is the critical damping - but the Koni yellows are rebound adjustable so we can play with it. Moving to non-runflat tires solved the bulk of the 'crashiness' I felt with the suspension so now it's just a matter of reducing the pitch and roll I feel under way.

PS - trying to come to grips with the math - the formulas I've found don't seem to be matching up with the numerical values I'm trying to use and the conversion process is defying the two working gray cells in my brain. Regardless, I'm not sure now that going up ~ 30 lb/in on the front springs will make a noticeable difference. What I think I'm seeing is that to noticeably change the natural frequency you have to significantly alter the spring rates, going from a natural frequency of 1 Hz to 2 Hz requires quadrupling the spring rate from what I can tell.

PPS - I think I have a handle on it now. The spring rate for the M Sport front is 120 lb/in, the corner weight is somewhere around 394 Kg. I think I understand the conversion issues from lb/in to N/m and believe, subsequently, that the M Sport has an ~ 1.15 Hz natural frequency front value and the Eibach Prokit has an ~ 1.25 Hz value. Phewww . . . . What I think I've seen in some sporting cars is that they are great at speed but ill-tempered (uncomfortable) at in-town speeds. I think that has to do with spring rates as much as shock valving. That's why I don't want to push the spring rates beyond 1.5 Hz

[atr_hugo]

So, here's what I worked out (and I would accept any corrections or updates, especially from fe1rx or Orb):

BMW 135i Calculations

To obtain a 1 Hz ride frequency at 900 lbs of weight on a corner requires a spring rate of 92.5 lb/in, then for 950 lbs of weight to keep the 1 Hz ride frequency the spring rate would be 97.64 lb/in. The weight at the rear of the car is 850 lbs – the position of the spring requires it to be multiplied by 3 – leads to a spring rate of 262.08 lb/in for a 1 Hz ride frequency.

Motion ratio of the springs hung on the 'McStrut' front suspension is somewhere around .94 to .98, the motion ratio of the rear springs between .3 and .4.

Multiply lb/in times motion ratio, then divide the result by 5.733 to get N/mm then times 1000 to get N/m. Convert the weight on the corner to Kg, then run the frequency equation.

The calculation is: frequency = 1/(2*3.14)*(square root of (spring rate (N/m) divided by mass (Kg)))

The 135i M Sport suspension uses 120 lb/in springs in front and 350 lb/in springs in the rear which calculate to a 1.1 Hz natural frequency front and 1.2 Hz natural frequency rear.

There are a couple of threads that address spring rates for aftermarket and BMW parts (many thanks to the original posters for getting this info, it has been INVALUABLE!):

http://www.1addicts.com/forums/showt...7#post22540257

http://www.1addicts.com/forums/showthread.php?t=1145748

Some sites that deal with natural frequency:

https://www.rqriley.com/suspensn.htm

http://www.autospeed.com/cms/article...ounce&A=112686

http://www.kaztechnologies.com/wp-co...m-Kasprzak.pdf

http://forum.wscc.co.uk/forum/topic/...rs-for-my-car/

[John_01]

There are a few things that can be refined to make the frequency calculation more accurate:
* Spring rate for 135i is more like 133lb/in Front and 365lb/in rear. These are the D6 / L3 spring set used in 135i. The other 120 / 350 figure is taken from TC Kline measurement of D4 / L2 springs used in 128i.
* The stiffness of suspension arm's rubber bushings can be added. fe1rx has estimated it adds around 7lb/in front, and 40lb/in rear, which is simply added to the effective wheel rate. When the M3 arms are installed it is reduced to 5lb/in front and 35lb/in rear.
* If you use the stock rear subframe bushings, it will reduce the effective spring rate. I guess it may be around 9% loss of mechanical spring rate for a stock 135i. Anyway don't use the stock subframe bushings because they suck!
* In the final frequency calculation, don't use the total corner weight of the car. You should subtract the unsprung weight including wheel and tires. For the front it may be around 120lb unsprung weight depending on the wheel and tire, and about 125lb for the rear. So the sprung front corner weight loading may be 920lb - 120lb = 800lb. For the rear it may be 795lb -125lb = 670lb. Probably a car with ZF auto transmission will be heavier still.

Due to the reduced weight in the calculation and added stiffness of suspension arm bushings, the frequency calculation will be higher that what you posted above. I calculated total effective wheel rate of 131.6lb/in front and 145lb/in rear. The frequencies then worked out to be 1.27Hz Front and 1.46Hz rear for a stock 135i.

[John_01]

Originally Posted by gjm120

Correction to this: the recommended springs for my car are T2/T2. I had the part number correct but the wrong associated T_ for the rear spring.

Since your car is a 128i it makes sense. T3 front springs are generally for 135i.

[atr_hugo]

Thanks!!!

This opens up another can of worms - if I'm already close to the far edge of the acceptable range (1 to 1.5 Hz) I may be looking at dampers only then.

John, thanks!!!

[gjm120]

Originally Posted by John_01

Since your car is a 128i it makes sense. T3 front springs are generally for 135i.

I was hoping you'd jump in here as you seem to have the best understanding of spring rates and their impact.

My car is AT and has about every option available. I'm currently on D6 / L3 from buying a used 135i setup. It was a very slight drop from the stock non-sport 128i suspension and I'm happy with the ride height.

From keeping notes reading about spring rates, I believe T2 / T2 are 146 / 420. This is only a 10% / 15% increase above the D6 / L3 spring2 already on my car. Is this worth the trouble and expense? Will I get much handling improvement due to the higher rate or just a drop? (assuming BMWP springs are shorter than sport springs). I know a drop by and in itself can improve handling by lower the center of gravity, but, I like my current ride height.

[atr_hugo]

OK - I think I'm pretty ready to move to actually purchasing components. First, I'm now running Pirelli P Zero All Season Plus in 225/40R18 and 255/35R18. Have had them on the car for about half a year and they grip as well (or slightly better) in the dry as the RE050s they replaced, they are MUCH better in the wet than the RE050s, and, not being run-flats, they are WORLDS more comfortable. (More comfortable, but slightly less dry grip than the Michelin Pilot Sport A/S3s I've run before.) Bear in mind this is to be a 'Blue Highway' car, no track days, no autocrosses.

Also, given the natural frequency values of the M Sport, I really see no need to get new springs. Spring rates aren't the problem with the car, it's the pitch and roll issues.

I'll get rid of the voided rear sub frame bushings with the Whiteline KDT 917 two piece solid bushings. Then on to dampers.

I hadn't entirely discounted the Koni FSD - soon to be Koni Special Active. However, one thing to note is that given the 1.27/1.46 F/R natural frequencies of the spring rates for the BMW 135i M Sport suspension, would the frequency be approaching the limits of the low speed damping of the FSDs? Also will the M Sport springs, with their slightly lower height, cause a pre-load condition with the FSDs?

So, for the time, I've eliminated the Koni Special Actives. I ran FSDs on the '08 135i and they were OK - better than stock dampers. What I think it comes down to now is:

Koni yellows or Bilstein B8s

[John_01]

I think Koni yellow is a good choice because they are adjustable. However two downsides:
* Koni dampers require OE style bump stops, so you will need to purchase the BMW performance bump stops, or otherwise cut a few mm off your stock bump stops. Bilstein B6 and B8 include bump stops internally to the Dampers.
* The adjustment on the top of the Koni strut prevents the dust caps from being re-fitted. That's a problem because it will lead to contamination of the strut top bearing. The ideal solution is to have BMW 1M strut brace and strut caps, but it will add hundreds of dollars to the cost.

I never tried Koni FSD, but I was thinking of getting a set for my E90 sedan. I already put koni yellows and a 1M strut brace on my E82, but I'm looking at a different setup for the E90 as the stock dampers are getting a bit tired.

[John_01]

Originally Posted by gjm120

My car is AT and has about every option available. I'm currently on D6 / L3 from buying a used 135i setup. It was a very slight drop from the stock non-sport 128i suspension and I'm happy with the ride height.

From keeping notes reading about spring rates, I believe T2 / T2 are 146 / 420. This is only a 10% / 15% increase above the D6 / L3 spring2 already on my car. Is this worth the trouble and expense? Will I get much handling improvement due to the higher rate or just a drop? (assuming BMWP springs are shorter than sport springs). I know a drop by and in itself can improve handling by lower the center of gravity, but, I like my current ride height.

I agree the T2 / T2 Performance springs would give spring rates around 146lb Front / 420lb Rear. The stiffness change will be fairly subtle, and still a nicely balanced setup. I think the ride height is a more important consideration if you are looking at the ride quality. Lower ride height will mean less suspension travel and more harshness. IMO it doesn't make sense to change unless you are ok with noticeably lower front ride height and increased harshness on bad roads.

If you change to D2 / L2 (128i M-sport), I expect the ride height will change:
-10mm front, -6.5mm rear.

If you change to T2 / T2 (128i Performance) , I expect the ride height will change:
-21mm front, -10.5mm rear.

If you change to T3 / T3 (Rates: 160 / 458) , I expect the ride height will change:
-9mm front, -0.5mm rear.

M-sport suspension is advertised as a 10mm drop, but I guess when you swapped in the D3 / L3 springs from a 135i, the front ride height probably didn't change very much or not at all. Please tell me if I'm wrong about that.

[gjm120]

Originally Posted by John_01

I agree the T2 / T2 Performance springs would give spring rates around 146lb Front / 420lb Rear. The stiffness change will be fairly subtle, and still a nicely balanced setup. I think the ride height is a more important consideration if you are looking at the ride quality. Lower ride height will mean less suspension travel and more harshness. IMO it doesn't make sense to change unless you are ok with noticeably lower front ride height and increased harshness on bad roads.

If you change to D2 / L2 (128i M-sport), I expect the ride height will change:
-10mm front, -6.5mm rear.

If you change to T2 / T2 (128i Performance) , I expect the ride height will change:
-21mm front, -10.5mm rear.

M-sport suspension is advertised as a 10mm drop, but I guess when you swapped in the D3 / L3 springs from a 135i, the front ride height probably didn't change very much or not at all. Please tell me if I'm wrong about that.

You're right - I certainly got less than 10mm drop. (For the front, the smallish drop was partially due to new strut mounts.)

I noticed earlier in this thread you had T2 up front and found it too low and soft. Is your car heavily optioned?

One mitigating factor to the less suspension travel I'd have with the BMWP springs is that different shorter bumpstops are recommended for them. I would expect not to be riding on the stops excessively. None the less I don't want to spend nearly $100/ spring to just be lowered and slightly firmer.

Many thanks for the input!

[atr_hugo]

Originally Posted by John_01

I think Koni yellow is a good choice because they are adjustable. However two downsides:

No free lunches, are there - and the externally adjustable rears are quite dear (from a price perspective ; -)

I never tried Koni FSD, but I was thinking of getting a set for my E90 sedan. I already put koni yellows and a 1M strut brace on my E82, but I'm looking at a different setup for the E90 as the stock dampers are getting a bit tired.

What I remember - it's been a few years since I had them on the car - is that they do roll and pitch less than the stock dampers. I just don't know how they'll work on the '13 - the '08 had progressive rate springs on the sport suspension, IIRC, and I don't think it was a low as the '13's M Sport suspension. But then again, I've been know to be wrong (just ask She Who Must Be Obeyed).

Thanks John

[John_01]

Originally Posted by gjm120

You're right - I certainly got less than 10mm drop. (For the front, the smallish drop was partially due to new strut mounts.)

I noticed earlier in this thread you had T2 up front and found it too low and soft. Is your car heavily optioned?

My car is N55, so the engine will be heavier that the N52 in 128i. The N55 engine block will be heavier and all the turbo components will add up. I don't have any options apart from DCT transmission. No Sunroof, and no navigation system, so it is a pretty basic spec.

The BMW dealership selected the T2 front and T1 rear springs for my car. BMW advertised a 10~15mm drop for the Performance springs compared to M-sport. I guess they are simply aiming to get in that range, but in my case it ended up at least 15mm lower at the front.

[John_01]

Originally Posted by atr_hugo

What I remember - it's been a few years since I had them on the car - is that they do roll and pitch less than the stock dampers. I just don't know how they'll work on the '13 - the '08 had progressive rate springs on the sport suspension, IIRC, and I don't think it was a low as the '13's M Sport suspension. But then again, I've been know to be wrong (just ask She Who Must Be Obeyed).

I've never seen a progressive rate spring for 1 series cars. It seems the "L2" and "L3" coded spring is what's know as the "Linear" spring that was supposed to come out in the later cars. Back to 2006, the 1 series hatchback used springs coded as "D0" or "D1" for the M-sport spec model, which seems to be part of the same numbering system used on the 3 series cars.

The way I look at it, all the E82 and E90 suspension systems actually have progressive spring rates. They use a long polyurethane bump stop that engages a short distance into the bump travel. When combined with the linear springs it provides a overall progressive rate.

[gjm120]

Originally Posted by John_01

I've never seen a progressive rate spring for 1 series cars. It seems the "L2" and "L3" coded spring is what's know as the "Linear" spring that was supposed to come out in the later cars. Back to 2006, the 1 series hatchback used springs coded as "D0" or "D1" for the M-sport spec model, which seems to be part of the same numbering system used on the 3 series cars.

The way I look at it, all the E82 and E90 suspension systems actually have progressive spring rates. They use a long polyurethane bump stop that engages a short distance into the bump travel. When combined with the linear springs it provides a overall progressive rate.

The springs I have are L3 rears. These came off of a 2008 135i which is supposed to be a progressive spring. For what it's worth, I could see nothing in the coil spacing or thickness that would indicate any progressiveness.

[gjm120]

I'm driving myself crazy with the BMWP spring question. Been told T2/T2 are the correct springs from BMW parts people. I'm sure they are correct. The problem is the spring selector is a black box, i.e. no way to add some weight to the car and see what the output is. So.... I found a MT 135i with the same options as my car and had found out it gets T2/T3 front / rear. The 135i has the N55 and MT, mine is N52 w/ Steptronic. Somewhat surprised the stiffer recommended spring is only for the rear.

I think the 135i MT is approx 20 kg heavier than my car. This shows that my vehicle is on the edge of needing stiffer springs. So I'm trying to decide whether to go with the higher rate rear spring. Will this unbalance my handling because of the front / rear relative rates. Or should I go higher rate springs on both ends? Or just take stick with recommended springs?

Another consideration is that using T2/T3 will reduce or eliminate the rake that I will get if John_01's calcs hold true. I might even create a nose up attitude which I would not like.

I'm about to say _ it and go with the recommended springs and see what happens. Any input will be greatly appreciated as always.

[atr_hugo]

Well - the 128i is going to be closer to a 50/50 balance than the 135i (which is around 52/48). So can it take a higher spring rate in back? Remember that the back needs to be just a bit stiffer than the front to make it deal with jolts - the front hits then, some time later - based on vehicle speed, the rear hits the same bump, so the rear needs to be stiffer so that it can catch up to the 'decay' of the front suspension's movement.

As always, there are many folks on here that are more competent than I to help . . .

[gjm120]

Originally Posted by atr_hugo

Well - the 128i is going to be closer to a 50/50 balance than the 135i (which is around 52/48). So can it take a higher spring rate in back? Remember that the back needs to be just a bit stiffer than the front to make it deal with jolts - the front hits then, some time later - based on vehicle speed, the rear hits the same bump, so the rear needs to be stiffer so that it can catch up to the 'decay' of the front suspension's movement.

As always, there are many folks on here that are more competent than I to help . . .

Interesting point about the weight distribution that would explain the slightly heavier 135i getting the stiffer rear spring. I got something very wrong in my post - T3 springs in the back with T2's up front will increase the rake. So T2/T3 is not the way to go. I probably should either go T2/T2 or T3/T3.

[gjm120]

Update on my suspension changeout with BMWP springs and Koni Yellows. After a lot of research and hand wringing, I stuck with the T2/T2 spring recommended by the BMW spring selector.

So far the drop is ~10mm front and 8mm rear. I used the old hardware except for the spring pads as it had less than 7,500 miles. The setup has been on the car for just over 2 weeks and not a lot of miles so there may be more drop coming.

I am really pleased with the ride. As others have stated - less crashy but tighter. Better dampers, stiffer springs and a lower center of gravity = big improvement. I've both front and rear set about 1/2 turn from full soft.

A trivial note - Koni put one adjusting knob in each box. One was the short nearly flat knob and the other was taller with more of a stem. The short knob fits under the strut brace and seems to be low enough to seal the top of the strut from dirt and other debris.

Thanks for all the input and guidance from John_01 and others.