Spring rates available for TTX-25

[jose_90]

Hello,
My name is Jose Maria Martin, and I'm a student in a new team, from Seville, Spain. I work in the suspension team, and I was wondering if anyone here could help. The thing is, we were going to use the Ohlins TTX 25 mkII, and have designed the suspension with it in mind.
However, we (in fact, I) made the mistake of not checking the available spring rates at the start, so now I have realized that with my motion ratio, weight, and desired ride frequencies, I would need a spring with a rate below 150 lb/in, which is the minimum. So, my question is, has anyone found and used other springs, with lower spring rates, that work with that damper? The other option would be to modify my motion ratio, but I am trying to avoid that.

Thank you for your help!

[Goost]

See if Cane Creek has what you need.
The ones we use have always proven linear within a percent or two through the operating travel range.

EDIT:
At one point we bought a kit with several sets of springs, there was a set of 150lbf/in in there but I think that was the lowest rate

[mech5496]

I will second Goost; we had the same kit a few years ago and 150lb/in is the lowest

[Claude Rouelle]

f = 1/2 sqrt (k/m). For a given target frequency, with lighter and lighter FS / FSAE cars this problem is raising. I have seen spring in FSAE as soft as 6.5 N/mm = 37 lb/in for car under 200 kg (DRIVER INCLUDED!)

Remember that Google is your best friend. Just look at spring but not only for race cars. If it is not in any catalogue, have the spring being manufactured.

There must be in your country or even your town a spring manufacturer. Race teams do have specific spring manufactured to their specific needs. That is what I did when I designed and manufactured my first race car (as the thesis of my engineering degree). And I was persuasive enough to have the spring manufacturer to help a "poor young entrepreneurial student" that he did not make me pay for it :). I am sure you can go through the same process.

[Drew Price]

Maybe look for springs that are actually designed to be 'helper springs' to keep the main spring from drooping? The little short ones that are meant to coil bind relatively quickly? They should be much softer.

Like these, the little blue one:

http://1.bp.blogspot.com/_NWBurkUfM0E/S3VexGu1mZI/AAAAAAAAAEQ/SkGXkouRe3o/s400/spring+seat+rings.JPG

[jose_90]

thank you for all the responses, I really appreciate it. I'll try google and if not there seems to be a company close to me that manufactures them.


f = 1/2 sqrt (k/m). For a given target frequency, with lighter and lighter FS / FSAE cars this problem is raising. I have seen spring in FSAE as soft as 6.5 N/mm = 37 lb/in for car under 200 kg (DRIVER INCLUDED!)


I wish my problem was caused by my car being 200 kg ;) it's quite closer to 300 (including the driver). It seems to me that the available spring rates for that damper are designed for use with high motion ratios (mr=wheel:spring), and ours is almost 1.

Thank you all again!

[JT A.]

Do a search for "1/4 midget springs". They are roughly the same free length, 5mm larger ID, and they come in rates from 70-150 lb/in in very small increments. I think they would probably work on the TTX25 but I haven't tried it. You might have to make a custom perch for the larger diameter, but the original spring perches may be large enough.

[Claude Rouelle]

FYI

Without driver, 135 kg for a mono cylinder without wings, 145 kg with wings is a good target. For a 4 cylinders 175 kg without wings, 185 with wings is another reasonable target.

If my informations are correct several teams will beat these numbers in 2014.

These numbers are for an experienced teams. Do not try to reach these low numbers and compromise structural integrity and reliability if you are lacking design, manufacturing and testing experience.

The 2013 specific car with 37 lb/in spring (it was a front spring) I have in mind was in 145 kg region and had a motion ratio of about 0.9 (wheel mvt / spring mvt) although that number did not take into account any compliance.

[Luniz]

The internal diameter of the springs for the TTX25 is 36mm, which comes in quite handy as the spec dampers for Formula 3 cars are the same ID. We used to get our springs from www.h-r.com , they provide springs for quite a lot of FS teams so they already have some FS-Specific stiffnesses in the shelf.

[Goost]

Claude,

Is your comment that they are front springs implying a high rear weight distribution or preloaded springs?
Or perhaps neither; I may not follow correctly but thought it was a curious clarification to make.

~~~

On a bit of a different note (but may still be relevant if it leads to design changes for jose_90):

Do any teams have experience with preloaded springs on cars of our size / down-force levels?
Maybe Danny has comments, I know he published some work on it at one point.

[Claude Rouelle]

Goost,

Slightly lighter front suspended mass (not much; lets' say about 48 % front) and slightly smaller front suspended mass natural frequency target makes slightly softer spring front.

Note: there is a more sophisticated way to speak about car ride and front / rear suspended mass natural frequency ratio. It takes into account a bit of the kinematics, the pitch inertia and the front and suspension stiffness and the notion of the spring center in heave and in pitch. But although rudimental the simple front / rear suspended mass natural frequency ratio is still helpful.

A smaller suspended mass frequency front than rear makes sense unless you have a very ride height sensitive car. I have not seen yet a FS / FSAE car exploiting ground effect well enough that 1 mmm of front ride height difference does make a significant balance difference so we won't explore that particular topic here but lt say that if the effect of 1 mm of front ride height change is as sensitive as 3 mm of rear ride height change on the aerobalance, at quasi equal front and rear suspended mass and quasi equal front and rear motion ratios, you will necessarily need a stiffer suspension (so a stiffer spring) front than rear.

If you want to speak about preload and droop limiting you just opened a can of worms. It could be useful to exploit a very ride height aerodynamically sensitive car but you need to first understand aeromaps effect on car grip and, balance (=yaw moment). You also need to understand your tires theoretically and practically. It deserves a specific, separate thread in this forum. If you want to start it go ahead.

I would just say that a first step would be calculate if the desired preload you will be creating on your spring by turning the spring platform against the fully extended damper (imagine you car on stands)will be bigger, equal or smaller than your suspended mass effect (at 1 G of vertical acceleration) on that spring. Do not forget to take into account the "preload" coming from the damper gas pressure * the shaft section (unless you have a shaft thru damper)

Have fun.