FAQ
Product & Compatibility Questions
Q: How do I know which brake kit is compatible with my BMW?
A: All our kits are vehicle-specific and sorted by BMW M platform. Select your model from the product menu—our kits are engineered for direct bolt-on fitment. If you’re unsure, contact us at sales@rennstop.com for confirmation. We do not currently support any non-M models but may in the future if there is demand, so get in touch if interested.
Q: What’s the difference between the CP9660 and CP9668 calipers?
A: The CP9660 is the lightest true 6-piston racing caliper at approximately 2.78 kg, using 18mm thick pads—ideal for sprint/track sessions where minimizing unsprung weight is key.
The CP9668 weighs around 3.1 kg and accepts thicker 25mm pads, offering significantly longer pad life (designed for endurance racing). It provides the same piston area and performance but requires more wheel clearance—always verify fitment using our wheel templates here. Both deliver excellent modulation and heat management in AP Racing’s legendary Radi-CAL design.
Q: Do your brake kits come with everything needed for installation?
A: Yes, our kits include calipers, discs, brackets, stainless braided lines, and mounting hardware. You’ll need to supply brake pads (we offer compatible options) and fresh brake fluid (we recommend high-performance options like Motul RBF600 or RBF660).
Q: Will an AP Racing brake kit fit under my stock BMW wheels?
A: It depends on the kit and wheel size. Our CP9660-based kits are designed to clear most 18”+ aftermarket wheels. The CP9668 requires more clearance due to the thicker pads—always use our wheel templates here to check fitment.
Q: Do I need to upgrade both front and rear brakes, or can I do front-only?
A: Front-only is a popular and effective upgrade. Our kits maintain near-OEM brake bias, so ABS/traction control work seamlessly without rears upgraded. Most track drivers start with fronts for the biggest gains in stopping power and fade resistance. Rear upgrades add balance for heavy track use, give additional weight saving, improve pedal feel and fade resistance, lower cost of replacement rotors, and visually match the front setup for those of us with OCD!
Q: Are any modifications needed to fit?
A: Our kits are a direct bolt-on; however, AP Racing recommends removal of dust shields. On the front, these unbolt easily—this improves airflow for better cooling and gives clearance for the larger calipers. On the rear, some cutting of the dust shield is required for removal. If you run 3rd party cooling ducts, some modification may be required.
Technical & Installation Questions
Q: Can I install these brake kits myself or do I need a professional?
A: While experienced DIY enthusiasts can install them (they’re direct bolt-on for most BMW M models), we strongly recommend professional installation by a qualified mechanic or BMW specialist. Torque specs provided must be followed—never use impact tools on discs and caliper brackets. Proper installation is critical for safety.
Q: What brake pad compounds do you recommend for track use vs. street use?
- Street/spirited driving: EBC Bluestuff or Ferodo DS2500—these offer excellent cold bite, low noise/dust, and good modulation without being too aggressive.
- Track/aggressive use: EBC RP-X/SR11, Ferodo DS1.11/DSUNO, Pagid RS29, or similar race compounds for consistent high-temperature performance and fade resistance.
We stock or can source pads matched to your caliper (CP9660, CP9668, or CP9449 pad shapes). Always match pads to your primary use—track pads will be noisy/dusty on the street.
Q: Does installing a big brake kit affect my brake bias or pedal feel?
A: Our kits are torque-matched to OEM specs for balanced bias. AP calipers will improve pedal feel due to the stiff design and anti-knockback springs. You get firmer, more consistent response with less fade. Proper bleeding and bedding are key for optimal feel.
Performance & Track Day Questions
Q: How much unsprung weight do I save with an AP Racing kit?
A: Up to 15kg for our F8X front kit depending on your current setup. If coming from the standard blue calipers with 380x30mm discs, expect a weight saving of approximately 4.5kg. If you're currently running the 2NH 400x36mm setup, expect a saving of around 15kg.
Q: Are AP Racing kits suitable for daily street driving?
A: Yes, especially with street-friendly pads (e.g., Ferodo DS2500). They offer better modulation and heat tolerance than stock, but expect more dust/noise with aggressive compounds. They will not be as quiet/low-maintenance as OEM for daily driving. Pro 5000 R calipers do not have dust boots so will require more maintenance if driven in harsh environments. When changing pads, always clean the pistons before pushing back in to avoid damage to the seals.
Q: What’s the difference between your kits and cheaper “big brake” options online?
A: Genuine AP Racing Radi-CAL calipers use aerospace-grade materials, proper piston sizing and proven endurance designs. Cheaper kits often use mismatched or low-quality parts that fail with heat or offer poor modulation. We focus on safety, longevity and real performance gains—we offer premium kits without the premium price tag.
Q: Will upgrading my brakes improve my lap times?
A: Yes, significantly for most drivers. Our AP Racing kits reduce unsprung weight (by up to 15kg per axle), provide better heat dissipation, stronger bite, and more consistent pedal feel under heavy use. This means less fade, better trail-braking and more confidence—often translating to much faster lap times depending on the car, driver skill and track.
Ordering & Shipping Questions
Q: Do you ship internationally?
A: Yes, we ship worldwide from the UK. International customers are responsible for any import duties, VAT, or customs fees. Contact us for a shipping quote to your location.
Q: How long does delivery typically take within the UK?
A: Most UK orders ship within 1–3 business days (many items in stock) and arrive in 1–2 days via tracked courier. Made-to-order or custom items may take 7–14 days.
Q: What if I order the wrong part?
A: Please see our returns policy here. As some components are supplied/manufactured to order, we may charge a re-stocking fee. If you contact us before ordering, we can make sure that you order the correct parts/kit for your application.
Q: Are all items in stock or made to order?
A: Popular components are held in stock for fast shipping. Kits are assembled to order (typically with a 7–21 days lead time). Product pages indicate an estimated ship date, but feel free to email for current availability.
Q: Why choose AP Racing over Brembo, StopTech, or OEM M Performance brakes?
A: AP Racing dominates motorsport (F1, endurance racing) for lightweight design, fade resistance and pad/disc availability. Our custom kits for BMW M cars offer superior heat management and unsprung weight savings compared to many alternatives—perfect for serious track enthusiasts.
About Renn Stop
Q: Where are you based?
A: We’re proudly based in the beautiful Cotswolds, UK—right in the heart of some of the country’s best driving roads!
Q: Do you offer technical support after purchase?
A: Absolutely. Every kit includes detailed installation instructions. We always recommend professional fitting, but we’re here for ongoing support—whether it’s troubleshooting, recommending spares, pads, or setup advice. Just email sales@rennstop.com or reach out via our contact form.
Q: Do you offer customisations or design services?
A:Yes, we can offer custom colours for all anodised parts with a 2-4 week lead time. We will also consider requests for new platforms or take on custom design / manufacturing projects. Just get in touch at sales@rennstop.com to discuss.
Q: Can I visit your workshop or showroom?
A: We don’t currently have a public showroom or walk-in facility, but we’re fully committed to customer support. Drop us a line at sales@rennstop.com with any questions—we respond quickly and love helping fellow BMW enthusiasts get the most from their AP Racing upgrades.